Plant Science Terms - H

Habit - general growth, shape, and form of a plant

"Hair Cut" - using a sharp pair of scissors or shears to trim the ends of the branches off a plant, cutting an even amount off all parts of the plants, like your stylist giving you a haircut.  If there are long pieces trailing or sticking out these would be trimmed back more so that at the end the plant is nicely even.  A haircut will neaten the plant and encourage branching

Half-hardy - plants able to withstand some cold, damp weather but will be damaged by frost

Halteres - modified hind wings that are reduced in size and used for stabilization during flight

Hardening - result of many changes that occur in a plant as it develops resistance to adverse conditions, especially cold

Hardening off - treatment of tender plants to enable them to survive a more adverse environment.  The process is often used to acclimate greenhouse grown plants and gradually exposes seedlings to outdoor conditions before transplanting; process of gradual preparation for winter weather. Treatments involve withholding nutrients, lowering temperatures, allowing temporary wilting, and other methods to slow growth rate and generally take several weeks

Hardiness - ability to withstand climatic changes

Hardiness Zone - temperature zones are based on the lowest average temperature each area is expected to receive during the winter.  Hardiness zones are used to determine whether a plant is likely to be perennial in your area

Hardpan - an impervious layer of soil or rock that prevents root growth and downward drainage of air and water

Hardwood cuttings - cuttings made from woody deciduous species and narrow-leaved evergreen species, such as grape and hemlocks

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

Harrow - an implement consisting of a heavy frame set with teeth or tines that is dragged over plowed land to break up clods, remove weeds, and cover seed

Haustorium - modified hyphal branch of a parasitic plant. Grows into a host plant’s cell to absorb food and water

Head - to cut off part of a shoot or limb rather than remove it completely at a branch point. (2) The part of a tree from which the main scaffold limbs originate

Heading back - type of pruning cut where the end portion of the shoot is removed but the portion at the base is not

Headspace - space left between the top of the soil and the top of the container.  This space helps keep soil from washing out and help channel water into the container when watering.  Without head space water can easily run off the top of the container

Heartwood - central cylinder, often dark colored, of xylem tissue in a woody stem

Heat Tolerant - plants that flourish despite hot temperatures

Heavy Feeders - plants that need a lot of fertilizer for optimal performance. Regular applications of fertilizer are necessary for continued performance

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

Height, Medium - plants that are between 10 and 24 inches in height.  These plants are often used in the center (or between the tall and short plants) in a mixed bed or as filler in a container 

Height, Short - plants that are 10 inches or less in height.  These plants are often used in the front of mixed beds or the edges of pots

Height, Tall - plants that are 24 inches or more in height.  These plants are often used at the back of mixed beds or in the center of containers

Hemiptera - major order of insects that have two pairs of wings and piercing-sucking mouthparts (bed bugs, stink bugs, cinch bugs)

Herbaceous - soft, green, non-woody, pliable, usually barkless shoot or plant. Distinct from stiff, woody growth

Herbaceous cuttings - cuttings made from herbaceous plants (see definition below), such as chrysanthemums, coleus, and geraniums

Herbaceous perennials - plants with soft, succulent stems whose tops are killed back by frost in many temperate and colder climates, but whose roots and crowns remain alive and send out top growth when favorable growing conditions return

Herbarium - collection of plant specimens that have been taxonomically classified, pressed, dried, and mounted on a sheet of herbarium paper

Herbicide - chemical used to eliminate undesirable plants. Herbicides may kill essentially all plants or be quite selective in their activity

Herbicide, contact - herbicide that injures only those portions of a plant with which it comes into contact

Herbicide, nonselective - herbicide that kills or injures all plants. Some plant species may exhibit more tolerance than others. Examples include glyphosate, and glufosinate

Herbicide, postemergence - herbicide that needs to be applied after weeds emerge to be effective

Herbicide, pre-emergence - herbicide that needs to be applied before weeds emerge to be effective. Can be applied before or after turf establishment. Rainfall or irrigation is often needed to move the chemical into the top few inches of the soil for best activity

Herbicide, selective - herbicide that kills or injures some plants without harming others

Herbicide, systemic - herbicide that is taken up through contact with the leaves or through the soil (via contact with the roots) and is moved throughout the plant to kill the whole plant

Heterozygous - having mixed hereditary factors, not a pure line

High-pressure sprayer - sprayer using a high-pressure pump to force the spray through nozzles for both creating a fine spray and delivery to the plant

Homoptera - major order of insects that have two pairs of wings, or are wingless, and piercing-sucking mouthparts (aphids, leafhoppers, scales, mealybugs)

Homozygous - pure for a trait; breeds true

High analysis - a fertilizer containing 30% or more active nutrients, such as ammonium nitrate 33-0-0. The cost per bag is usually more, but the cost per pound of nutrient is less, lowering the cost for fertilizing a given area

Hirsute - leaf surfaces that are pubescent with coarse, stiff hairs

Hispid - leaf surfaces that are rough with bristles, stiff hairs, or minute prickles

Honeydew - sticky substance excreted by aphids and some other insects

Hormone - naturally occurring compound that alters plant growth in a specific manner. (See also 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 - science of growing fruits, vegetables, flowers, and other ornamental plants through intensive cultivation

Host - plant on which an insect or disease completes all or part of its life cycle

Host plant - plant that is invaded by a parasite

Host range - various plants that may be attacked by a parasite

Hotbed - an enclosed bed for propagating or protecting plants. Has a source of heat to supplement solar energy

Hot composting - fast composting process that produces finished compost in 4 to 8 weeks. High temperatures are maintained by mixing balanced volumes of energy materials and bulking agents, by keeping the pile moist, and by turning it frequently to keep it aerated

Humus - end product of decomposing animal or vegetable matter. (See also Compost)

Humus - dark-colored organic material that is the product of organic matter breakdown by both microorganisms and chemical reactions

Hybrid - results of a cross between two different species or well-marked varieties within a species. Hybrids grown in a garden situation will not breed true to form from their own seed

Hybrid - a first-generation cross between two genetically diverse parents

Hydathode - modified pore, especially on a leaf, that exudes drops of water

Hydrophobic - having little or no affinity for water

Hydroponics - method of growing plants without soil. Plants usually are suspended in water or polymers, and plant nutrients are supplied in dilute solutions

Hymenoptera - major order of insects that have two pairs of wings, or are wingless, and chewing mouthparts (wasps, bees, ants, sawflies)

Hypha (or hyphae) - single filament of a fungus

Hypocotyl - seedling stem that develops below the cotyledons


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