| 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. |
