habit - a plant's habit is its natural size, shape and form. Terms such as upright, rounded, horizontal, spreading, prostrate and vertical are used as descriptive terms.
haploid - having a single complete set of chromosomes (n chromosomes).

See haploid, diploid, triploid and tetraploid.

hand pollination - manually moving the pollen from one plant to the pistil of another plant . Many types of tools such as paint brushes, cotton swabs, dead bee bodies and others are used by hybridizers to assist in making the transfer.
hardening off - plants that have been grown under ideal conditions of heat and humidity in a greenhouse may not be immediately adapted to the more variable conditions found outdoors. To help seedlings or plants make this adjustment, a process of "hardening off" may be used. This may involve taking plants out in the spring on a cool morning and bringing them back in if the night is to be frosty. Other ways to harden plants include withholding water and altering the temperature within the greenhouse.
hardiness - a general term for the ability of a plant to withstand environmental stresses. In temperate zones, the main concern is cold temperature hardiness while in other regions, the challenge may be hot, dry summers.

The most famous way of designating survivability of plants is the cold hardiness zone map of the USDA. However, other organizations have begun to develop maps based on other criteria such as heat, drought conditions and urban factors.

hardiness zone - this term is most often used when referring to the temperature maps developed by the U.S. Department of Agriculture and, more recently, by Arnold Arboretum.

These maps show the United States and Canada divided into zones based on the average low temperature experienced within that zone. The zones start with number 1 in the extreme northern areas of the continent and proceed to the south. The lower the number, the colder the climate.

Remember that the zones only relate to minimum air temperatures. A plant is given a zone designation based on practical experience as to which plants historically survive those temperatures. Some ratings are based on scientific cold hardiness studies at universities but this is by far the minority of cases.

hardscape - all the non-living parts of a landscape such as fences, paths, structures, statuary, trellis, arbors, etc.

See Softscape.
hardwood cutting - propagation method in which the plant segments for rooting are collected during the dormant season. It usually refers to mature twigs that have been hardened off for winter or wood that is older than one year.

See Softwood Cutting.

headhouse - a structure attached to greenhouses which is used for propagating, potting, storage, etc.
heading back - there are two basic types of pruning cuts: heading back and thinning cuts.

Heading back is the removal of ends of branches to achieve a reduction in plant height or span. The most common form of heading is when plants are trimmed into hedges or into forms such as squares or balls. It is generally give the plant an "unnatural" look.

heal in - this is putting plants into the soil temporarily until they can be transplanted to their permanent location. This  helps to prevent the plant from drying out or may be used if the plant must be moved from its current location but the new site is not yet ready for it.
heart shaped - shaped like a valentine heart. It is a leaf having an acute point or apex, and a flat to cordate leaf base.
heart wood - in the cross-section of a dicotyledon type of tree's trunk, the heart wood is found just inside the sapwood and outside the center pith. It consists of dead xylem and phloem cells and no longer transport water or sap in the tree. 
heavy soil - term for a soil composed predominately of clay particles. These soils are generally poorly drained and lack oxygen spaces needed for plant growth. They are also more prone to compaction.
hedge - a grouping of plants that have dense foliage and are spaced close enough so that they grow together forming a solid mass. They may be precisely trimmed and shaped or they may be left "natural" in their native growth habit. Although many types of plants may be used for hedges, perhaps the most common species are yews, boxwood and privet.
herbaceous - these plants do not form a woody stem and, in temperate zones, will die back to the ground when exposed to a heavy frost. These plants are also commonly called herbaceous perennials

See Woody.

herbaceous perennial - this term applies to plants that live more than two years (perennial) which do not form woody tissue (herbaceous). These are plants whose foliage dies back to the ground each year generally as the result of cold temperatures or dry seasons. In the home garden, they are often just called "perennials" by the gardening public.

In the case of bulb plants, the foliage dies back to allow a rest period at some point during the year.

herbicide - a substance that either kills or inhibits the growth of plants. These may be of natural origin or synthetic chemicals.

Herbicides may be classified as pre-emergent, post-emergent, selective or non-selective.

herbs - plants which do not develop much woody tissue and which usually have rather succulent annual stems. This term also refers to a much smaller group of plants (chives, dill, sage, etc.) grown for culinary, medicinal, functional, or other properties.
hoar frost - a frost that leaves ice crystals on soil and plants owing to condensation of humidity at a decreased temperature.
holdfast - a suction-cup-like organ on some climbing vines, which anchors them to walls, fences and other surfaces.
honeybees - the bees we associate with honey, hives and plant pollination. These insects are not native to North America having been brought here by early European settlers.

Native bee species tend to be more solitary insects that do not form large hives and do not produce quantities of honey. Still, they contribute extensively to pollination of fruit and other plants.

honeydew - the droppings of sucking insects such as aphids, scale, whiteflies, etc. is called honeydew. It is often seen as a shiny coating on leaves that are located below where the insects are feeding. It is a sign of their presence.

Often, a black fungi called sooty mold will develop using the honeydew as a food supply.

hose-end sprayer - a garden hose attachment used for applying pesticide or fertilizer.
horizon (Soil) - typical soils are divided into more or less distinct layers (horizons) moving down toward the center of the earth.
hormone - these are compounds that control general growth and developmental process in the plant. They are sometimes also called growth regulators.

The common plant hormones include gibberellins, auxins, ethylene, abscisic acid and cytokinins.

horticulture - a form of intensive agriculture that includes the cultivation of fruits, vegetables, ornamental trees and shrubs, turfgrass, groundcovers, flowers, and floriculture plants. Horticulture is both an art and a science.
hose-in-hose - Describes a flower that has one perfect set of petals within another. It is generally an abnormality, but varieties of plants such as azaleas with attractive hose-in-hose flowers have been bred.
Hosta Journal - The Hosta Journal is the official publication of the The American Hosta Society. This beautiful, full-color magazine type publication is included with membership in AHS. For further information on the Journal or how to join the Society, visit the AHS website.
humus - highly decomposed, stable organic matter in the soil.
hybrid - (v. hybridize) generally, a hybrid organism results from sexual reproduction with two distinct parents. It is the combination of the genetic material of both parents resulting in genotypic variation.

In plants, a hybrid may be the result of the mixture of pollen (male) from one plant with the ovary (female) of a separate plant. However, in plants with both male and female flower parts on the same plant (monoecious or perfect flowers), the hybrid may be the result of "self-pollination".

Some of the resulting hybrid plants are sterile and incapable of reproduction. Seed from fertile hybrids may produce plants that vary widely from the parents in the next generation.

Through carefully controlled breeding, hybridizers are often capable of combining the exactly correct plants to produce a plant that has the best qualities of each parent. Hybrid seed corn companies have this down to a pretty exact science.

Hybrid Origin Ever see an "x" in the middle of a plant name such as Clematis x jackmanii? Well, what this indicates is that this species is of hybrid origin. That is, it is an entirely new species that resulted from cross-breeding of two existing species. The new species does not exist in the wilds as do other species.

For example, the Judd Viburnum (Viburnum x juddii), is the result of crossing the species, V. carlesii x V. bitchiuense raised at the Arnold Arboretum by William H. Judd, plant propagator, in 1920.

Much more rarely, you might see an "X" at the beginning of a genus name. An example is X Heucherella which is a new genus that resulted from the cross of plants in the genus, Heuchera with plants in the genus, Tiarella. Again, like the species created by humans, X Heucherella does not exist in the wilds.

hybrid tea rose - When most people think of a "rose" they are really picturing the classic hybrid tea roses with their large, delicate blooms that come from the florist. Although not everyone agrees, it appears that the first hybrid tea cultivar was 'La France' which was introduced in 1867.

Of course, this all started with the so-called tea roses that came to Europe from China along with shiploads of tea. These were cross bred with hybrid perpetual roses to come up with the hybrid tea.

Unlike earlier types of roses which had small, five-petaled flowers which bloomed only during June, hybrid teas can bloom the entire season if pruned properly. On the basic hybrid tea, each flower is borne singly or in small clusters on the end of the stem.

Perhaps one of the key traits of the hybrid tea is that it is not very winter hardy. Generally, they can be grown without winter protection only in areas that do not dip to 10 degrees or less. That is one reason why most hybrid teas sold today are grafted onto another type of more hardy root stock.

hybridizer - people who actively transfer the pollen from the anther of one plant to the pistil of another or to the same plant. They often have a specific goal in mind when they make the cross such as improved fragrance or different color flowers.
hydrogel - a chemical compound that absorbs many times its weight in water and is used to keep media moist to reduce the number of waterings required. These are often used in potted plants or containers to help minimize irrigation demands.
hygroscopic - when soils get very dry, they get to a point where the only water is that which is held very tightly to the surface of individual particles. This water is so tightly bound that it is not available for use by plant roots.

See field capacity.

hypocotyl - the part of the embryo or seedling situated between the cotyledon and the radicle.


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