Glossary for Gardeners

glossary for gardeners title image over yellow rose

Scroll down or click letter to bring the alphabetic categories into view.

| A | B | C | D | E | F | G | H | I | J | K | L | M |

| N | O | P | Q | R | S | T | U | V | W | X | Y | Z |

~ A ~

  • 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:
    accumulated heat units formula = max temp plus minimum temp divided by 2 - 50 in degrees Farenheight
    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. See pH.
  • actinomorphic flower
    A flower possessing radial symmetry. Any cut through the center divides the flower into two equal parts.
  • active ingredient
    The chemical in a pesticide formulation that actually kills the target pest.
  • additive
    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 an adjuvant or surfactant.
  • adjuvant
    See additive (above).
  • 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 cannot 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.
  • after-ripening
    The seed maturation process that must be completed before germination can occur.
  • aggregate 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 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.
  • 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.
  • 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.
  • 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.
  • 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).
  • 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.

~ B ~

  • bacillus thuringiensis (Bt)
    Bacillus thuringiensis (Bt) is a bacterium used as a biological control agent for many insect pests.
  • bacterium
    A single-celled microscopic organism having a cell wall but no chlorophyll. They reproduce by cell division.
  • balled and burlapped (B&B)
    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 its 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.
  • biosolids
    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.
  • 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.
  • BTU (British Thermal Unit)
    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.
  • 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.
~ C ~
  • 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.
  • 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.
  • 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.
  • 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.
  • 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 integrated. See pollinizer.
  • 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 An inflorescence composed of many tightly packed, small, ray and disc flowers.
  • compostThe 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.
  • 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.
  • 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 bulbil 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.
  • cross-pollination The fertilization of an 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 epidermis 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.

~ D ~

  • 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 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.
  • 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 (above)
  • 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 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.

~ E ~

  • 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.
  • 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.
  • etiolation
    Development of yellow, long, spindly growth on a plant as a result of insufficient light.
  • 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.
  • 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.

~ F ~

  • 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.
  • 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.

~ G ~

  • 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.
  • 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 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.
  • gymnosperm
    A member of a class of plants that forms seeds in an exposed condition, often in cones.
~ H~
  • 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 undesirable plants.
  • heterotrophic nutrition A form of nutrition in which the organism depends on organic matter for food.
  • 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 genera.
  • 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 polymers, and plant nutrients are supplied in dilute solutions.
  • 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.

~ I ~

  • 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 keep growing in length indefinitely. For example, indeterminate tomatoes are tall, late-fruiting, and require staking for improved yield. See determinate.
  • 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).
  • 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
    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 (1) – below.
  • interplanting
    See intercropping (above).
  • 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 (2) – above.
  • 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.

~ J ~

  • 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 ~

  • K
    The chemical symbol for potassium.
  • 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.

~ L ~

  • 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.
  • 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.
  • 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 short-day plant, day-neutral plant.

~ M ~

  • macronutrient
    Collectively, the primary nutrients (nitrogen, phosphorus, and potassium) and secondary nutrients (sulfur, calcium, and magnesium) required for plant growth.
  • 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.
  • 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 (above).
  • 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 (above).
  • monoecious
    A species having both male and female flowers on the same plant. Pecans, avocados, and squash are examples on monecious plants. See dioecious.
  • 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 ~

  • 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.
  • 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.
  • 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.
  • nonpoint 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.
  • nonselective pesticide
    A pesticide that kills most plants or animals.
  • 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, and 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.
  • nymph
    The immature stage of an insect that undergoes simple metamorphosis. Usually similar in form to the adult.

~ O ~

  • 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.
  • 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
    Passage of materials through a membrane from an area of high concentration to an area of lower concentration.
  • 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.

~ P ~

  • P
    The chemical symbol for phosphorus. (see below)
  • 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.
  • 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.
  • pathogen
    Any organism that causes disease. Generally applied to bacteria, viruses, fungi, nematodes, and parasitic plants.
  • pathology
    The study of diseases.
  • 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 two or more years and produces new foliage, flowers, and seeds each growing season.
  • 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
    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 process in green plants of using sunlight to convert water and carbon dioxide into sugar.
  • 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 macronutrient, micronutrient.
  • 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 (see above).
  • pleno
    See plena (see above).
  • 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.
  • pollinizer
    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, which 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.
  • post emergent
    A product applied after crops or weeds emerge from the soil. See preemergent, preplant (see below).
  • 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.
  • preemergent
    A product applied before crops or weeds emerge from the soil. See post emergent (above), preplant (below).
  • 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 post emergent, preemergent (above).
  • 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.

~ Q ~

  • quarantine
    A regulation forbidding sale or shipment of plants or plant parts, usually to prevent disease, insect, nematode, or weed invasion of an area.
  • quick-release fertilizer
    A fertilizer that contains nutrients in plant-available forms such as ammonium and nitrate.

~ R ~

  • 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.
  • 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.
  • 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.
  • root
    Generally, the underground portion of a plant. It anchors the plant and absorbs water and nutrients.
  • 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.
  • rootstock
    The root or stem onto which a scion or interstock is grafted.
  • 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.

~ S ~

  • 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 (above).
  • 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 pollinizer.
  • 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 twelve 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 three to twelve 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.
  • 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 Thuja 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.
  • 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.
  • 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 (above).
  • 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 additive.
  • 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.

~ T ~

  • 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 common purpose.
  • tissue culture
    The process of generating new plants by placing small pieces of plant material onto a sterile medium. Also called embryo culture.
  • 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 twelve 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 (one 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.

~ U ~

  • umbel
    A group of flowers growing from a common point on a stem.
  • understock
    See rootstock.

~ V ~

  • 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 occurring 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 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).
  • vertical 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. Multiplies only within a living host cell.

~ W ~

  • 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.

~ X ~

  • xanthophyll
    A yellow or almost colorless photosynthetic pigment.
  • xeric
    A plant or 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
    Water- and nutrient-conducting tissue. See phloem.

~ Y ~

  • There are no entries for Y

~ Z ~

  • zygomorphic flower
    An irregular shaped flower divisible into two similar halves along one plane only.
  • zygote
    A fertilized egg.

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