HORTICULTURE

branch of agriculture concerned with the cultivation of garden crops, generally fruits, vegetables, flowers, and ornamentals such as plants used for landscaping. 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 achieved 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 heterozygote seed possessing unwanted, recessive characteristics not apparent in the parent plants. The latter may be controlled in 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 structuresrunners, 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 uses other 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 rotting or drying 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. Plant growth may be regulated through use of plant hormones, auxins, which may stimulate 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 nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, and various trace elements must be present in usable form in the soil for plants to survive and grow. If any of 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 plants' 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, fruit 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. the branch of plant agriculture dealing with garden crops, generally fruits, vegetables, and ornamental plants. The word is derived from the Latin hortus, garden, and colere, to cultivate. As a general term, it covers all forms of garden management, but in ordinary use it refers to intensive commercial production. In terms of scale, horticulture falls between domestic gardening and field agriculture, though all forms of cultivation naturally have close links. Horticulture is divided into the cultivation of plants for food (pomology and olericulture) and plants for ornament (floriculture and landscape horticulture). Pomology deals with fruit and nut crops. Olericulture deals with herbaceous plants for the kitchen, including, for example, carrots (edible root), asparagus (edible stem), lettuce (edible leaf), cauliflower (edible flower), tomatoes (edible fruit), and peas (edible seed). Floriculture deals with the production of flowers and ornamental plants; generally, cut flowers, pot plants, and greenery. Landscape horticulture is a broad category that includes plants for the landscape, including lawn turf, but particularly nursery crops such as shrubs, trees, and climbers. The specialization of the horticulturist and the success of the crop are influenced by many factors. Among these are climate, terrain, and other regional variations. Roy Perrott Additional reading American Horticultural Society, North American Horticulture (1982); Liberty Hyde Bailey and Ethel Zoe Bailey, Hortus Third (1976); Thomas H. Everett, New York Botanical Garden Illustrated Encyclopedia of Horticulture, 10 vol. (198082); Jules Janick, Horticultural Science, 3rd ed. (1979), and Plant Science: An Introduction to World Crops, 3rd ed. (1981). Environmental control Control of the natural environment is a major part of all forms of cultivation, whatever its scale. The basic processes involved in this task have already been described in a preceding section on the principles of gardening, and these also apply to horticulture. The scale, intensiveness, and economic risk in commercial gardening and nurseries, however, often require approaches markedly different from those of the small home garden; and some of these are described here. The intensive cultivation practiced in horticulture relies on extensive control of the environment for all phases of plant life. The most basic environmental control is achieved by location and site: sunny or shady sites, proximity to bodies of water, altitude, and latitude. Structures Various structures are used for temperature control. Cold frames, used to start plants before the normal growing season, are low enclosed beds covered with a removable sash of glass or plastic. Radiant energy passes through the transparent top and warms the soil directly. Heat, however, as long-wave radiation, is prevented from leaving the glass or plastic cover at night. Thus heat that builds up in the cold frame during the day aids in warming the soil, which releases its heat gradually at night to warm the plants. When supplemental heat is provided, the structures are called hotbeds. At first, supplemental heat was supplied by respiration through the decomposition of manure or other organic matter. Today, heat is provided by electric cables, steam, or hot-water pipes buried in the soil. Greenhouses are large hotbeds, and in most cases the source of heat is steam. While they were formerly made of glass, plastic films are now extensively used. Modern greenhouse ranges usually have automatic temperature control. Summer temperatures can be regulated by shading or evaporative fan-and-pad cooling devices. Air-conditioning units are usually too expensive except for scientific work. Greenhouses with precise environmental controls are known as phytotrons. Other environmental factors are controlled through automatic watering, regulation of light and shade, addition of carbon dioxide, and the regulation of fertility. Shade houses are usually walk-in structures with shading provided by lath or screening. Summer propagation is often located in shade houses to reduce excessive water loss by transpiration. Horticultural education and research Scholarly works in horticulture appear continuously in scientific literature. Specific institutions devoted to horticultural research, however, go back to the beginning of the experiment-station system, the first being a private laboratory of John Bennet Lawes, with the later collaboration of Joseph Henry Gilbert, in Rothamsted, Eng. (1843). Horticultural education and research in the United States was given great impetus by Justin S. Morrill, a supporter of the Morrill Act (1862), which provided educational institutions in agricultural and mechanical arts for each state. State experimental stations and the federal experimental stations of the U.S. Department of Agriculture, with its centre at Beltsville, Md., carry out systematic research efforts in horticulture. Although much research is carried out on horticultural food crops, there has been an increasing emphasis on ornamentals. Horticultural research is also conducted by private companies among the seed industry, canning and processing firms, and private foundations and botanical gardens. Horticultural education is an established part of professional agricultural education worldwide. Training in horticulture up to the Ph.D. degree is offered in universities. There are relatively few schools devoted to the training of gardeners and horticultural technicians in the United States, although a number of state universities have two-year programs in horticulture. Vocational horticultural training is more highly developed in Europe. There are a great number of national and international societies devoted to horticulture. These include community organizations such as garden clubs, specialty organizations devoted to a particular plant or group of plants (e.g., rose and orchid societies), scientific societies, and trade organizations. The first society devoted to horticulture originated in 1804 with the establishment in England of the Royal Horticultural Society. There are similar organizations in other European countries. The American Pomological Society, dedicated to the science and practice of fruit growing, was formed in 1848. The American Horticultural Society, established in 1945, is devoted largely to ornamentals. The American Society for Horticultural Science was established in 1903 and became perhaps the most widely known scientific society devoted to horticulture. The International Society for Horticultural Science, formed in 1959 with permanent headquarters in The Hague, sponsors international congresses every four years. Most societies and horticultural organizations publish periodicals. There are thousands of publications in the world devoted to some aspect of horticulture. The scientific and technical horticultural literature since 1930 is abstracted in Horticultural Abstracts, prepared by the Commonwealth Bureau of Horticulture and Plantation Crops, East Malling, Kent, Eng. Jules Janick Roy Perrott Ornamental horticulture Ornamental horticulture consists of floriculture and landscape horticulture. Each is concerned with growing and marketing plants and with the associated activities of flower arrangement and landscape design. The turf industry is also considered a part of ornamental horticulture. Although flowering bulbs and flower seed represent an important component of agricultural production for the Low Countries of Europe, ornamentals are relatively insignificant in world trade. Floriculture has long been an important part of horticulture, especially in Europe and Japan, and accounts for about half of the nonfood horticultural industry in the United States. Because flowers and pot plants are largely produced in plant-growing structures in temperate climates, floriculture is largely thought of as a greenhouse industry; there is, however, considerable outdoor culture of many flowers. The industry is usually very specialized with respect to its crop; the grower must provide precise environmental control. Exact scheduling is imperative since most floral crops are seasonal in demand. Because the product is perishable, transportation to market must function smoothly to avoid losses. The floriculture industry involves the grower, who mass-produces flowers for the wholesale market, and the retail florist, who markets to the public. The grower is often a family farm, but, as in all modern agriculture, the size of the growing unit is increasing. There is a movement away from urban areas, with their high taxes and labour costs, to locations with lower tax rates and a rural labour pool and also toward more favourable climatic regions (milder temperature and more sunlight). The development of airfreight has emphasized interregional and international competition. Flowers can be shipped long distances by air and arrive in fresh condition to compete with locally grown products. The industry of landscape horticulture is divided into growing, maintenance, and design. Growing of plants for landscape is called the nursery business, although a nursery refers broadly to the growing and establishment of any young plant before permanent planting. The nursery industry involves production and distribution of woody and herbaceous plants and is often expanded to include ornamental bulb cropscorms, tubers, rhizomes, and swollen roots as well as true bulbs. Production of cuttings to be grown in greenhouses or for indoor use (foliage plants), as well as the production of bedding plants, is usually considered part of floriculture, but this distinction is fading. While most nursery crops are ornamental, the nursery business also includes fruit plants and certain perennial vegetables used in home gardens, for example, asparagus and rhubarb. Next to ornamental trees and shrubs, the most important nursery crops are fruit plants, followed by bulb crops. The most important single plant grown for outdoor cultivation is the rose. The type of nursery plants grown depends on location; in general (in the Northern Hemisphere) the northern areas provide deciduous and coniferous evergreens, whereas the southern nurseries provide tender broad-leaved evergreens. The nursery industry includes wholesale, retail, and mail-order operations. The typical wholesale nursery specializes in relatively few crops and supplies only retail nurseries or florists. The wholesale nursery deals largely in plant propagation, selling young seedlings and rooted cuttings, known as lining out stock, of woody material to the retail nursery. The retail nursery then cares for the plants until growth is complete. Many nurseries also execute the design of the planting in addition to furnishing the plants. Bulb crops The bulb crops include plants such as the tulip, hyacinth, narcissus, iris, daylily, and dahlia. Included also are nonhardy bulbs used as potted plants indoors and summer outdoor plantings such as amaryllises, anemones, various tuberous begonias, caladiums, cannas, dahlias, freesias, gladioli, tigerflowers, and others. Hardy bulbs, those that will survive when left in the soil over winter, include various crocuses, snowdrops, lilies, daffodils, and tulips. Many bulb crops are of ancient Old World origin, introduced into horticulture long ago and subjected to selection and crossing through the years to yield many modern cultivars. One of the most popular is the tulip. Tulips are widely grown in gardens as botanical species but are especially prized in select forms of the garden tulip (which arose from crosses between thousands of cultivars representing several species). Garden tulips are roughly grouped as early tulips, breeder's tulips, cottage tulips, Darwin tulips, lily-flowered tulips, triumph tulips, Mendel tulips, parrot tulips, and others. The garden tulips seem to have been developed first in Turkey but were spread throughout Europe and were adopted enthusiastically by the Dutch. The Netherlands has been the centre of tulip breeding ever since the 18th century, when interest in the tulip was so intense that single bulbs of a select type were sometimes valued at thousands of dollars. The collapse of the tulipmania left economic scars for decades. The Netherlands remains today the chief source of tulip bulbs planted in Europe and in North America. The Netherlands has also specialized in the production of related bulbs in the lily family and provides hyacinth, narcissus, crocus, and others. The Dutch finance extensive promotion of their bulbs to support their market. Years of meticulous growing are required to yield a commercial tulip bulb from seed. Thorough soil preparation, high fertility, constant weeding, and careful record keeping are part of the intensive production, which requires much hand labour. Bulbs sent to market meet specifications as to size and quality, which assure at least one year's bloom even if the bulb is supplied nothing more than warmth and moisture. The inflorescence (flowering) is already initiated and the necessary food stored in the bulb. Under less favourable maintenance than prevails in The Netherlands, a subsequent year's bloom may be smaller and less reliable; it is not surprising therefore that tulip-bulb merchants suggest discarding bulbs after one year and replanting with new bulbs to achieve maximum yield. Propagation Propagation, the controlled perpetuation of plants, is the most basic of horticultural practices. Its two objectives are to achieve an increase in numbers and to preserve the essential characteristics of the plant. Propagation can be achieved sexually by seed or asexually by utilizing specialized vegetative structures of the plant (tubers and corms) or by employing such techniques as cutting, layering, grafting, and tissue culture. (A detailed discussion of the methods of controlling sexual propagation can be found in the article plant breeding.) Seed propagation The most common method of propagation for self-pollinated plants is by seed. In self-pollinated plants, the sperm nuclei in pollen produced by a flower fertilize egg cells of a flower on the same plant. Propagation by seed is also used widely for many cross-pollinated plants (those whose pollen is carried from one plant to another). Seed is usually the least expensive and often the only means of propagation and offers a convenient way to store plants over long periods of time. Seed kept dry and cool normally maintains its viability from harvest to the next planting season. Some can be stored for years under suitable conditions. Seed propagation also makes it possible to start plants free of most diseases. This is especially true with respect to virus diseases, because it is almost impossible to free plants of virus infections and because most virus diseases are not transmitted by seed. There are two disadvantages to seed propagation. First, genetic variation occurs in seed from cross-pollinated plants because they are heterozygous. This means that the plant grown from seed may not exactly duplicate the characteristics of its parents and may possess undesirable characteristics. Second, some plants take a long time to grow from seed to maturity. Potatoes, for example, do not breed true from seed and do not produce large tubers the first year. These disadvantages are overcome by vegetative propagation. The practice of saving seed to plant the following year has developed into a specialized part of horticulture. Seed technology involves all of the steps necessary to ensure production of seed with high viability, freedom from disease, purity, and trueness to type. These processes may include specialized growing and harvesting techniques, cleaning, and distribution. Relatively little tree and shrub seed is grown commercially; it is generally harvested from natural stands. Rootstock seed for fruit trees is often obtained as a by-product in fruit-processing industries. Seed growing and plant improvement are related activities. Thus many seed-producing firms actively engage in plant-breeding programs to accomplish genetic improvement of their material. Harvesting of dry seed is accomplished by threshing. Seed from fleshy fruits is recovered through fermentation of the macerated (softened by soaking) pulp or directly from screening. Machines have been developed to separate and clean seed, based on size, specific gravity, and surface characteristics. Extended storage of seed requires low humidity and cool temperature. Trade in seed requires quality control. For example, U.S. government seed laws require detailed labeling showing germination percentage, mechanical purity, amount of seed, origin, and moisture content. Seed testing is thus an important part of the seed industry. While most vegetable seed germinates readily upon exposure to normally favourable environmental conditions, many seed plants that are vegetatively (asexually) propagated fail to germinate readily because of physical or physiologically imposed dormancy. Physical dormancy is due to structural limitations to germination such as hard impervious seed coats. Under natural conditions weathering for a number of years weakens the seed coat. Certain seeds, such as the sweet pea, have a tough husk that can be artificially worn or weakened to render the seed coat permeable to gases and water by a process known as scarification. This is accomplished by a number of methods including abrasive action, soaking in hot water, or acid treatment. Physiologically imposed dormancy involves the presence of germination inhibitors. Germination in such seed may be accomplished by treatment to remove these inhibitors. This may involve cold stratification, storing seed at high relative humidity and low temperatures, usually slightly above freezing. Cold stratification is a prerequisite to the uniform germination of many temperate-zone species such as apple, pear, and redbud.