[From Encyclopedia Britannica, 15th ed, Micropedia Vol 6 p. 75, 1990]
Propagation, the controlled perpetuation of plants, is the most basic horticultural practice. Its objectives are to increase the numbers of a plant and to preserve its essential characteristics. Propagation may be achieve sexually by using seeds or asexually by utilizing techniques such as cutting, layering, grafting and tissue culture.
Seeds are the most commonly used means of growing new plants. Seeds are inexpensive, easy to transport and store, may be kept for long periods, and produce plants that are disease-free. Their disadvantages are the long time required for new plants to develop, and the possibility of the heterozygous seed possessing unwanted, recessive characteristics not apparent in the parent plants. The latter may be controlled by selective breeding programs. Vegetative (asexual) reproduction is based on the plant’s ability to regenerate tissues and parts. It ensures that a superior plant may be reproduced endlessly without variation, and is faster and easier than growth from seed. Many plants reproduce naturally by specialized vegetative structures--runners, rhizomes, corms, bulbs, offshoots, tubers, stems, or roots. Layerage and cuttage are horticultural practices that induce plants to propagate by removing a shoot or adventitious root. Grafting involves joining an added plant part, called the scion.to a rootstock by tissue regeneration. Grafting has other uses than propagation; it promotes plant vigour, hardiness, and disease resistance.
Successful horticulture depends upon extensive control of the environment. The most basic control, of course, is the site where the plant is grown, which determines the amounts of sunlight and water and the type of soil available to the plant.
Temperature of the plant's environment may be controlled by using hot caps, cloches, and similar coverings, mulches, and greenhouses. The storage temperature of perishable fruits, vegetables, and flowers must be carefully monitored to prevent rooting or dying out. Frost damage to outdoor crops may be prevented by minimizing radiation heat loss, conserving or adding heat. To prevent frost damage to transplanted plants, a process known as hardening off is used, which deters formation of vulnerable growth tissue by withholding water and fertilizer.
Light has a tremendous effect on plant growth. In addition to providing the energy for photosynthesis, it affects flowering, dormancy, tuber formation, and seed and stalk development. In many cases these processes depend on the length of day, or photoperiodicity. Out-of-doors light may be controlled through location, density, and distribution of planting. In greenhouses artificial illumination may be used, although unless the crop is especially valuable this may be too expensive to be practical.
Two important horticultural techniques are training (the orientation of plants in space) and pruning (judicious removal of plant parts). They are frequently used in conjunction with each other to improve the appearance or usefulness of plants. Familiar examples include trimmed privet hedges and Japanese dwarf potted trees, known as bonsai. [Note: think of these as means to control plant form. pj ]
Plant growth may be regulated through use of plant hormones, auxins, which may stimulated budding, flowering and fruit set and growth. Other hormones and chemicals promote ripening of fruit and inhibit sprouting in onions and potatoes.
Soil management is essential in horticulture. Plants are dependent upon soil for 13 of the 16 minerals necessary to their growth. Carbon, oxygen, and hydrogen are derived from air and water, but N, P, K, Ca, Mg, S, and various trace elements must be present in usable form in the soil for plants t survive and grow. If any f these elements are missing, they must be added to the soil (or directly to the plant) as fertilizer.
The acidity or alkalinity of the soil also influences plants. Extremes of soil acidity or alkalinity are toxic in themselves, but even slight variations from neutrality affect availability of soil nutrients and must be corrected.
Soil structure determines the plant’s supply of water and oxygen, the ease of root growth and the availability of nutrients. In general, adding decaying organic matter to soil improves its texture.
Water is essential for growth. It may be added to the soil through irrigation. too much water in the soil suffocates or rots plants and must be drained off.
Soil is not the only medium for plant growth. In hydroponics plants are grown either in water to which all necessary nutrients have been added, or in sand or gravel with nutrients added.
Plants are subject to a wide variety of injuries and diseases caused by viruses, bacteria fungi, other green plants, worms, insects, mites, birds, and animals. Control of these pests is divided into three categories: plant pathology, entomology (study of insects) and weed control. Pest control involves such different techniques as using chemicals toxic to pests, breeding disease resistant plants, and introducing natural predators to kill pests.
Ornamental horticulture consists of floriculture and landscaping, and the marketing associated with each. Floriculture is the growing of potted plants and cut flowers and their subsequent arrangement and sale by a florist, usually not the grower. Commercial flowers may be grown out-of -doors but usually require the controlled environment of the greenhouse. Air freight has made it possible for flowers to be grown long distances from their markets.
The nursery business grows plants for landscaping. These plants include ornamental shrubs and trees, bulb crops such as tulips, herbaceous perennials, and roses. Since these plants are usually partially grown when they are sold, provisions must be made for transporting them in viable condition. With trees and shrubs the ball of soil surrounding their roots is removed with the roots and wrapped in burlap. Smaller plants may be sold in pots. Production of turf grass has become a major specialized industry.
Landscape design is a distinct profession, only a portion of which involves horticulture. In its broadest sense it is concerned with all aspects of human use of land. The landscape architect uses growing plants as one element in creating an attractive and functional outdoor area.