| 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 | Daily values are then totaled for the season, with values less than zero ignored (but not deducted from the total). [formula = max temp plus minimum temp divided by 2 – 50 in degrees Farenheight] |
| 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. |
| 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. |
| 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: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. |
| 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. |
| 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. |
| 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. |
| 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. |
| 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. |
| 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. |
| 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. |
| 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. |
