Hydroponic Glossary of Terms

abscisic acid

–
A growth-inhibiting hormone.

abscission

–
The dropping of leaves, flowers, or fruit by a plant. This can result from natural growth processes (e.g., fruit ripening) or from external factors such as temperature or chemicals.

abscission layer

–
Specialized cells, usually at the base of a leaf stalk or fruit stem, that trigger both the separation of the leaf or fruit and the development of scar tissue to protect the plant.

absorption

–
The intake of water and other materials through root or leaf cells.

accumulated heat units

–
The number of hours in a growing season. Usually calculated at temperatures above 50°F, but can be calculated at other temperatures, depending on the crop. A day’s heat units (above 50°F) are calculated as: Daily values are then totaled for the season, with values less than zero ignored (but not deducted from the total).

acid soil

–
Soil with a pH below 7 on a pH scale of 0 to 14. The lower the pH, the more acid the soil. Also see possible Hydrogen.

actinomorphic flower

–
A flower possessing radial symmetry. Any cut through the center divides the flower into two equal parts.

active ingredient

–
As it relates to pesticides, it is the chemical in a pesticide formulation that actually kills the target pest.
In relation to synthetic nutrients like Emerald Harvest, an active ingredient would be “Mono Ammonium Phosphate”

adjuvant

–
A substance that, when added to a pesticide, reduces the surface tension between two unlike materials (e.g., spray droplets and a plant surface), thus improving adherence. Also called surfactant.

adventitious

–
Growth not ordinarily expected, usually the result of stress or injury. A plant’s normal growth comes from meristematic tissue, but adventitious growth comes from nonmeristematic tissue.

adventitious bud

–
A bud in an unusual place on a plant, often on an internode. This may be the result of an injury. Suckers and water sprouts usually grow from adventitious buds.

adventitious root

–
A root in an unusual place, often where a branch contacts soil or damp material. A plant can not be reproduced from cuttings or layering unless adventitious roots develop.

aeration

–
Mechanically loosening or puncturing soil to increase permeability to water and air.

aerial root

–
A root emerging above the soil level. aerobic Active in the presence of free oxygen.

aeroponics

–
A variation of hydroponics that involves the misting of plant roots with nutrient solution.

air

–
the invisible gaseous substance surrounding the earth, a mixture mainly of oxygen and nitrogen.

Airflow

–
or air flow, is the movement of air. The primary cause of airflow is the existence of air. Air behaves in a fluid manner, meaning particles naturally flow from areas of higher pressure to those where the pressure is lower. Atmospheric air pressure is directly related to altitude, temperature, and composition.

In engineering, airflow is a measurement of the amount of air per unit of time that flows through a particular device.

The flow of air can be induced through mechanical means (such as by operating an electric or manual fan) or can take place passively, as a function of pressure differentials present in the environment.

Air Exchange Rate or Air Changes per Hour (ACH)

–
The air exchange rate in your house tells you important things about the effectiveness of your HVAC system as well as the health of your indoor air quality.

Your heating and cooling system delivers new airflow into rooms through supply ducts while removing air through return ducts. How rapidly the system completely replaces the existing air volume in a room with fresh air is known as the air exchange rate, or air changes per hour (ACH). If the rate for a room is calculated at 7, for example, that means the entire volume of air inside that room is replaced with new air 7 times per hour.
How is Air Exchange Rate Determined?

Using equipment to measure airflow through the supply vent, an HVAC technician can determine the air volume entering a room expressed as CFM (cubic feet per minute). The CFM is multiplied by 60 minutes per hour, then that figure is divided by the total volume of the room—length x width x height. The result is the present air exchange rate for that room.

after-ripening

–
The seed maturation process that must be completed before germination can occur.

aggergate fruit

–
A group of small fruits derived from several ovaries within a single flower.

aggregation

–
The process by which individual particles of sand, silt and clay cluster and bind together to form soil peds.

alkaline

–
Refers to medium or nutrient solution with a high pH; any pH over 7 is considered alkaline.

alkaline soil

–
Soil with a pH above 7 on a pH scale of 0 to 14. The higher the reading, the more alkaline the soil. See pH.

alkaloid

–
A nitrogen-containing compound frequently used as a chemical defense by plants.

allele

–
Different forms of the same gene; allele “A” may produce a tall plant, while allele “a” gives a short plant.

allelopathy

–
The excretion by some plants of compounds from their leaves and/or roots that inhibit the growth of other plants.

ammonium (NH4+)

–
A plant-available form of nitrogen contained in many fertilizers and generated in the soil by the breakdown of organic matter. See nitrogen cycle.

anaerobic

–
Active in the absence of free oxygen.

angiosperm

–
A member of a class of plants characterized by the formation of flowers and seeds in fruits.

anion

–
A negatively charged ion. Plant nutrient examples include nitrate (NO3-), phosphate (H2PO4-), and sulfate (SO42-). See cation.

annual

–
A plant that completes its life cycle in one growing season.

annual ring

–
A cylinder of secondary xylem added to the wood in a single growing season.

antagonism

–
The effect of a deficiency or toxicity of an element that restricts or interferes with the uptake.

anther

–
The pollen-bearing part of a flower’s male sexual organ. The filament supports the anther; together they are referred to as the stamen.

anthocyanin

–
A blue, violet, or red flavonoid pigment found in plants.

anvil pruner

–
A pruning tool that cuts a branch between one sharpened blade and a flat, anvil-shaped piece of metal. These have a tendency to crush rather than make a smooth cut.

aquaponics

–
The integration of aquaculture (the raising of marine animals, such as fish) with hydroponics; the waste products from the fish are treated and then used to fertilize hydroponically growing plants.

apex

–
The tip of a stem or root.

apical bud

–
A bud at the tip of a stem.

apical dominance

–
The inhibition of lateral bud growth by the presence of the hormone auxin in a plant’s terminal bud. Removing the growing tip removes auxin and promotes lateral bud break and subsequent branching, usually directly below the cut.

apical meristem

–
A region of actively dividing cells at the tip of a growing stem or root.

arboretum

–
An area devoted to specimen plantings of trees and shrubs.

asexual reproduction

–
See vegetative propagation.

aspect

–
Direction of exposure to sunlight.

assimilation

–
The building of cell matter from inorganic and organic materials (carbohydrates and sugars).

atomic weight

–
The relative weight of an atom.

attractant

–
A material that lures pests.

autotrophic nutrition

–
A form of nutrition in which complex food molecules are produced by photosynthesis from carbon dioxide, water, and minerals.

auxin

–
One of the best known and most important plant hormones. Most abundantly produced in a plant’s actively growing tips. Generally stimulates growth by cell division in the tip region and by cell elongation lower down the shoot. Growth of lateral buds is strongly inhibited by the normal concentration of auxin in the growing tip.

available water supply

–
Soil water that is available for plant uptake. Excludes water bound tightly to soil particles.

axil

–
The upper angle formed by a leaf’s stalk (petiole) and the internodes above it on a stem.

axillary bud

–
A bud that forms on an axil.

axillary bud primordium

An immature axillary bud.

bacillus thuringiensis (Bt)

–
A bacterium used as a biological control agent for many insect pests; primarily mosquitoes, fungus gnats, and caterpillars. Take a look at Mosquito Dunks.

bacterial soft rot

–
See botrytis.

bacterium

–
A single-celled microscopic organism having a cell wall but no chlorophyll. They reproduce by cell division.

balled root plant

A plant dug with soil. The root ball is enclosed with burlap or a synthetic material.

band

–
To apply a pesticide or fertilizer in a strip over or along each crop row.

bare-root (BR)

A plant with little or no soil around it’s roots; a common method of selling deciduous plants and small evergreens.

bark

–
All the tissues, collectively, formed outside the vascular cambium of a woody stem or root.

basal

(1) At or near the base of a branch or trunk.
(2) At or near a plant’s crown.

basal break

–
New growth that develops at the base of a branch or near a plant’s crown.

beneficial insect

–
An insect that helps gardening efforts. May pollinate flowers, eat harmful insects or parasitize them, or break down plant material in the soil, thereby releasing nutrients. Some insects are both harmful and beneficial. For example, butterflies can be pollinators in their adult form, but destructive in their larval (caterpillar) form.

berry

–
The fleshy fruit of cane fruits, bush fruits, and strawberries.

biennial

–
A plant that germinates and produces foliage and roots during its first growing season, then produces flowers and seeds and usually dies during its second growing season.

biennial bearing

–
Producing fruit in alternate years.

bio-solids

–
A by-product of wastewater treatment sometimes used as a fertilizer.

blade

–
The flattened part of a leaf.

blanch

–
To exclude light from plants or parts of plants to render them white or tender. Often done to cauliflower, endive, celery, and leeks.

blight

–
Rapid, extensive discoloration, wilting, and death of plant tissue.

bloom booster

–
Fertilizer high in phosphorus (P) that increases flower yield.

blossom-end-rot (BER)

–
A physiological and nutritional disorder on fruit creating a black, leathery, sunken appearance on the blossom end of the fruit – often associated with poor watering, root death, and calcium deficiency.

blotch

–
A blot or spot (usually superficial and irregular in shape) on leaves, shoots, or fruit.

bole

–
See trunk.

bolting

–
Producing seeds or flowering prematurely, usually due to heat. For example, cool-weather crops such as lettuce bolt during the summer. Leaf crops are discouraged from bolting by removal of flower heads. See deadhead.

bonsai

–
One of the fine arts of gardening; growing carefully trained, dwarfed plants in containers selected to harmonize with the plants. Branches are pruned and roots trimmed to create the desired effect. botanical insecticide An insecticide, such as rotenone or pyrethrum, derived from a plant. Most botanicals biodegrade quickly. Most, but not all, have low toxicity to mammals.

botrytis

–
A fungal disease promoted by cool, moist weather. Also known as gray mold or fruit rot.

bract

–
A modified leaf, usually small, but sometimes large and brightly colored, growing at the base of a flower or on its stalk. Clearly seen on dogwoods and poinsettias.

bramble

–
A spiny cane bush with berry fruits (e.g., raspberries and blackberries).

branch

–
A subsidiary stem arising from a plant’s main stem or from another branch.

break

–
(1) Any new growth coming from a bud.
(2) See bud break.

broadcast

–
(1) To sow seed by scattering it over the soil surface.
(2) To apply a pesticide or fertilizer uniformly to an entire, specific area by scattering or spraying it.

broadleaf evergreen

–
A non-needled evergreen.

British Thermal Unit (BTU)

–
Amount of heat required to raise the temperature of 1 pound of water 1°F.

bud

–
A small protuberance on a stem or branch, sometimes enclosed in protective scales, containing an undeveloped shoot, leaf, or flower.

bud break

–
The resumption of growth by a resting bud.

bud head

–
A swollen or enlarged area where a bud was grafted to a stock.

bud scale

–
A modified leaf that forms a protective covering for a bud.

bud sport

–
See mutation.

bud union

–
The suture line where a bud or scion was grafted to a stock. Sometimes called the graft union.

budding

–
The grafting of a bud onto stock of a different plant. The bud is the scion.

budstick

–
A shoot or twig used as a source of buds for budding.

buffering action

–
The ability of a nutrient solution or raw water to resist changes in pH

bulb

–
An underground storage organ consisting of a thin, flattened stem surrounded by layers of fleshy, dried leaf bases. Roots are attached to the bottom. See corm, tuber, rhizome.

bulbil

–
A small bulb-like organ that sometimes forms in place of flowers.

bulblet

–
(1) An underground bulbil.
(2) A tiny bulb produced at the base of a mother bulb.

buttress root

–
An enlarged, aboveground root giving support to a tree trunk.

calcium carbonate (CaCO3)

–
A compound found in limestone, ashes, bones, and shells; the primary component of lime.

callus

–
Tissue that develops when cambium or other meristematic tissue is wounded.

calorie

–
Amount of heat required to raise the temperature of 1 cubic centimeter of water 1°C.

calyx

–
The collective term for the sepals (the cup, usually green, between a flower and its stem).

cambium

–
The living, growing layer of cells between the xylem and phloem. In woody plants, it is located just beneath the bark.

candelabrum

–
A strong, dominant rose cane with accelerated growth that originates from a bud union and explodes with many blooms.

candle

–
On a pine tree, new terminal growth from which needles emerge.

cane

–
The externally woody, internally pithy stem of a bramble or vine.

canker

–
A localized lesion on a limb or trunk, usually due to disease or injury. Part of the bark or wood appears to be eaten away or is sunken.

canopy

–
(1) The top branches and foliage of a plant.
(2) The shape-producing structure of a tree or shrub.

capillary force

–
The action by which water molecules bind to the surfaces of soil particles and to each other, thus holding water in fine pores against the force of gravity.

capillary water

–
Water held in the tiny spaces between soil particles or between plant cells.

capitulum

–
(1) A dense, short, compact cluster of sessile flowers (stalkless and attached directly at the base), as in composite plants or clover.
(2) A very dense grouping of flower buds, as in broccoli.

carotene

–
A orange-yellow pigment located in the chloroplasts.

caterpillar

See larva. Also a type of excavation equipment.

catfacing

–
Disfigurement or malformation of a fruit. Fruits typically affected include tomatoes and strawberries. Although not fully understood, catfacing is thought to be caused by insects or adverse weather during fruit development.

cation

–
A positively charged ion. Plant nutrient examples include calcium (Ca++) and potassium (K+). See anion.

Cation Exchange Capacity (CEC)

–
A soil’s capacity to hold cations as a storehouse of reserve nutrients.

cell

–
The smallest structure in a plant.

central leader

–
(1) A trunk or stem extending up through the axis of a tree or shrub and clearly emerging at the top.
(2) A system of pruning that uses the central leader as a basic component.

cell wall

–
The outer covering of a plant cell.

cellular respiration

–
The chemical breakdown of food substances, resulting in the release of energy.

cellulose

–
A plant substance forming part of the cell wall.

cercus

–
A thread-like or sometimes forceps-like tail near the tip of an insect’s abdomen (usually a pair). Plural: cerci.

chelate

–
A complex organic substance that holds micronutrients, usually iron, in a form available for absorption by plants.

chlorophyll

–
The green pigment in plants. Responsible for trapping light energy for photosynthesis.

chloroplast

–
A specialized component of certain cells. Contains chlorophyll and is responsible for photosynthesis.

chlorosis

–
An abnormal yellowing of a leaf.

chiller

–
A refrigeration unit used to reduce the temperature of water or nutrient solution.

chromosome

–
A threadlike structure within each living cell which contains the cell’s genetic material.

cladode

–
A flattened stem performing the function of a leaf, as in a cactus pad.

cladosporium

–
Any of several fungal diseases that afflict plants; commonly called leaf mold.

clay

–
The smallest type of soil particle (less than 0.002 mm in diameter).

climber

–
A plant that climbs on its own by twining or using gripping pads, tendrils, or some other method to attach itself to a structure or another plant. Plants that must be trained to a support are properly called trailing plants, not climbers.

cloche

–
A plastic, glass, or plexiglas plant cover used to warm the growing environment or protect plants from frost.

clone

–
A plant group whose members have all been derived from a single individual through constant propagation by vegetative (asexual) means, e.g. by buds, bulbs, grafts, cuttings, or laboratory tissue culture.

closed system

–
A hydroponic system, like nutrient film technique (NFT) systems, that recirculates the nutrient solution. C:N ratio The ratio of carbon (C) to nitrogen (N) in organic materials. Materials with a high C:N ratio (high in carbon) are good bulking agents in compost piles, while those with a low C:N ratio (high in nitrogen) are good energy sources.

cold composting

–
A slow composting process that involves simply building a pile and leaving it until it decomposes. This process may take several months or longer. Cold composting does not kill weed seeds or pathogens.

cold frame

–
A plastic-, glass-, or plexiglas-covered frame that relies on sunlight as a source of heat to warm the growing environment for tender plants.

cold hardening

–
The process where plants prepare for low temperatures.

cole crops

–
A group of vegetables belonging to the cabbage family; plants of the genus Brassica, including cauliflower, broccoli, cabbage, turnips, and brussels sprouts.

coleoptera

–
An insect family made up of species having horny front wings that fit over their hindwings. Includes beetles and weevils.

collar

–
A swollen area at the base of a branch where it connects to a trunk. Contains special tissue that prevents decay from moving downward from the branch into the trunk. See shoulder ring.

compaction

–
Pressure that squeezes soil into layers that resist root penetration and water movement. Often the result of foot or machine traffic.

companion planting

–
The practice of growing two or more plants together in the hope that the combination will discourage disease and insect pests.

compatible

–
Different varieties or species that set fruit when cross-pollinated or make a successful graft union when intergrafted. See pollenizer.

complete flower

–
A flower having all of the normal flower parts.

complete metamorphosis

–
A type of insect development in which the insect passes through the stages of egg, larva, pupa, and adult. The larva usually is different in form from the adult. See simple metamorphosis.

composite head

–
A inflorescence composed of many tightly-packed, small, ray and disc flowers.

compost

–
The product created by the breakdown of organic waste under conditions manipulated by humans. Used to improve both the texture and fertility of garden soil. See humus.

compound bud

–
More than one bud on the same side of a node. Usually, unless growth is extremely vigorous, only one of the buds develops, and its branch may have a very sharp angle of attachment. If it is removed, a wider angled shoot usually is formed from the secondary (accessory) bud. Ashes and walnuts are examples of plants that typically have compound buds.

compound leaf

–
A leaf in which the blade is divided into separate leaflets.

conductivity

–
The scale, described as electrical conductivity (EC) or conductivity factor (CF), that is used to measure the strength of nutrient solution.

conifer

–
A cone-bearing tree or shrub, usually evergreen. Pine, spruce, fir, cedar, yew, and juniper are examples.

conk

–
A fungal fruiting structure (e.g., shelf or bracket fungi) formed on rotting woody plants.

contact pesticide

–
A pesticide that kills on contact.

Controlled Environmental Agriculture (CEA)

–
The growing of plants in structures as greenhouses that permit the regulation of optimum environmental conditions for the crop year-round regardless of ambient weather conditions.

cordon

–
(1) A method of espaliering fruit trees, vines, etc. to horizontal, vertical, or angles wire or wooden supports so maximum surface is exposed to the sun, resulting in maximum fruit production.

(2) A branch attached to such a support.

cork

–
The protective outer tissue of bark.

cork cambium

–
A layer of cells in the cambium that gives rise to cork.

corm

–
An underground storage organ consisting of the swollen base of a stem with roots attached to the underside. Crocus and gladiolus are examples of plants that form corms. See bulb, tuber, rhizome.

cormel

–
A small, underdeveloped corm, usually attached to a larger corm. See bubil and bulblet.

cornicle

–
A short, blunt horn or tube (sometimes button-like) on the top and near the end of an aphid’s abdomen. Emits a waxy liquid that helps protect against enemies.

corolla

–
Collectively, all of a flower’s petals.

cortex

–
Cells that make up the primary tissue of roots or stems.

corymb

–
A usually flat-topped flower cluster in which the individual flower stalks grow upward from various points on the main stem to approximately the same level.

cotyledon

–
A seed leaf; the first leaf from a sprouting seed. Monocots have one cotyledon; dicots have two.

cover crop

–
A crop dug into the soil to return valuable organic matter and nitrogen to the soil. Legumes such as clover, cowpeas, and vetch are common cover crops. Also called green manure.

critical photoperiod

–
The maximum day length a short-day plant, and the minimum day length a long-day plant, require to initiate flowering.

cropping cycle

–
The time period during which the plant grows from seeding until final harvest and its subsequent removal.

cross-pollination

–
The fertilization of a ovary on one plant with pollen from another plant, producing an offspring with a genetic makeup distinct from that of either parent. See pollenizer.

crotch angle

–
The angle formed between a trunk and a main scaffold limb. The strongest angle is 45° to 60°.

crown

(1) Collectively, the branches and foliage of a tree or shrub.
(2) The thickened base of a plant’s stem or trunk to which the roots are attached.

cultivar

–
A specially cultivated variety of a plant that most often is reproduced vegetatively. For example, ‘Transparent’ is a cultivar of apple. See variety.

cuticle

–
(1) A relatively impermeable surface layer on the spidermis of leaves and fruits.
(2) The outer layer of an insect’s body.

cutin

–
(1) A waxy substance on plant surfaces that tends to make the surface waterproof and can protect leaves from dehydration and disease.
(2) A waxy substance on an insect’s cuticle that protects the insect from dehydration.

cutting

–
A piece of leaf, stem, or root removed from a plant and prompted to develop into a new plant that is genetically identical to the parent plant.

cyme

–
A flower stalk on which the florets start blooming from the top of the stem and progress toward the bottom.

cyst

–
The swollen, egg-containing female body of certain nematodes. Can be seen on the outside of infected roots.

cytokinin

–
A plant hormone primarily stimulating cell division.

cytoplasm

–
The living protoplasm of a cell, excluding the nucleus.

cytoplasmic membrane

–
The membrane enclosing the cytoplasm.

damping-off

–
A disease caused by many different organisms. In the most conspicuous cases, a seedling’s stem collapses at or near the soil surface, and the seedling topples. Another type rots seedlings before they emerge from the soil or causes seeds to decay before germinating.

day-neutral plant

–
A species capable of flowering without regard to day length. See also short-day plant, long-day plant.

deadhead

–
To remove individual, spent flowers from a plant for the purpose of preventing senescence (going dormant) and prolonging blooming. For effective results, the ovary behind the flower must be removed as well.

deciduous

–
A plant that sheds all of its leaves annually.

decomposition

–
The breakdown of organic materials by microorganisms.

deep flow/raft culture

–
A hydroponic system commonly used for lettuce production in hot climates where the plants are supported on top of a bed of nutrient solution by Styrofoam boards floating on the solution.

defoliation

–
The unnatural loss of a plant’s leaves, generally to the detriment of its health. Can be caused by high winds, excessive heat, drought, frost, chemicals, insects, or disease.

dehorning

–
A drastic method of pruning a neglected tree or shrub. Entails the removal of large branches, especially high in the crown, a few at a time over several seasons.

dermaptera

–
An insect family made up of species having chewing mouthparts and a pair of large, forceps-like appendages near the tail. Wingless or with one or two pairs of inconspicuous wings. Earwigs are an example.

desiccation

–
Drying out of tissue. determinate
A plant growth habit in which the stems stop growing at a certain height and produce a flower cluster at the tip. Determinate tomatoes, for example, are short, early-fruiting, have concentrated fruit set, and do not require staking. See indeterminate.

dethatch

–
To remove thatch (a tightly intermingled layer of stems, leaves, and roots, living and dead, that forms between the soil surface and green vegetation of grass).

diageotropic

–
Horizontal growth of a plant part.

diatomaceous earth

–
The fossilized remains of diatoms (a type of tiny algae).

dicot

–
A plant having two cotyledons (seed leaves).

dicotyledon

–
See dicot.

dieback

–
Progressive death of shoots, branches, or roots, generally starting at the tips.

differentiation

–
A change in composition, structure, or function of cells and tissues during growth.

dioecious

–
A plant species having male and female flowers on separate plants. An example is holly. See monoecious.

disbud

–
The selective removal of some flower buds so the remaining buds receive more of the plant’s energy and produce larger, showier flowers. Roses, chrysanthemums, and camellias often are disbudded.

disc flower

–
A small, tubular flower in the center of a composite head.

division

–
The breaking or cutting apart of a plant’s crown for the purpose of producing additional plants, all genetically identical to the parent plant.

DNA Deoxyribonucleic acid

–
The substance that the genes which carry genetic information is made of.

dominate species

–
The most abundant species in a plant community.

dormancy

–
The annual period when a plant’s growth processes greatly slow down.

dormant bud

–
A bud formed during a growing season that remains at rest during the following winter or dry season. If it does not expand during the following growing season, it is termed a latent bud.

dormant oil

–
A horticultural oil applied during the dormant season to control insect pests and diseases.

double, semidouble

–
A flower with more than the normal number of petals, sepals, bracts, or florets. May be designated botanically by the terms flore pleno, plena, or pleniflora. double worked Grafted twice, i.e. grafted to an intermediate stock.

drainage

–
The ability of soil to transmit water through the surface and subsoil.

drip irrigation

–
A type of irrigation system by which each plant is fed individually with a small drip tube and the flow is regulated by an emitter commonly used in most hydroponic systems.

drip tip

–
A pointed leaf tip helping to drain water from the leaf surface.

drip zone

–
The area from the trunk of a tree or shrub to the edge of its canopy. Most, but not all, of a plant’s feeder roots are located within this area.

dripline

–
An imaginary line on the ground directly beneath the outermost tips of a plant’s foliage. Rain tends to drip from leaves onto this line.

drupe fruit

–
See stone fruit.

dwarfed

–
Restricted plant size without loss of health and vigor.

deep water culture (DWC)

–
A hydroponic method of plant production by means of suspending the plant roots in a solution of nutrient-rich, oxygenated water, also known as bubbleponics. ebb-and-flow a hydroponic system in which the plants are sub-irrigated periodically and the nutrient solution drains back to a central cistern for subsequent cycles.

ebb-and-flow

–
A hydroponic system in which the plants are sub-irrigated periodically and the nutrient solution drains back to a central cistern for subsequent cycles

ecology

–
The science of relationships between organisms and their environment.

economic threshold

–
The level at which pest damage justifies the cost of control. In home gardening, the threshold may be aesthetic rather than economic.

egg

–
A female sex cell.

Electrical Conductivity (EC)

A measure of the ability of a nutrient solution to conduct electricity, which is dependent upon the ion concentration and nature of the elements present.

emasculate

–
To remove a flower’s anthers.

embryo

–
The dormant, immature plant within a seed; the “germ” referred to in wheat germ.

embryo culture

–
See tissue culture.

enation

–
Epidermal outgrowths on leaves or stems.

endodermis

–
A layer of cells in roots between the cortex and the vascular tissues.

endosperm

The nutritive tissue within the seed of a flowering plant. Surrounds and is absorbed by the embryo.

enzyme

–
A biological catalyst that aids in a specific biochemical process, such as converting food from one form to another.

epidermal hair

–
A filament of cells arising from an epidermal cell.

epidermis

–
The outermost layer of cells covering a plant’s leaves, roots, and young parts.

epigeous germination

–
Seed germination in which the cotyledons are raised above the soil surface.

epinasty

–
An abnormal downward-curving growth or movement of a leaf, leaf part, or stem.

epiphyte

–
A plant growing on another plant for support.

espalier

–
The training of a tree or shrub to grow flat on a trellis or wall. Espalier patterns may be very precise and formal or more natural and informal.

ethylene

–
A gaseous plant hormone (C2H4) produced in abundance by ripening fruits and damaged tissues.

etiolation

–
The condition where a plant is grown in darkness, resulting in pale and elongated stems and underdeveloped leaves.

evapotranspiration

–
The loss of water from a plant through evaporation and transpiration; critical to the uptake of minerals and cooling of the plant through movement of water within the entire plant.

evergreen

–
A plant that never loses all of its foliage at the same time.

excise

–
To remove or extract, as an embryo from a seed or ovule.

excurrent

–
A tree form in which the main trunk remains dominant with small, more or less horizontal branches. Fir and sweetgum are examples.

exfoliating

–
Peeling off in shards or thin layers, as in bark from a tree.

exoskeleton

–
The outer support structure of an insect.

exotic

–
Non-native. See also Joe Exotic of Oklahoma

F1, F2, F3, etc.

Filial generation – the F1 generation is the result of crossing two different varieties; a cross of two F1 plants produces F2 seed; and so on.

fallow

–
To keep part of a garden unplanted or in a cover crop during the growing season.

family

–
A broad group of plants with common characteristics.

fasciation

–
Distortion of a plant that results in thin, flattened, and sometimes curved shoots.

feeder roots

–
Fine roots and root branches with a large absorbing area (root hairs). Responsible for taking up the majority of a plant’s water and nutrients from the soil.

fermentation

–
The partial breakdown of food molecules to yield ethyl alcohol, carbon dioxide, and energy. Fermentation occurs in the absence of oxygen.

fertility (soil)

–
The presence of minerals necessary for plant life.

fertilization

–
(1) The fusion of male and female germ cells following pollination.
(2) The addition of plant nutrients to the environment around a plant.

fertilizer

–
A natural or synthetic product added to the soil or sprayed on plants to supply nutrients.

fertilizer analysis

–
The amount of nitrogen (N), phosphorous (as P2O5), and potassium (as K2O) in a fertilizer, expressed as a percentage of total fertilizer weight. On the N-P-K fertilizer label, the percentage by weight of nitrogen (N) is always listed first, phosphorous (P) second, and potassium (K) third.

fiber

–
A long, thick-walled cell that dies at maturity.

fibrous root

–
A root system that branches in all directions, often directly from the plant’s crown, rather than branching in a hierarchical fashion from a central root. See taproot.

filament

–
The stalk supporting a flower’s anthers.

flagging

–
Loss of turgor and drooping of plant parts, usually as a result of water stress.

floricane

–
Second-year growth of caneberries. Produces fruit on laterals.

flower

–
The reproductive branch or structure of an angiosperm plant.

flower cluster/truss

–
A group of flowers that form from the stem of tomato plants which when pollinated produce the fruit.

foliar fertilization, foliar feeding

–
Fertilization of a plant by applying diluted soluble fertilizer, such as fish emulsion or kelp, directly to the leaves.

food

–
An organic substance that provides energy and body-building materials, especially carbohydrates, fats, and proteins.

force

–
To bring a plant into early growth, generally by raising the temperature or transplanting it to a warmer situation. Tulips and paperwhites are examples of plants that often are forced.

form

–
(1) A naturally-occurring characteristic different from other plants in the same population. (2) The growth habit (shape) of a plant.

formal

–
(1) A garden that is laid out in precise symmetrical patterns.
(2) A flower, such as some camellias, that consist of layers of regularly overlapping petals.

frond

–
Specifically, the foliage of ferns, but often applied to any foliage that looks fern-like, such as palm leaves.

fruit

–
The edible portion of a plant that is closely associated with a flower. Botanically, a fruit is a ripened, mature ovary.

fruiting habit

–
The location and manner in which a fruit is borne on woody plants.

fungicide

–
Any material capable of killing fungi. Sulfur and copper sulfate are two common mineral fungicides.

fungus

–
A plant organism that lacks chlorophyll, reproduces via spores, and usually has filamentous growth. Examples are molds, yeasts, and mushrooms.

fusarium

–
Any of several fungal diseases that afflict plants; commonly called dry rot or wilt.

gall

–
A growth on plant stems or leaves caused by abnormal cell growth stimulated by the sucking of some insects (e.g., aphids) or by viral, fungal, or bacterial infection.

gamete

–
A sex cell, male or female.

gene

–
A unit of genetic inheritance. generative growth reproductive phase of a plant in which it produces flowers and fruit.

genus

–
A group of related species, each of which is distinct and unlikely to cross with any other. A group of genera forms a family, and a group of families forms an order. See species.

geotropism

–
The turning or curving of a plant’s parts in response to gravity. A root growing downward is an example. Geotropism is controlled largely by the hormone auxin.

germination

–
The initial sprouting stage of a seed.

germination inhibitor

–
A chemical substance preventing seed germination.

girdling

–
The cutting, removing, or clamping of bark all the way around a trunk or branch. Sometimes, girdling is done deliberately to kill an unwanted tree, but often it results from feeding by insects or rodents. Wires and ties used to support a tree can cause girdling, as can string trimmers.

glabrous

–
Hairless, but not necessarily smooth.

glaucous

–
Covered with a grayish, bluish, or whitish waxy coating that is easily rubbed off. Blue spruce needles are an example of glaucous leaves.

gradual metamorphosis

–
See simple metamorphosis.

graft union

–
See bud union.

grafting

–
The act of inserting a shoot or bud of one plant into the trunk, branch, or root of another, where it grows and becomes a permanent part of the plant.

gravitational water

Water in excess of a soil’s capacity. Drains downward to groundwater.

green cone

–
An enclosed composting unit often used for composting food waste.

green manure

–
See cover crop.

groundcover

–
Plants used in lieu of grass for holding soil and providing leaf texture.

growing medium

–
Materials that are sometimes used in hydroponic growing to support the plant’s roots and, sometimes, to hold nutrient.

growing season

–
The period between the beginning of growth in the spring and the cessation of growth in the fall.

growth regulator

–
A compound applied to a plant to alter its growth in a specific way. May be a natural or synthetic substance. See hormone.

guard cell

–
Leaf epidermal cells that open and close to let water, oxygen, and carbon dioxide pass through the stomata.

gum

–
A sticky, water-soluble plant secretion that hardens on contact with air. guttation plants having high root pressure under high relative humidity conditions will exude water at their leaf margins through specialized cells.

gutter NFT

–
A nutrient film technique water culture system in which plants are grown in small gutters or channels.

gymnosperm

–
A member of a class of plants that forms seeds in an exposed condition, often in cones.

haltere

–
A small, knob-like organ (sometimes shaped like a baseball bat or bowling pin) located on the thorax of insects of the order Diptera. These take the place of hindwings and helps balance the insect in flight.

haploid

–
Having one set of unpaired chromosomes.

hardening off

–
The process or gradually exposing seedlings started indoors to outdoor conditions before transplanting.

hardpan

–
An impervious layer of soil or rock that prevents root growth and the downward drainage of water.

hardy

–
Frost- or freeze-tolerant. In horticulture, this term does not mean tough or resistant to insect pests or disease.

haustorium

–
A modified hyphal branch of a parasitic plant which grows into a host plant’s cell to absorb food and water.

head

(1) To cut off part of a shoot or limb rather than removing it completely at a branching point.
(2) The part of a tree from which the main scaffold limbs originate.

heartwood

–
The center cylinder of xylem tissue in a woody stem.

heeling in

–
The temporary burying of a newly dug plant’s roots to prevent their drying until a new planting site is prepared. Nurseries heel in bare-root berries, trees, and shrubs.

hemiptera

An insect family made up of species generally having sucking mouthparts and four wings. Wings are thick at the base and membranous at the free end. Include true bugs, stink bugs, assassin bugs, and back swimmers.

herbaceous

–
A soft, pliable, usually barkless shoot or plant. Distinct from stiff, woody growth.

herbaceous perennial

–
A herbaceous plant that dies back in the winter and regrows from the crown in the spring.

herbicide

–
A chemical used to kill undesireable plants.

heterotrophic nutrition

–
A form of nutrition in which the organism depends on organic matter for food, such as humans.

heterozygous

–
Having mixed hereditary factors; not a pure line. See homozygous.

homoptera

–
An insect family made up of species having sucking mouthparts and usually two pairs of wings. Includes aphids, scales, leafhoppers, and cicadas.

homozygous

Having purity of type; a pure line. See heterozygous.

honeydew

–
A sticky substance excreted by aphids and some other insects.

hormone

–
A naturally occurring compound that alters plant growth in a specific manner. See growth regulator.

horticultural oil

–
An oil made from petroleum products, vegetable oil, or fish oil, used to control insect pests and diseases. Oils work by smothering insects and their eggs, and by protectively coating buds against pathogen entry.

horticulture

–
The branch of the science of agriculture that relates to cultivating gardens and orchards, including the growing of vegetables, fruits, flowers, and ornamental shrubs and trees.

host

–
A plant on which an insect or disease completes all or part of its life cycle.

hot composting

–
A fast composting process that produces finished compost in 6 to 8 weeks. High temperatures are maintained by mixing balanced volumes of energy materials with bulking agents, by keeping the pile moist, and by turning it frequently to keep it aerated.

hotbed

–
An enclosed bed for propagating or protecting plants. Has a source of heat to supplement solar energy.

humus

–
The end product of decomposed animal or vegetable matter. See compost.

hybrid

–
A cross between two varieties or species, whether of the same genus or two generations.

hybrid vigor

–
The increased vigor, size and fertility of a hybrid compared to its parents.

hydroponics

–
A method of growing plants without soil. Plants usually are suspended in water or inert growing media, and plant nutrients are supplied in dilute solutions.

hygrometer

–
An instrument for measuring relative humidity in the atmosphere.

hymenoptera

An insect family made up of species having four membranous wings, of which the front pair are larger. Includes bees, wasps, sawflies, and ants.

hypha

A single filament of a fungus.

hypocotyl

–
The first leaf-like structure that appears on a germinating seed. Grows upward in response to light.

hypogeous germination

–
Seed germination in which the cotyledons remain below the soil surface.

imbibition

–
The process of water absorption, causing swelling.

immobilization

–
The process by which soil microorganisms use available nitrogen as they break down materials with a high C:N ratio, thus reducing the amount of nitrogen available to plants.

immune

–
A plant that does not become diseased by a specific pathogen. See resistant, tolerant.

incompatible

–
Kinds or varieties of a species that do not successfully cross-pollinate or intergraft.

incomplete flower

–
A flower lacking one or more of the normal flower parts.

incomplete metamorphosis

See simple metamorphosis.

incubation

–
A period of development during which a pathogen changes to a form that can penetrate or infect a new host plant.

indeterminate

–
A plant growth habit in which stems continue growing in length indefinitely. For example, indeterminate tomatoes are tall, late-fruiting, and require staking for improved yield. See determinate.

indicator paper

–
A litmus type paper that changes color with specific levels of acid or base and is used to check pH indicator solution a solution that changes color with pH changes.

infection

–
The condition reached when a pathogen has invaded plant tissue and established a parasitic relationship between itself and its host.

infiltration

The movement of water into soil.

inflorescence

–
A shoot bearing clusters of flowers.

inflorescence collective

–
A group of individual flowers. The grouping can take many forms, such as a spike (flowers closely packed along a vertical stem, e.g. snapdragons), an umbel or corymb (flowers forming a flattened dome, e.g. yarrow), a panicle (a complex hierarchical arrangement of flowers, e.g. hydrangeas), or a capitulum (tightly packed disc flowers, e.g. the center of a daisy).

injector

–
A piece of equipment which proportions a concentrated nutrient stock solution with a precise amount of water in a ratio to give the resulting solution the exact nutrient formulation calculated by the operator for feeding his crop.

inoculation

–
The introduction of a pathogen to a host plant’s tissues.

inoculum

–
The parts of a pathogen that infect plants.

insectary plant

–
A plant that attracts beneficial insects.

insecticidal soap

–
A specially formulated soap that is only minimally damaging to plants, but kills insects. These usually work by causing an insect’s outer shell to crack, resulting in drying out of its internal organs.

insecticide

–
Any material that kills insects. Includes numerous botanical products, both organic and synthetic.

insectivorous plant

–
A plant that captures and digests insects as a source of nutrition.

instar

–
The stage of an insect’s life between molts.

Integrated Pest Management (IPM)

–
A method of managing pests that combines cultural, biological, mechanical, and chemical controls, while taking into account the impact of control methods on the environment.

intensive gardening

–
The practice of maximizing use of garden space, for example by using trellises, intercropping, succession planting, and raised beds.

intercalary meristem

–
A meristem located between nondividing tissues such as at the base of a leaf.

intercropping

–
The practice of mixing plants to break up pure stands of a single crop.

internode

–
The portion of a stem between two nodes. See interstem.

interplanting

–
See intercropping.

interstem

–
(1) The portion of a stem between two nodes.
(2) The middle piece of a graft combination made up of more than two parts, i.e., the piece between the scion and the root stock.

interstock

–
The middle piece of a graft combination made up of more than two parts, i.e., the piece between the scion and the root stock. See interstem.

invasive

–
Growing vigorously and outcompeting other plants in the same area; difficult to control. ion An atom or molecule with either positive or negative charges. See anion, cation.

isoptera

An insect family made up of species having soft bodies, strong mandibles (mouth parts), and well-developed claws. Workers and soldiers are wingless and sterile. Termites are an example.

Joint

–
A node; the place on a stem where a bud, leaf, or branch forms.

juvenile stage

–
(1) The early or vegetative phase of plant growth characterized by carbohydrate utilization.
(2) The first stage of an insect’s life cycle, either as a larva or a nymph.

k

–
The chemical symbol for postassium.

key

–
A tool for plant or animal classification and identification. Consists of a series of paired statements that move from general to specific descriptions.

knot garden

–
A formal garden in which two or more kinds of plants with different-colored foliage, often herbs, are planted and pruned so they interweave and form a knot pattern.

larva

–
The immature form of an insect that undergoes complete metamorphosis. Different from the adult in form. Also called a caterpillar.

latent bud

–
A bud that does not break during the season after it is formed. Usually found on the lower portion of a shoot, it does not expand under normal growth stimuli, but will break if the growth above it is damaged or pruned away.

lateral

–
A branch attached to and subordinate to another branch or trunk.

lateral bud

–
A bud on the side, rather than the tip, of a stem.

lateral meristem

–
A region where cells divide, located along the length of a stem or root.

latex

–
A thick, white, fluid secretion of many plant species.

laticifer

–
A cell producing latex. layering A method of stimulating adventitious roots to form on a stem. There are two primary methods of layering. In ground layering, a low-growing branch is bent to the ground and covered with soil. In air layering, moist rooting medium is wrapped around a node on an aboveground stem.

leaching

–
Movement of water and soluble nutrients down through the soil profile.

leader

–
A developing stem or trunk that is longer and more vigorous than laterals. See central leader.

leachate

–
the drainage from a substrate such as perlite, rockwool, peatlite, rice hulls, etc. contained in slabs, bags or pots; the spent part of a nutrient solution after passing through the plant roots.

leaf

–
An outgrowth of a stem, usually the principal organ of photosynthesis.

leaf primordium

–
An immature leaf, located at a stem tip.

leaf rosette

–
A group of leaves radiating from a short stem.

leaf scar

–
A visible, thickened crescent or line on a stem where a leaf was attached.

leaf tendril

–
A modified leaf or leaf part used as a grasping organ.

leaflet

–
A single division of a compound leaf.

lenticel

–
A small opening on the surface of fruits, stems, and roots that allows exchange of gases between internal tissues and the atmosphere.

lepidoptera

–
An insect family made up of species having four wings covered with minute scales. Members undergo complete metamorphosis through the egg, larva, pupa, and adult stages. Includes butterflies and moths.

lignin

A tough, durable plant substance deposited in cell walls, especially in wood and coconut.

lime

–
A rock powder consisting primarily of calcium carbonate. Used to raise soil pH (decrease acidity).

loam

–
A soil with roughly equal influence from sand, silt, and clay particles.

lodge

–
To fall over, usually due to rain or wind. Corn and tall grasses are examples of plants susceptible to lodging.

long-day plant

–
A plant requiring more than 12 hours of continuous daylight to stimulate a change in growth, e.g., a shift from the vegetative to reproductive phase. See also a short-day plant, and day-neutral plant.

macronutrient

–
The major minerals that are used by plants in large amounts, consisting of nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca), and magnesium (Mg).

macropore

–
A large soil pore or opening. Macropores include earthworm and root channels, and control a soil’s permeability and aeration.

mandible

–
The first pair of jaws on insects. These are stout and tooth-like in chewing insects, needle- or sword-shaped in sucking insects, and the lateral (left and right) upper jaws of biting insects.

maturity

–
Ripeness, usually the state of development that results in maximum quality.

meiosis

–
A cell division process where the chromosome number is reduced by half.

meristem

–
Plant tissue in the process of formation; vegetative cells in a state of active division and growth, e.g., those at the apex of growing stems and roots.

mesophyll

–
A leaf’s inner tissue, located between the upper and lower epidermis, where raw materials (carbon dioxide and water vapor) are held for use in photosynthesis.

metabolism

–
The sum of the biochemical processes of a living cell.

metamorphosis

–
The process by which an insect develops. See complete metamorphosis, simple metamorphosis.

microclimate

–
The climate of a small area within a larger climate area. For example, a backyard can have a different microclimate from that of the surrounding neighborhood, or there may be different microclimates within a single backyard. Microclimates can significantly influence plant growth and should be considered in plant selection and care.

microfibril

A fine thread of cellulose in a cell wall.

micronutrient

A nutrient used by plants in small amounts, less than 1 part per million.
Micronutrients include boron, chlorine, copper, iron, manganese, molybdenum, and zinc. Also called trace elements.

micropore

–
A fine soil pore, typically a fraction of a millimeter in diameter. Micropores are responsible for a soil’s ability to hold air and water.

middle lamella

–
A layer of pectin binding two adjacent cell walls.

mineral deficiency

–
When a plant is not receiving a required nutrient–at all or in an insufficient amount–a disorder will result.

mitochondria

–
Cellular bodies in which cellular respiration occurs.

mitosis

–
A cellular division process in which the chromosomes are duplicated.

modified central leader

–
A system of pruning used primarily on fruit trees. The tree’s central leader is encouraged for the first few years, then suppressed. This system allows for well-placed scaffolds and strong crotches, but keeps the tree’s crown relatively close to the ground for easy harvesting. Also called modified leader.

modified leader

–
See modified central leader.

molecular weight

–
The relative weight of a molecule.

molecule

–
A chemically bonded group of atoms.

molt

–
The shedding of an insect’s outer covering during growth. The form assumed between molts is called an instar.

monocot

A plant having one cotyledon (seed leaf). Also called monocotyledon.

monocotyledon

See monocot.

monoecious

–
A species having both male and female flowers on the same plant.
Pecans, avocados, and squash are examples on monecious plants.

morphology

–
The study of the form of plants or plant parts.

mosaic

–
Nonuniform foliage coloration with more or less distinct intermingling of normal and abnormal colored patches.

mottle

–
An irregular pattern of light and dark areas.

mulch

–
Any material placed on the soil surface to conserve soil moisture, moderate soil temperature, and/or control weeds. Wood chips, bark, and shredded leaves are examples; inorganic materials such as rocks, plastics, or newspapers are also used.

multiple fruit

–
A cluster of mature ovaries from several flowers on a single stem.

mutation

–
A genetic change within an organism or its parts that changes its characteristics. Also called a bud sport or sport.

mycelia

Masses of fungal threads (hyphae) that make up the vegetative body of the fungus.

mycology

The study of fungi.

mycoplasma

See phytoplasma.

mycorrhizae

–
Beneficial fungi that infect plant roots and increase their ability to take up nutrients from the soil.

N

–
The chemical symbol for nitrogen.

nastic movement

–
A movement of a plant part (such as a leaf) not caused by an external stimulus.

native plant

–
A plant indigenous to a specific habitat or location.

natural selection

–
The action of the environment on organisms such that those better able to survive environmental stress are more likely to survive and reproduce.

naturalize

–
(1) To design a garden with the aim of creating a natural scene. Planting generally is done randomly, and space is left for plants to spread at will. (2) The process whereby plants spread and fill in naturally.

necrosis

–
Tissue death, browning of leaf tissue due to a nutritional disorder.

nectar

–
A sugary fluid secreted by some flowers.

nectar guide

–
A contrasting color pattern on a flower that guides pollinators to the nectar.

nectary

–
A gland secreting nectar.

nematocide

–
A material that kills or protects against nematodes.

nematode

–
A microscopic roundworm, usually living in the soil. May feed on plant roots and can be disease pathogens or vectors. Others are beneficial parasites of insect pests.

Nutrient Film Technique (NFT)

A water culture system based upon constant flow of the nutrient solution past the plant roots; must be a thin film of water flowing through the roots to provide adequate moisture and aeration.

nitrate (NO3-)

A plant-available form of nitrogen contained in many fertilizers and generated in the soil by the breakdown of organic matter. Excess nitrate in soil can leach into groundwater. See nitrogen cycle.

nitrifier

–
A microbe that converts ammonium to nitrate.

Nitrogen (N)

A primary plant nutrient, especially important for foliage and stem growth.

nitrogen cycle

–
The sequence of biochemical changes undergone by nitrogen as it moves from living organisms, to decomposing organic matter, to inorganic forms, and back to living organisms.

nitrogen fixation

–
The conversion of atmospheric nitrogen into plant-available forms by Rhizobia bacteria living on the roots of legumes.

node

–
The point on a plant where a branch, bud, or leaf develops. On younger branches, it usually is marked by a slight swelling. The space on the stem between nodes is called an internode.

non-point source

–
A relatively small, nonspecific source of pollutants that, when added to other sources, may pose a significant threat to the environment. See point source.

non-selective pesticide

–
A pesticide that kills most plants or animals. For example DDT.

N-P-K

–
The acronym for the three primary nutrients contained in manure, compost, and fertilizers.

The N stands for Nitrogen,
the P stands for Phosphorus,
the K stands for Potassium.

On a fertilizer label, the N-P-K numbers refer to the percentage of the primary nutrients (by weight) in the fertilizer.
For example, a 5-10-5 fertilizer contains 5% nitrogen, 10% phosphorous, and 5% potassium.

nucleus

–
The organelle (structure) within most living cells that contains the chromosomes which controls various cellular processes, including division into new cells. Bacteria and viruses do not have a nucleus; their chromosomes are distributed throughout the entire organism.

nutrient

–
Any substance, especially in the soil, that is essential for and promotes plant growth. See macronutrient, micronutrient.

nutrient solution

–
The water solution containing all of the essential plant elements in their correct ratios; the basic nutrient supply to plant roots.

nymph

–
The immature stage of an insect that undergoes simple metamorphosis. Usually similar in form to the adult.

offset

–
A new shoot that forms at the base of a plant or in a leaf axil.

oil

–
See horticultural oil.

open-pollinated seed

–
Seed produced from natural, random pollination so that the resulting plants are varied.

open (non-recirculating) system

–
A hydroponic system in which the nutrient solution passes only once through the plant roots; the leachate is not collected and returned to a cistern for a repeated cycle.

organ

–
A part of a plant, composed of different tissues, that acts as a functional unit.

organelle

–
A structure within a cell, such as a chloroplast, that performs a specific function.

organic

–
Referring to substances that contain both carbon and nitrogen.

organic fertilizer

–
A natural fertilizer material that has undergone little or no processing. Can include plant, animal, or mineral materials.

organic matter

–
Any material originating from a living organism (peat moss, plant residue, compost, ground bark, manure, etc.).

organism

–
A living plant or animal.

ornamental plant

–
A plant grown for beautification, screening, accent, specimen, color, or other aesthetic reasons.

orthoptera

–
An insect family made up of species having a tough, leathery shell, membranous hindwings, and straight forewings. Includes locusts, crickets, grasshoppers, and katydids.

osmosis

–
The flow or diffusion that takes place through a semipermeable membrane typically separating a solvent and a solution that strives to bring about a condition of equilibrium.

ovary

–
The part of a flower containing ovules that will develop into seeds upon fertilization. Along with the style and stigma, it makes up the pistil (female sexual organ).

ovule

–
Within the ovary, a body that will develop into seeds after fertilization.

oxidation

–
The chemical process by which sugars and starches are converted into energy. In plants, this is also known as respiration.

oxygenation

–
The supplying of oxygen; usually refers to the needs of plants’ roots oxygen deficit when oxygen is inadequate to support normal plant physiological processes.

P

–
The chemical symbol for phosphorus.

palisade mesophyll

–
The cells just beneath a leaf’s upper epidermis that contain most of the leaf’s chlorophyll and are responsible for most photosynthesis.

palmate

–
A form of espalier training. palmate venation A leaf whose veins radiate outward from a single point somewhat like the fingers of a hand.

palmately compound leaf

–
A leaf in which the leaflets radiate from one point.

panicle

–
A highly branched inflorescence.

parallel venation

–
A vein pattern in which the veins are parallel to each other.

parasite

–
Any animal or plant that lives in, or on, another animal or plant and withdraws nutrients from its host. Some parasites are financial ones, like music managers, who are not musicians themselves, but take 20% of all income from a band, just to “manage” their time.

parasitic seed plant

–
A plant that lives parasitically on other seed plants. An example is mistletoe.

parenchyma

–
A thin-walled, undifferentiated cell.

parterre

–
A formal garden in which shrubs, flowers, and paths form a geometric pattern of matched pairs.

parthenocarpic

–
Development of fruit without fertilization.

parts per million (PPM)

–
A ratio figure that represents the amount of one substance that is in one million parts of another substance; commonly used to describe the relative concentrations of nutrient solutions.

pathogen

–
Any organism that causes disease. Generally applied to bacteria, viruses, fungi, nematodes, and parasitic plants.

pathology

–
The study of diseases.

peat

–
a soilless medium that is partially decomposed aquatic, marsh, bog or swamp vegetation.

peatlite mix

–
A soilless medium consisting of a mixture of peat, sand, vermiculite and/or perlite.

pectin

–
A substance in cell walls binding cells together.

ped

–
A cluster of individual soil particles.

pedicel

–
The stem of an individual flower.

peduncle

–
The main stem supporting a cluster of flowers (as opposed to a pedicel, which is the stem of an individual flower).

pendulous

–
More or less hanging or declined.

perennial

–
A plant that lives 2 or more years and produces new foliage, flowers, and seeds each growing season.

perlite

–
A soilless medium that is fired volcanic pumice.

perianth

–
Collectively, all external flower parts.

pericarp

–
The fruit wall; derived from the ovary wall.

pericycle

–
A root tissue giving rise to branch roots.

permeability

–
The rate at which water moves through the soil.

persistent

–
(1) Adhering to a position instead of falling, whether dead or alive, e.g., flowers or leaves.
(2) A pesticide that retains its chemical properties in the environment for a long time.

petals

–
The usually showy structures around a flower’s reproductive organs.

petiolate leaf

–
A leaf in which the blade is attached to a stem by a petiole.

petiole

–
The stalk of a leaf.

pH or Possible Hydrogen

–
A scale measuring the acidity or alkalinity of a sample. What the pH scale actually measures is the hydrogen ion (H+) concentration present. pH values run from 0 (the most acidic value possible) to 14 (the most alkaline value possible). pH values from 0 to 7 indicate acidity, a pH of 7 is considered to be neutral, while pH values from 7 to 14 indicate alkalinity. The scale is logarithmic, thus a difference of 1 pH unit is equal to a 10-fold change in acidity or alkalinity (depending on the direction), a difference of 2 pH units indicates a 100-fold change, and a difference of 3 pH units indicates a 1,000-fold change.

phenological stage

–
Crop development stage.

phenotype

–
The physical appearance of an organism.

pheromone

–
A vapor or liquid emitted by an insect that causes a specific response from a receiving insect. Some pheromones are used to attract a mate. Synthetic pheromones are used as attractants in insect traps.

phloem

–
Photosynthate-conducting tissue. See xylem.

phosphate

–
The form of phosphorous listed in most fertilizer analysis (P2O5).

phosphorous (P)

–
A primary plant nutrient, especially important for flower production. In fertilizer, usually expressed as phosphate (P2O5).

photoinduce

–
To initiate a physiological process as a result of being exposed to a specific photoperiod.

photoperiod

–
The amount of time a plant is exposed to light.

photosynthate

–
A food product (sugar or starch) created through photosynthesis.

photosynthesis

–
The formation of carbohydrates from carbon dioxide (CO2) and a source of hydrogen (H)–such as water–in chlorophyll-containing cells exposed to light involving a photochemical release of oxygen through the decomposition of water.

phototropism

–
A growth response to light. Growth of a plant toward a light source is the most common example.

phytoplasma

–
A microscopic, bacteria-like organism that lacks a cell wall. Previously called mycoplasma.

phytotoxic

–
Toxic to a plant. picotee A pattern of flower petal coloration in which the edges of the petal are a contrasting color to the body.

pilose

–
Having long, soft hairs.

pinch

–
To remove a growing tip from a stem, thus causing axillary shoots or buds to develop. See deadhead, shear.

pinnately compound leaf

–
A leaf in which the leaflets are arranged on both sides of a common axis.

pinnate venation

–
A leaf vein pattern in which the major veins are arranged in rows on each side of the midrib.

pistil

–
The female sexual organ of a flowering plant, made up of the stigma, style, and ovary.

pit

–
A small opening in a cell wall.

pith

–
A region of parenchyma cells at the center of a stem.

plagiotrophic

–
Growth of a branch at an angle.

plant growth regulator

–
See growth regulator. plant nutrition
A plant’s need for and use of basic chemical elements. See macronutrients, micronutrients.

plasmolysis

–
Shrinkage of cytoplasm away from cell walls due to water loss.

pleach

–
To intertwine branches of a tree, vine, or shrub to form an arbor or hedge.

plena

–
A term used in botanical names to indicate a double-flowered variety. See double.

pleniflora

–
See plena.

pleno

–
See plena.

point source

–
A single, identifiable source of pollutants such as a factory or municipal sewage system. See nonpoint source.

pollard

–
A method of tree pruning that involves heading back severely to main branches each year so as to produce a thick, close growth of young branches.

pollen

–
A plant’s male sex cells, which are held on the anther for transfer to a stigma by insects, wind, or some other mechanism.

pollenizer

–
A plant whose pollen sets fruit on another plant. See cross-pollination.

pollination

–
The transfer of pollen from a male anther to a female stigma, enabling fruits to set and develop.

pollinator

–
An agent, such as an insect, that transfers pollen from a male anther to a female stigma.

polyploid

–
Having three or more sets of chromosomes per cell.

pome fruit

–
A fruit having a core, such as an apple, pear, or quince.

pomology

–
The science of fruits and the art of fruit cultivation, especially fruit trees.

postemergent

–
A product applied after crops or weeds emerge from the soil. See preemergent, preplant.

potash

–
The form of potassium listed in most fertilizer analysis (K2O).

potassium (K)

–
A primary plant nutrient, especially important for developing strong roots and stems. In fertilizers, usually expressed as potash. See potash.

predator

–
An animal that eats another animal, or an insect that eats another insect, such as predator mites.

pre-emergent

–
A product applied before crops or weeds emerge from the soil. See postemergent, preplant.

preharvest interval

–
The period of time that must pass from the time a pesticide is applied to a crop until the crop is safe to pick and use.

preplant

–
A product applied before a crop is planted. See postemergent, preemergent.

prickle

–
A rigid, straight, or hooked outgrowth of bark or stems. Often called a thorn, but technically different. Roses are examples of plants with prickles. See thorn.

primary growth

–
Growth arising from cellular activities in apical meristems.

primary nutrient

–
A nutrient required by plants in a relatively large amount (nitrogen, phosphorus, and potassium). See macronutrient.

primocane

–
First-year growth, usually vegetative, on caneberries. Only fall-bearing raspberries produce fruit on primocanes late in summer.

processed fertilizer

–
A fertilizer that is manufactured or is refined from natural ingredients to be more concentrated and more available to plants.

prop root

–
A supportive root growing from an aboveground stem.

propagate

–
To start new plants by seeding, budding, grafting, dividing, etc. protoplasm The living substance of cells, including cytoplasm and nucleus.

prune

–
To remove plant parts to improve a plant’s health, appearance, or productivity.

pseudobulb

–
A thickened, aboveground, modified stem that serves as a storage organ. Found in some orchids.

pubescent

–
Having short hairs.

pupa

–
The stage between larva and adult in insects that go through complete metamorphosis.

pyrethrum

–
Natural insecticide made from the blossoms of various chrysanthemums.

quarantine

–
A regulation forbidding sale or shipment of plants or plant parts, usually to prevent disease, insect, nematode, or weed invasion of an area.
Or an active quarantine of a plant in a secluded area where it is isolated so it can be spot treated for insects, fungal, or bacterial infestations.

quick-release fertilizer

–
A fertilizer that contains nutrients in plant-available forms such as ammonium and nitrate.

raceme

–
A flower stalk on which the florets start blooming from the bottom of the stem and progress toward the top.

radial spacing

–
The horizontal spacing of branches around a trunk.

radicle

–
The first part of a seedling to emerge from the seed. Grows downward and develops into the primary root.

raphide

–
A needle-shaped crystal of calcium oxalate in certain species’ cells that deter herbivores.

ray flower

–
One of several small flowers often forming a ring around the disc flowers in a composite head.

receptacle

–
The enlarged end of a flower stalk to which the flower parts are attached.

recessive trait

–
A genetic characteristic that is masked by a dominant gene.

relative humidity

–
The ratio of water vapor in the air to the amount of water vapor the air could hold at the current temperature and pressure.

reservoir

–
The container in a hydroponic system which holds nutrient solution in reserve for use.

resin

–
A viscous, protective secretion of many conifers that is insoluble in water and hardens in contact with air.

resistant

–
A plant having qualities that make it retard the activities of a pathogen or insect pest. See immune, tolerant.

respiration

–
The process within plants where sugars and starches are converted into energy.

reticulate venation

–
A net-like vein pattern in some leaves.

reversion growth

–
A stem that originates from, and has the characteristics of, the plant’s rootstock. See sucker.

reverse osmosis

–
The process of removing minerals from water, which is forced by pressure through a differentially permeable membrane, filtering out the minerals; can happen when growers accidentally apply too strong of a nutrient to a plant’s roots, leeching life out of the plant.

rhizobia bacteria

–
Bacteria that live in association with the roots of legumes and convert atmospheric nitrogen to plant-available forms, a process known as nitrogen fixation.

rhizome

–
A thickened underground stem that grows horizontally with bud eyes on top and roots below. Bearded iris is an example of a plant that produces rhizomes.

rhizosphere

–
The thin layer of soil immediately surrounding plant roots.

ribosome

–
A celluar organelle which is the site of protein synthesis.

ripeness-to-flower

–
The minimum vegetative size a plant must achieve before it is capable of flowering.

rockwool

–
A substrate used to grow plants hydroponically; an extruded wool-like product formed through a process of melting rock and extruding it into threads and pressing it into loosely woven sheets at high temperatures.
Below is a video of how rockwool insulation is made, and food grade rockwool from Dan that Gros is made from a different process, with no binders and different elmental inputs.

root

–
Generally, the underground portion of a plant. It anchors the plant and absorbs water and nutrients.

rootstock

–
A plant which has an established healthy root system used for grafting, a cutting, or budding from another plant. Rootstocks are most commonly used with fruiting and flowering plants that are susceptible to root diseases. By grafting to a healthy vigorous rootstock, typical root diseases for a cultivar are prevented.

root cap

–
A protective cover over a root tip.

root cutting

–
A section of root prepared for the purpose of vegetative propagation.

root hair

–
A delicate, elongated epidermal cell that occurs just behind a root’s growing tip. Root hairs increase the root’s surface area and absorptive capacity.

root nodule

–
A small swelling on a root resulting from invasion of nitrogen-fixing bacteria.

root pressure

–
The pressure developed by living cells in the root pushing water up the xylem.

root pruning

–
The cutting or removal of some of a plant’s roots.

root tuber

–
An enlarged, food-storage root bearing adventitious shoots.

root-bound

–
A condition in which a plant’s roots have completely filled its container. Typically, the roots begin to encircle the pot’s outer edge. Further growth is prevented until the plant is removed from the container.

rosette

–
A small cluster of leaves radially arranged in an overlapping pattern.

rot

–
Decomposition and destruction of tissue.

rotation

–
The practice of growing different plants in different locations each year to prevent the buildup of soil-borne diseases and insect pests, or the depletion of specific nutrients.

row cover

–
A sheet of synthetic material used to cover plants in order to retain heat and exclude insect pests.

rugose

–
Wrinkled.

runner

–
See stolon.

russet

–
Yellowish-brown or reddish-brown scar tissue on a fruit’s surface.

sand

–
The coarsest type of soil particle (0.05 to 2.0 mm in diameter).

sanitation

–
The process of removing sources of plant pathogens from a growing area, for example, by cleaning up plant debris and sterilizing tools and growing media.

saprophyte

–
An organism that obtains food from dead organic matter.

sapwood

–
The outer, light-colored, water conducting region of secondary xylem.

scab

–
(1) A crust-like disease lesion.
(2) A specific disease that causes scab lesions.

scaffold

–
The principal branches of a tree or shrub arising from the trunk or another main branch to for the plant’s framework.

scaffold branch

–
See scaffold.

scale

–
(1) A modified leaf that protects a bud.
(2) A type of insect pest.

scarification

–
Nicking, sanding, or otherwise compromising the hard outer coating of a seed to increase its water intake and thus promote germination. Sometimes incorrectly called scarfing.

scion

–
A cutting or bud that is grafted to the stock of another plant.

secondary growth

–
The growth resulting from the activities of lateral meristems (vascular and cork cambium).

secondary nutrient

–
A nutrient needed by plants in a moderate amount: calcium, magnesium, and sulfur. See macronutrient, primary nutrient.

secondary phloem

–
Food-conducting tissue formed by the vascular cambium.

secondary xylem

–
Water-conducting tissue formed by the vascular cambium.

seed

–
A reproductive structure formed from the maturation of an ovule and containing an embryo and stored food.

seed coat

–
A hard outer covering that protects a seed from disease and insects. Also prevents water from entering the seed and initiating germination before the proper time.

seed leaf

–
See cotyledon.

seedling

–
A young plant, shortly after germination.

selective pesticide

–
A pesticide that kills or controls only certain kinds of plants or animals. For example, 2,4-D kills broadleaf lawn weeds but leaves grass largely unharmed.

self-fruitful

–
A plant that bears fruit through self-pollination.

self-pollination

–
The transfer of pollen from the anther to the stigma of the same flower.

self-unfruitful

–
A plant that requires another variety for pollination. See pollenizer.

senescense

–
(1) The aging process. (2) A descriptive term for a plant that is in the process of going dormant for the season, although technically only the parts that are dying (the leaves) are becoming senescent.

sepal

–
An appendage at a flower’s base, typically green or greenish and more or less leafy in appearance. Collectively, the calyx.

separation

–
The process of removing new bulbs or corms from their parent for the purpose of propagation.

sessile

–
Stalkless and attached directly at the base, as in sessile leaves.

shade-tolerant

–
Having the ability to live in low light conditions.

shear

–
To cut back a plant (as opposed to selective pruning or deadheading). Often used to regenerate plants with many small stems, where deadheading would be too time-consuming.

shoot

–
One season’s branch growth. The bud scale scars (ring of small ridges) on a branch mark the start of a season’s growth.

short-day plant

–
A plant requiring more than 12 hours of continuous darkness to stimulate a change in growth, e.g., a shift from the vegetative to reproductive phase. See long-day plant, day-neutral plant.

shoulder ring

–
One of the ridges around the base of a branch where it attaches to a trunk or to another branch, See collar.

shrub

–
A woody plant that grows to a height of 3 to 12 feet. May have one or several stems with foliage extending nearly to the ground.

side-dress

–
To apply fertilizer to the soil around a growing plant.

sieve plate

–
The perforated end-wall of a sieve tube member.

sieve tube

–
A food conducting cell. sign
Direct evidence of a damaging factor (for example, a pest or pathogen itself, secretions, insect webbing, or chemical residue).

signal word

–
An indication of toxicity on pesticide labels. Pesticides labeled “Caution” are the least toxic, those labeled “Warning” are more toxic, and those labeled “Danger” or “Danger – Poison” are the most toxic.

silt

–
A type of soil particle that is intermediate in size between sand and clay (0.002 to 0.05 mm in diameter).

simple fruit

–
A fruit formed from a single ovary.

simple leaf

A leaf in which the blade is not divided into smaller units (leaflets).

simple metamorphosis

–
A type of insect development in which the insect passes through the stages of egg, nymph, and adult. The nymph usually resembles the adult. See complete metamorphosis.

slow-release fertilizer

–
A fertilizer material that must be converted into a plant-available form by soil micro-organisms.

soft pinch

–
To remove only the succulent tip of a shoot, usually with the fingertips.

soil

–
A natural, biologically active mixture of weathered rock fragments and organic material at the earth’s surface.

soil salinity

–
A measure of the total soluble salts in a soil.

soil solution

–
The solution of water and dissolved minerals found in soil pores.

soil structure

–
The arrangement of aggregates (peds) in a soil.

soil texture

–
How coarse or fine a soil is. Texture is determined by the proportions of sand, silt, and clay in the soil.

soilless mix

–
A sterile potting medium consisting of ingredients such as sphagnum peat moss and vermiculite.

soluble salts

–
A mineral residue often remaining in soil from irrigation water, fertilizer, compost, or manure applications.

solubility

–
The ability of a fertilizer to dissolve in water to form a solution.

sonic repeller

–
A sonic-wave emitting unit said to disrupt the activities of small mammals. Not proven to be effective.

sorus

–
An area of spore production on the underside of a fern leaf.

spandix

–
A spike of flowers enclosed in a spathe.

spathe

–
A large bract enclosing a spandix.

species

–
The basic unit of plant or animal classification. Plants within a individual species have several characteristics in common. Most importantly, they can cross with one another, but normally not with members of other species. Classification of species is quite fluid, with periodic revision by botanists.

specific epithet

–
The second word in a binomial (scientific) name, following the genus name. For example, in the binomial name Thuga plicata (The tree commonly known as Western Redcedar), “Thuga” is the genus name and “plicata” is the specific epithet.

sperm

–
A male sex cell.

spike

–
An inflorescence in which the flowers are attached to the main stem without stalks.

spine

–
A modified leaf part that is hard and pointed.

spore

–
(1) The reproductive body of a fungus or other lower plant, containing one or more cells.
(2) A bacterial cell modified to survive in an adverse environment.

sport

–
See mutation.

spot treatment

–
To apply a pesticide to a small section or area of a crop.

spur

–
Commonly seen on fruit trees, spurs are a short, compact twig with little or no internodal development on which flowers and fruit are borne.

stalk

–
On a male flower, the portion of the stamen that supports the anther.

stamen

–
The male, pollen-producing part of a flower consisting of the anther and its supporting filament.

standard

–
A plant pruned so that it consists of a single bare, vertical stem, atop which a shaped mass of foliage, usually globular, is maintained.

starch

–
The principal food-storage substance (a carbohydrate) of higher plants.

stem

–
The leaf and flower bearing part of a plant.

stem cutting

–
A section of a stem prepared for vegetative propagation.

stem tuber

–
An enlarged tip of a rhizome containing stored food.

sterile

–
(1) Material that is free of disease organisms (pathogens), as in potting medium.
(2) A plant that is unable to produce viable seeds.

sterilization

–
The act of rendering something free from living cells. In hydroponics it is essential that all materials (especially any growing medium) used are sterile to avoid contaminating the hydroponic system. Steam and chemical agents are often used in this process.

stigma

–
The part of a female sex organ that receives pollen. Supported by the style, through which it is connected to the ovary. Often sticky when receptive.

stinging hair

–
A multicellular hair containing an irritating fluid.

stipules

–
A pair of appendages found on many leaves where the petiole meets the stem.

stock

–
See rootstock.

stolon

A horizontal stem running along, but above, the soil surface and producing roots and leaves where its nodes contact the soil. Strawberries are an example of a plant that produces stolons. Also called a runner.

stoma

Tiny openings in a leaf’s epidermis that allow water, oxygen, and carbon dioxide to pass in and out of a plant. (pl. stomata)

stomates

See stoma.

stone cell

A hard, thick-walled plant cell.

stone fruit

–
A fleshy fruit, such as peach, plum, or cherry, usually having a single hard stone that encloses a seed. Also called a drupe.

strain

–
A variation within a cultivar or variety.

stratification

–
The exposure of seeds to moisture or low temperature to overcome dormancy.

style

–
The part of a plant’s female sex organ that supports the stigma and connects it to the ovary.

stylet

–
A nematode’s lance-like or needle-like mouthpart used to puncture and feed from plant cells.

suberin

–
A fatty plant substance present in the walls of cork cells.

subspecies

–
A major division of a species, more general in classification than a cultivar or variety.

succession

–
The progression of a plant community to a stable mixture of plants.

succession planting

–
The practice of planting new crops in areas vacated by harvested crops.

sucker

–
A shoot or stem that originates underground from a plant’s roots or trunk, or from a rootstock below the graft union. See reversion growth.

summer oil

–
A light, refined horticultural oil used during the growing season to control insect pests and diseases.

sunscald

–
Winter or summer injury to the trunk of a woody plant caused by hot sun and fluctuating temperatures. Typically, sunscalded bark splits and separates from the trunk.

surfactant

–
See also adjuvant.

sustainable gardening

–
Gardening practices that allow plants to thrive with minimal inputs of labor, water, fertilizer, and pesticides.

symbiotic

–
Mutually beneficial.

symptom

–
A change in plant growth or appearance in response to living or nonliving damaging factors.

systemic pesticide

–
A pesticide that moves throughout a target organism’s system to cause death.

tannin

–
A substance occurring in the bark and leaves of some plants that helps protect against predators.

taproot

–
A thick central root attached directly to a plant’s crown. Taproots branch little, if at all.

taxonomy

–
Classification or naming of plants or animals.

temporary branch

–
(1) A small shoot or branch left on a young tree’s trunk for protection and nourishment.
(2) A low lateral allowed to remain until a tree is tall enough to have scaffolds at the desired height.

tender

–
Not tolerant of frost or cold temperatures. In horticulture, tender does not mean weak or susceptible to insect pests or disease.

tendril

–
A slender projection used for clinging, usually a modified leaf. Easily seen on vines such as grapes and clematis.

terminal

–
The tip (apex), usually of a branch or shoot.

terminal bud

–
The bud at the tip of a stem, trunk, or branch. Its development extends the plants growth.

thatch

–
A tightly intermingled layer of stems, leaves, and roots, living and dead, that forms between the soil surface and green vegetation of grass.

thermoperiod

–
The change in temperature from day to night.

thermophillic

–
High temperature, as in microorganisms that break down organic matter in a hot compost pile.

thigmotrophism

–
A growth response to touch.

thin

–
(1) To remove an entire shoot or limb where it originates.
(2) To selectively remove plants or fruits to allow remaining plants or fruits to develop.

thorn

–
A hard, sharp-pointed, leafless branch. Hawthorne is an example of a plant that produces thorns. See prickle.

tiller

–
A shoot that arises from a plant’s crown. Generally associated with grass species.

tissue

–
A group of cells of the same type having a comon purpose. tissue analysis a laboratory analysis of plant tissue to determine levels of nutrients present.

tissue culture

–
The process of generating new plants by placing small pieces of plant material onto a sterile medium. Also called embryo culture.

Total Dissolved Solutes (TDS)

–
The concentration of all the elemental ions present in a nutrient solution; electrical conductivity (EC) is a measure of TDS generally expressed as mS (milliSiemens) or mMho (millimhos).

tolerant

–
A plant that will produce a normal yield even if infested by a disease or insect pest. See immune, resistant.

topiary

–
A tree or shrub shaped and sheared into an ornamental, unnatural form, usually a geometric shape or the shape of an animal.

totipotency

–
The ability of any cell to develop into a complete plant.

trace element

–
See micronutrient.

transpiration

–
The process of losing water in the form of vapor through stomata.

transpirational pull

–
The force exerted by transporation from the leaves which draws water up through the plant.

tree

–
A woody plant that typically grows more than 12 feet tall and has only one main stem or trunk.

tropism

–
The tendency of a plant to turn in response to an external stimulus, either by attraction or repulsion, as a leaf turns toward the light. See geotropism, phototropism.

trunk

–
The main stem of a tree. Also called a bole.

truss

–
A flower cluster, usually growing at the terminal of a stem or branch.

tuber

–
An underground storage organ made up of stem tissue. Contains buds on the surface, from which shoots may arise. Potatoes are an example.

tuberous root

–
An underground storage organ made up of root tissue. Sprouts only from the point where it was attached to the parent plant. Dahlias are an example.

turgor

–
Cellular water pressure; responsible for keeping cells firm.

twig

–
A young stem (1 year old or less) that is in the dormant winter stage (has no leaves).

twiner

–
A stem growing in a spiral fashion around a support.

umbel

–
A group of flowers growing from a common point on a stem.

understock

See rootstock.

vacuole

–
A fluid-filled sac within a cell.

vaporization

–
The evaporation of the active ingredient in a pesticide during or after application.

variegation

–
An inherited, irregular pattern of color on a leaf or petal.

variety

–
A strain of a plant having distinctive features that persist over successive generations in the absence of human intervention. Generally, variety applies to naturally-occuring strains, while cultivar applies to horticulturally-developed strains.

vascular cambium

–
A narrow cylinder of cells that gives rise to secondary xylem and phloem. A lateral meristem.

vascular plant

–
A plant which has water and food conducting tissues.

vascular tissue

–
Water-, nutrient-, and photosynthate-conducting tissue. See also xylem, phloem.

vector

–
A transmitter or carrier of disease.

vegetative propagation

–
The increase of plants by asexual means using vegetative parts. Normally results in a population of identical individuals. Can occur by either natural means (e.g., bulblets, cormels, offsets, plantlets, or runners), or by artificial means (e.g., cuttings, division, budding, grafting, or layering).

vein

–
A strand of xylem and phloem in a leaf blade.

venation

–
The arrangement of veins in a leaf.

vernalization

–
A low-temperature treatment promoting flowering.

vernation

–
The arrangement of new leaves within an older leaf sheath (e.g., on a grass plant).

verticillium

–
Any of several fungal diseases that afflict plants; commonly called wilt. See fusarium.

verticle spacing

–
The vertical space between branches on a tree.

viability

–
A seed’s ability to germinate.

virus

–
An infectious agent too small to see with a compound microscope. Multipies only within a living host cell.

water-holding capacity (WHC)

–
The ability of a soil’s micropores to hold water for plant use.

water sprout

–
A vigorous shoot originating above ground on a plant’s trunk, older wood, or bud union. Usually breaks from a latent bud. Often the result of heavy pruning.

water-soaking

–
Lesions that appear wet and dark and usually are sunken and/or translucent. Often a symptom of bacterial disease.

weed-and-feed

–
A combination fertilizer and herbicide sometimes used on lawns.

whorled

–
Arranged in a ring.

wilt

–
(1) Lack of freshness, turgor, and the drooping of leaves from a lack of water.
(2) A vascular disease that interrupts a plant’s normal uptake and distribution of water.

wilting point (WP)

–
The point at which water content within plant cells is low enough that cellular turgor is lost and the plant wilts.

witches’ broom

–
Abnormal brush-like development of many weak shoots.

wood

–
The dense tissue composed of secondary xylem in stems and roots.

woody perennial

–
A plant that goes dormant in the winter and begins growth in the spring from aboveground stems.

xanthophyll

–
A yellow or almost colorless photosynthetic pigment.

xeric

–
A plant ot landscape that conserves water. Most xeric plants need minimal supplemental water after an establishment period (18 to 24 months after planting) unless there is extreme drought.

xylem

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Water- and nutrient-conducting tissue. See phloem.

Y

There is nothing here

zygomorphic flower

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An irregular shaped flower divisible into two similar halves along one plane only.

zygote

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A fertilized egg.