| 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. |
