Horticulture: Growing Vegetables and Such

Horticulture is 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.

Propagation

Propagation is the controlled perpetuation of plants, 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.

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.

Vegetative propagation

Asexual or vegetative reproduction is based on the ability of plants to regenerate tissues and parts. In many plants vegetative propagation is a completely natural process; in others it is an artificial one. Vegetative propagation has many advantages. These include the unchanged perpetuation of naturally cross-pollinated or heterozygous plants and the possibility of propagating seedless progeny. This means that a superior plant may be reproduced endlessly without variation. In addition, vegetative propagation may be easier and faster than seed propagation, because seed dormancy problems are eliminated and the juvenile nonflowering stage of some seed-propagated plants is eliminated or reduced.

Vegetative propagation is accomplished by use of (1) apomictic seed; (2) specialized vegetative structures such as runners, bulbs, corms, rhizomes, offshoots, tubers, stems, and roots; (3) layers and cuttings; (4) grafting and budding; and (5) tissue culture.

Apomixis

Apomixis, the development of asexual seed (seed not formed via the normal sexual process), is a form of vegetative propagation for some horticultural plants including Kentucky bluegrass, mango, and citrus. Virus-free progeny can be produced in oranges from a seed that is formed from the nucellus, a maternal tissue.

Vegetative structures

Many plants produce specialized vegetative structures that can be used in propagation. These may be storage organs such as tubers that enable the plant to survive adverse conditions or organs adapted for natural propagation—runners or rhizomes—so that the plant may rapidly spread.

Bulbs consist of a short stem base with one or more buds protected by fleshy leaves. They are found in such plants as the onion, daffodil, and hyacinth. Bulbs commonly grow at ground level, though bulblike structures (bulbils) may form on aerial stems in some lilies or in association with flower parts, as in the onion. Buds in the axils (angle between leaf and stem) of the fleshy leaves may form miniature bulbs (bulblets) that when grown to full size are known as offsets. Corms are short, fleshy, underground stems without fleshy leaves. The gladiolus and crocus are propagated by corms. They may produce new cormels from fleshy buds. Rhizomes are horizontal, underground stems that are compressed, as in the iris, or slender, as in turf grasses. Runners are specialized aerial stems, a natural agent of increase and spread for such plants as the strawberry, strawberry geranium, and bugleweed (Ajuga). Tubers are fleshy enlarged portions of underground stem. The edible portion of the potato, the tuber, is also used as a means of propagation.

A number of plants form lateral shoots from the stem, which when rooted serve to propagate the plant. These are known collectively as offshoots but are often called offsets, crown divisions, ratoons, or slips.

Roots may also be structurally modified as propagative and food-storage organs. These tuberous roots, fleshy swollen structures, readily form shoots (called adventitious, because they do not form from nodes). The sweet potato and dahlia are propagated by tuberous roots. Shoots that rise adventitiously from roots are called suckers. The red raspberry is propagated by suckers.

Layering and cutting

Propagation can be accomplished by methods in which plants are induced to regenerate missing parts, usually adventitious roots or shoots. When the regenerated part is still attached to the plant the process is called layerage, or layering; when the regenerating portion is detached from the plant the process is called cuttage, or cutting.

Layering often occurs naturally. Drooping black raspberry stems tend to root in contact with the soil. The croton, a tropical plant, is commonly propagated by wrapping moist sphagnum enclosed in plastic around a stem cut to induce rooting. After rooting, the stem is detached and planted. Though simple and effective, layering is not normally adapted to large-scale nursery practices.

Cutting is one of the most important methods of propagation. Many plant parts can be used; thus cuttings are classified as root, stem, or leaf. Stem cuttings are the most common.

The ability of stems to regenerate missing parts is variable; consequently plants may be easy or difficult to root. The physiological ability of cuttings to form roots is due to an interaction of many factors. These include transportable substances in the plant itself: plant hormones (such as auxin), carbohydrates, nitrogenous substances, vitamins, and substances not yet identified. Environmental factors such as light, temperature, humidity, and oxygen are important, as are age, position, and type of stem.

Although easy-to-root plants such as willow or coleus can be propagated merely by plunging a stem in water or moist sand, the propagation of difficult-to-root species is a highly technical process. To achieve success with difficult-to-root plants special care is taken to control the environment and encourage rooting. A number of growth regulators stimulate rooting. A high degree of success has been achieved with indolebutyric acid, a synthetic auxin that is applied to the cut surface. A number of materials known as rooting cofactors have been found that interact with auxin to further stimulate rooting, and these are sold as a hormone rooting compound.

Humidity control is particularly important to prevent death of the stem from desiccation before rooting is complete. The use of an intermittent-mist system in propagation beds has proved to be an important means of improving success in propagation by cuttings. These operate by applying water to the plant for a few seconds each minute.

Grafting

Grafting involves the joining together of plant parts by means of tissue regeneration. The part of the combination that provides the root is called the stock; the added piece is called the scion. When more than two parts are involved, the middle piece is called the interstock. When the scion consists of a single bud, the process is called budding. Grafting and budding are the most widely used of the vegetative propagation methods.

Stock cambium and scion cambium respond to being cut by forming masses of cells (callus tissues) that grow over the injured surfaces of the wounds. The union resulting from interlocking of the callus tissues is the basis of graftage. In dicots (e.g., most trees) cambium—a layer of actively dividing cells between xylem (wood) and phloem (bast) tissues—is usually arranged in a continuous ring; in woody members, new layers of tissue are produced annually. Monocot stems (e.g., lilacs, orchids) do not possess a continuous cambium layer or increase in thickness; grafting is seldom possible.

The basic technique in grafting consists of placing cambial tissues of stock and scion in intimate association, so that the resulting callus tissue produced from stock and scion interlocks to form a living continuous connection. A snug fit can be obtained through the tension of the split stock and scion or both. Tape, rubber, and nails can be used to achieve close contact. In general, grafts are only compatible between the same or closely related species. Success in grafting depends on skill in achieving a snug fit. Warm temperatures (80°–85° F [27°–30° C]) increase callus formation and improve take in grafting. Thus grafts using dormant material are often stored in a warm, moist place to stimulate callus formation.

In grafting and budding, the rootstock can be grown from seed or propagated asexually. Within a year a small amount of scion material from one plant can produce hundreds of plants.

Grafting has uses in addition to propagation. The interaction of rootstocks may affect the performance of the stock through dwarfing or invigoration and in some cases may affect quality. Further, the use of more than one component can affect the disease resistance and hardiness of the combination.

Grafting as a means of growth control is used extensively with fruit trees and ornamentals such as roses and junipers. Fruit trees are normally composed of a scion grafted onto a rootstock. Sometimes an interstock is included between the scion and stock. The rootstock may be grown from seed (seedling rootstock) or asexually propagated (clonal rootstock). In the apple, a great many clonal rootstocks are available to give a complete range of dwarfing; rootstocks are also available to invigorate growth of the scion cultivar.

Tissue-culture techniques utilizing embryos, shoot tips, and callus can be used as a method of propagation. The procedure requires aseptic techniques and special media to supply inorganic elements; sugar; vitamins; and, depending on the tissue, growth regulators and organic complexes such as coconut milk, yeast, and amino-acid extract.

Embryo culture has been used to produce plants from embryos that would not normally develop within the fruit. This occurs in early-ripening peaches and in some hybridization between species. Embryo culture can also be used to circumvent seed dormancy.

A shoot tip, when excised and cultured, may produce roots at the base. This technique is employed for the purpose of producing plants free of disease. Certain orchids are rapidly multiplied by this method. Cultured shoot tips form an embryo-like stage that can be sectioned indefinitely to build up large stocks rapidly. These bulblike bodies left unsectioned develop into small plantlets. A similar procedure is used with the carnation, in which the shoot tip forms a cell mass that can be subdivided.

Callus-tissue culture—a very specialized technique that involves growth of the callus, followed by procedures to induce organ differentiation—has been successful with a number of plants including carrot, asparagus, and tobacco. Used extensively in research, callus culture has not been considered a practical method of propagation. Callus culture produces genetic variability because in some cases cells double their chromosome number. In rice and tobacco, mature plants have been obtained from callus formed from pollen. These plants have half the normal number of chromosomes.

Breedeing

The isolation and production of superior types known as cultivars are the very keystones of horticulture. Plant breeding, the systematic improvement of plants through the application of genetic principles, has placed improvement of horticultural plants on a scientific basis. The raw material of improvement is found in the great variation that exists between cultivated plants and related wild species. The incorporation of these changes into cultivars adapted to specific geographical areas requires a knowledge of the theoretical basis of heredity and art and the skill to discover, perpetuate, and combine these small but fundamental differences in plant material.

The goal of the plant breeder is to create superior crop varieties. The cultivated variety, or cultivar, can be defined as a group of crop plants having similar but distinguishable characteristics. The term cultivar has various meanings, however, depending on the mode of reproduction of the crop. With reference to asexually propagated crops, the term cultivar means any particular clone considered of sufficient value to be graced with a name. With reference to sexually propagated crops, the concept of cultivar depends on the method of pollination. The cultivar in self-pollinated crops is basically a particular homozygous genotype, a pure line. In cross-pollinated crops the cultivar is not necessarily typified by any one plant but sometimes by a particular plant population, which at any one time is composed of genetically distinguishable individuals.

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.

Temperature control

A number of temperature-control techniques are used in the field, including application of hot caps, cloches, plastic tunnels, and mulches of various types. Hot caps are cones of translucent paper or plastic that are placed over the tops of plants in the spring. These act as miniature greenhouses. In the past small glass sash called cloches were placed over rows to help keep them warm. Polyethylene tunnels supported by wire hoops that span the plants are now used for the same purpose. As spring advances the tunnels are slashed to prevent excessive heat buildup. In some cases the plastic tunnels are constructed so that they can be opened and closed when necessary. This technique is widely used in Israel for early production of vegetables.

Mulching is important in horticulture. Whether in the form of a topdressing of manure or compost or plastic sheeting, mulches offer the grower the various benefits of economical plant feeding, conservation of moisture, and control of weeds and erosion. Winter mulches are commonly used to protect such sensitive and valuable plants as strawberries and roses.

The storage of perishable plant products is accomplished largely through the regulation of their temperature to retard respiration and microbial activity. Excess water loss can be prevented by controlling humidity. Facilities that utilize the temperature of the atmosphere are called common storage. The most primitive types take advantage of the reduced temperature fluctuations of the soil by using caves or unheated cellars. Aboveground structures must be insulated and ventilated. Complete temperature-regulated storages utilizing refrigeration and heating are now common for storage of horticultural products. The regulation of oxygen and carbon dioxide levels along with the regulation of temperature is known as controlled-atmosphere storage. Rooms are sealed so that gaseous exchange can be effectively controlled. Many horticultural products, such as fruit, can be kept fresh for as long as a year under these controlled conditions.

Frost control

Frost is one of the high-risk elements for commercial growers, and the problem is accentuated by the fact that growers are striving to produce early-season crops. The precautions are consequently far more elaborate and costly than those of the domestic garden. Frost is especially damaging to perennial fruit crops in the spring—because flower parts are sensitive to freezing injury—and to tender transplants. The two weather conditions that produce freezing temperatures are rapid radiational cooling at night and introduction of a cold air mass with temperatures below freezing. Radiation frost occurs when the weather is clear and calm; air-mass freezes occur when it is overcast and windy.

Frost-control methods involve either reduction of radiational heat loss or conservation or addition of heat. Radiational heat loss may be reduced by hot caps, cold frames, or mulches. Heat may also be added from the air. Wind machines that stir up the air, for example, provide heat when temperature inversions trap cold air under a layer of warm air. These have been used extensively in citrus groves. Heat may be added directly by using heaters, usually fueled with oil. Sprinkler irrigation can also be used for frost control. The formation of ice is accompanied by the release of large amounts of heat, which maintains plants at the freezing temperature as long as the water is being frozen. Thus continuous sprinkling during frosty nights has been used to protect strawberries from frost injury.

Frost injury to transplants can be prevented through processes that increase the plant's ability to survive the impact of unfavourable environmental stress. This is known as hardening off. Hardening off of plants prior to transplanting can be accomplished by withholding water and fertilizer, especially nitrogen. This prevents formation of succulent tissue that is very frost-tender. Gradual exposure to cold is also effective for hardening. Induced cold resistance in crops such as cabbage, for example, can have a considerable effect; unhardened cabbages begin to show injury at 28° F (−2.2° C), while hardened plants withstand temperatures as low as 22° F (−5.6° C).

Light control

Light has a tremendous effect on plant growth. It provides energy for photosynthesis, the process by which plants, with the aid of the pigment chlorophyll, synthesize carbon compounds from water and carbon dioxide. Light also influences a great number of physiological reactions in plants. At energy values lower than those required for photosynthesis, light affects such processes as dormancy, flowering, tuberization, and seed-stalk development. In many cases these processes are affected by the length of day; the recurrent cycle of light is known as the photoperiod.

The control of light in horticultural practices involves increasing energy values for photosynthesis and controlling day length. Light is controlled in part by site and location. In the tropics day length approaches 12 hours throughout the year, whereas in polar regions it varies from zero to 24 hours. Light is also partly controlled by plant distribution and density.

Supplemental illumination in greenhouses increases photosynthesis. The cost of power to supply the artificial light, however, makes this impractical for all but crops of the highest value. Fluorescent lights are the most efficient for photosynthesis; special lights, rich in the wavelengths required, are now available.

Extension of day length through supplemental illumination and shading is common practice in the production of greenhouse flower crops, which are often induced to flower out of season. Artificial lengthening of short days, or interruption of the dark period, promotes flowering in long-day plants such as lettuce and spinach and prevents flowering of short-day plants such as chrysanthemums. Similarly, during naturally long days, shading to reduce day length prevents flowering of long-day plants and promotes flowering of short-day plants. The manipulation of day length is standard practice to control flowering of greenhouse chrysanthemums throughout the year. Tungsten lights have proved very effective for extending day length because they are rich in the red end of the spectrum that affects the photoperiodic reaction. Extending the day length is a relatively affordable practice because only a low light intensity is required. The same effects can be obtained through interruption of the dark period, even with light flashes. Decreasing day length is usually accomplished by simply covering the plants with black shade cloth.

Soil management

The principles involved here are again similar to those of home gardening. But the financial considerations of horticulture naturally require a more scientific approach to soil care. To be successful, the grower must ensure the economic use of every square yard of ground, especially because the cost of sound horticultural land is among the highest of any in agriculture. Crop rotation is planned to ensure that the soil is not depleted of essential chemicals by repeated use of one type of plant in the same plot. Soil analysis is employed so that any such depletion can be rectified promptly. Fertilizers are applied in a precise routine and, of course, in a variety beyond the reach or needs of the ordinary gardener. They are frequently applied through leaves or stems in the form of chemical sprays.

Water management

Depending on the terrain, water management may involve extensive works for irrigation and drainage. While the home gardener may well be content with a rough-and-ready appraisal of the wetness or dryness of the soil, horticulture is more exacting. Production of the high-quality fruits and vegetables demanded by the modern market requires a precise all-year balance of soil moisture, adjusted to the needs of the particular crop. These considerations apply whether the grower is situated in a high-rainfall area of Europe or in the parched land of the southwestern United States or Israel.

There are a number of general methods of land irrigation. In surface irrigation water is distributed over the surface of soil. Sprinkler irrigation is application of water under pressure as simulated rain. Subirrigation is the distribution of water to soil below the surface; it provides moisture to crops by upward capillary action. Trickle irrigation involves the slow release of water to each plant through small plastic tubes. This technique is adapted both to field and to greenhouse conditions.

Removal of excess water from soils can be achieved by surface or subsurface drainage. Surface drainage refers to the removal of surface water by development of the slope of the land utilizing systems of drains to carry away the surplus water. In subsurface drainage open ditches and tile fields intercept groundwater and carry it off. The water enters the tiling through the joints, and drainage is achieved by gravity feed through the tiles.

Pest control

Horticultural plants are subject to a wide variety of injuries caused by other organisms. Plant pests include viruses, bacteria, fungi, higher plants, nematodes, insects, mites, birds, and rodents. Various methods are used to control them. The most successful treatments are preventive rather than curative.

Control of pests is achieved through practices that prevent harm to the plant and methods that affect the plant's ability to resist or tolerate intrusion by the pathogen. These can be classified as cultural, physical, chemical, or biological.

Traditional practices that reduce effective pest population include the elimination of diseased or infected plants or seeds (roguing), cutting out of infected plant parts (surgery), removal of plant debris that may harbour pests (sanitation), and alternating crops unacceptable to pests (rotation). Any of a number of techniques can be employed to render the environment unfavourable to the pest, such as draining or flooding and changing the soil's level of acidity or alkalinity.

Physical methods can be used to protect the plant against intrusion or to eliminate the pest entirely. Physical barriers range from the traditional garden fence to bags that protect each fruit, a common practice in Japan. Heat treatment is used to destroy some seed-borne pathogens and is a standard soil treatment in greenhouses to eliminate soil pests such as fungi, nematodes, and weed seed. Cultivation and tillage are standard practices for weed control.

The horticultural industry is now dependent upon chemical control of pests through pesticides, materials toxic to the pest in some stage of its life cycle. Commercial growers of practically all horticultural crops rely on complete schedules utilizing many different compounds. Pesticides are usually classed according to the organism they control: for example, bactericide, fungicide, nematicide, miticide, insecticide, rodenticide, and herbicide.

Selectivity of pesticides, the ability to discriminate between pests, is a relative concept. Some nonselective pesticides kill indiscriminately; most are selective to some degree. Most fungicides, for example, are not bactericidal. The development of highly selective herbicides makes it possible to destroy weeds from crops selectively. Selectivity can be achieved through control of dosage, timing, and method of application.

Plant pests can also be controlled through the manipulation of biological factors. This may be achieved through directing the natural competition between organisms or by incorporating natural resistance to the whole plant. The introduction of natural parasites or predators has been a successful method for the control of certain insects and weeds. Incorporation of genetic resistance is an ideal method of control. Thus breeding for disease and insect resistance is one of the chief goals of plant breeding programs. A major obstacle to this method of control is the ability of pathogens (disease-producing organisms) to mutate easily and attack previously resistant plants.

Growth regulation by chemicals

Control of plant growth through growth-regulating materials is a modern development in horticulture. These materials have resulted from basic investigations into growth and development, as well as systematic screening of materials to find those that affect differentiation and growth. This field was given great impetus by the discovery of a class of plant hormones known as auxins, which affect cell elongation.

Auxins have been correlated with inhibition and stimulation of growth as well as differentiation of organs and tissues. Such processes as cell enlargement, leaf and organ separation, budding, flowering, and fruit set (the formation of the fruit after pollination) and growth are influenced by auxins. In addition, auxins have been associated with the movement of plants in response to light and gravity. Auxin materials are used in horticulture for the promotion of rooting, fruit setting, fruit thinning, and fruit-drop control.

Gibberellins are a group of related, naturally occurring compounds of which only one, gibberellic acid, is commercially available. Gibberellins have many effects on plant development. The most startling is the stimulation of growth in many compact or dwarf plants. Minute applications transform bush to pole beans or dwarf to normal corn. Perhaps the most widespread horticultural use has been in grape production. The application of gibberellin is now a regular practice for the culture of the Thompson seedless cultivar (Sultanina) of grapes to increase berry size. In Japan applications of gibberellic acid are used to induce seedlessness in certain grapes.

Cytokinins are a group of chemical substances that have a decisive influence on the stimulation of cell division. In tissue culture high auxin and low cytokinin give rise to root development; low auxin and high cytokinin encourage shoot development.

Ethylene, a hydrocarbon compound, acts as a plant hormone to stimulate fruit ripening as well as rooting and flowering of some plants. An ethylene-releasing compound, 2-chloroethylphosphonic acid, has many horticultural applications, of which the most promising may be uniform ripening of tomatoes and the stimulation of latex flow in rubber.

Many compounds that inhibit growth hormones have application in horticulture. For example, a number of materials that inhibit formation of gibberellins by the plant cause dwarfing. These include chlorinated derivatives of quaternary ammonium and phosphonium compounds. Many of these have applications in floriculture. Growth retardants such as succinic acid–2,2-dimethylhydrazide, a gibberellin suppressor, have applications in horticulture from a wide array of effects that include dwarfing and fruit maturity. The growth inhibitor maleic hydrazide has been effective in preventing the sprouting of onions and potatoes.

Autumn~Fall

For many gardeners, summer is peak vegetable gardening season. The warm weather and long days are needed to coax tomatoes, peppers, and other heat lovers into ripeness. But fall has some qualities that make it extremely favorable for crops that prefer cooler temperatures or wetter conditions. In fact, in some areas it the best season for growing vegetables.

(From https://www.thespruce.com/preparing-your-vegetable-garden-for-fall-1402167)

Why Plant Vegetables in Fall?

(From https://www.thespruce.com/preparing-your-vegetable-garden-for-fall-1402167)

If you've had an abundant growing season, you might prefer to look at fall as the season to rest and get the garden cleaned up. However, there are a lot of advantages to a fall vegetable garden that might make you want to reconsider. For instance:

The weather is milder
Many insect pests are getting ready to hide out for winter
The breeze keeps flying insects at bay
The soil actually becomes that elusive moist, but well-draining

Preparing Your Vegetable Garden for Fall

How to Keep Your Vegetable Garden Going into Winter

Although many vegetables grow and mature well into the fall, most need to be started before the nights turn cold. In climates with early frost dates, your fall garden will need to be started in mid-summer, late July through August. Even though the daytime temperatures remain high, evening temps will start to fall and the length of daylight is decreasing. So choose varieties with short days to maturity and get them in the ground on time.

As for gardeners in areas that experience extreme heat and infrequent rain during the summer, fall is the ideal vegetable gardening season. This is when you can grow your warm season vegetables, like tomatoes and peppers. Many can continue growing throughout the winter, switching over to the cool weather vegetables as your rainy season moves in.

Clean-Up the Garden, Before Replanting in the Fall

If you've been tending your vegetable garden all summer, keeping it weeded, removing diseased or spent plants, not stepping on the soil... your garden won't need much prep for the fall. Just clear the space, maybe add some compost and start planting. But if you're like most of us, your garden probably got a little ahead of your best efforts and could use some attention before you start a second season of planting. Don't worry, it's not a lot of heavy lifting and it's not terribly time-consuming.

There may already be holes left from pulling or harvesting plants, but try and clear out as much as you can.

Weeds - If they've been hiding out under plants, get rid of them now and do it before they go to seed.
Spent Plants - Get rid of everything possible. You can leave your tomatoes and peppers to ripen, but many other plants, like early planted beans, cucumbers and lettuce, are pretty much done for the season and they're just harboring disease and pests. Dispose of anything diseased, compost the rest.
Fallen Fruits - If you've been lax about cleaning up small fruits that have dropped off the plant, make sure you get them out now. Rotting fruits attract pests.

One last note, if this will be your 2nd planting season, make note of what was where, so you can rotate your crops as best as possible.

Replenish the Soil in the Planting Beds

You are going to want to freshen the garden soil and the easiest way to do that is to remove the layer of mulch. If it's still in good condition, you can re-use it for the fall. Most likely a lot of it has decomposed already and you'll need to add a bit more. But before you do, you will want to make sure the soil is ready for planting.

Loosen Compacted Soil

If your soil has gotten compacted during the course of the summer, fluff it up a bit with a garden fork. You don't need to do major tilling. Just enough to allow the new plant roots to move around and for water to get through.

Amend the Soil Before Planting

If you are feeling diligent, fall is a great time to test your soil. A lot of amendments take time to have an effect on the soil.

At the very least, replenish the soil by working in some compost. You can top dress with it or work it in while you are loosening the soil. Have your planting layout done before you add the compost, so you add it where the plants will be growing and not in the paths.

If you choose to use manure, make sure it has thoroughly composted for at least 6 months. Fresh compost can burn plant roots and can pose a serious health risk to humans when used on vegetable plants.

It's also good to add some slow-acting organic fertilizer when you loosen the soil. You can add individual amendments, like green sand or blood meal, if that's what your soil test recommends. But if you're plants aren't showing signs of nutrient deficiencies, a general purpose fertilizer will do the job.

When you're done adjusting your soil, rake it out. Raking evens out the surface, breaks up any remaining clumps and creates furrows to catch water.

Replace the Mulch

If you still have mulch from the summer, you can reuse it on the freshened planting beds. If you need to add more, straw makes an excellent mulch for vegetable gardens because it is easily scattered and moved about. It also makes a wonderful home for spiders, who will assist in controlling your pest population.

Another good choice for mulching in the fall is shredded leaves. If you have a supply of fallen leaves, either run them through a shredder, pill them up and mow them or corral them and run the string weeder through them like a blender. Wet them down well, after you spread them, or they will blow away. A light top dusting with compost will also help control them. Unshredded leaves tend to form a mat that doesn't let water through to the soil.

Preparing Your Vegetable Garden for Winter

If you are planning on using any kind of frost protection, like a cold frame or hoop cover, consider getting your structures in place now. Putting them out early will guarantee they are there when you need them and will help prevent hurting the plants and their roots, once they are growing. Don't put the covers in place yet, just the framing.

Ready

Now you're ready to start planting. Here are some best bet vegetables for a fall garden.

Of course, if this all sounds like too much work or if you'd like a rest for the fall, you can always sow a green manure or cover crop and let your garden tend itself until spring.

Tips for Planting a Fall Vegetable Gardens

(From https://www.thespruce.com/best-vegetables-for-a-fall-garden-4141720)

When direct seeding, be sure to choose varieties that mature in the shortest amount of time. Count back from your first expected frost date and plan accordingly.
Plant seeds deeper in the fall than in the spring. The ground is warmer in fall. Planting them a little deeper than the package suggests will get them down where it is cool and moist.
Transplants can handle swings in temperature and water better than seeds and newly emerged seedlings.
Pelletized seeds that are coated in clay are sometimes available for vegetables that take a long time to germinate, like carrots. They retain water better than uncoated seed, which means less watering and TLC for you. However, they usually need to be mail ordered and are more expensive.

Best Vegetables to Grow in a Fall Garden

Fall Vegetables to Start from Seed or Transplant

(From https://www.thespruce.com/best-vegetables-for-a-fall-garden-4141720)

Fall vegetable gardening will vary greatly across hardiness zones. Gardeners in zones 8 and up, who have waited patiently through the hazy heat of summer, can finally get all sorts of vegetable plants started, including tender tomatoes and eggplant.

Gardeners in northern climates will have to content themselves growing vegetables that enjoy the cooler, shorter days of fall, like leafy greens, root vegetables, cabbages, broccoli, and kale.

Fall Vegetables that Can be Started from Seed

To have a fall garden mature before frost, you'll need to get it started in mid-summer. That means late July to August, depending on your zone. Many plants will grow well in cool weather, but they need to be started while it is still a bit warm and the days are longer.

Fall vegetable gardens are often just an extension of summer succession planting. As one plant is finished, another vegetable is planted in its place. And a few vegetables are hardy enough to direct seed, some are even better grown from seed and are noted, below.

Fall Vegetables that are Better when Grown from Transplants

Although some cool-season vegetables need about 3 months to mature from seed and might not seem ideal for a fall garden, you can either start seed in pots, in a cool shady spot, or look for seedlings at the garden center and then transplant out when there is space freed up in your vegetable garden in the fall.

Even the plants recommend to start from seed, can be started as transplants, if you prefer. Starting seed in pots and moving them into your garden as space opens up is a great way to maximize your harvest.

List of Best Vegetables to Grow in a Fall Garden

Beets: As with all root vegetables, beets are best grown from seed. You don't want to disturb the root, once it is growing. Beets are easy to grow in succession, allowing you a staggered harvest from spring into summer. Although beets will still grow in the summer heat, they can have a tendency to get bitter and woody quickly. But late summer is a great time to resume succession planting, at 2-3 week intervals. The bulbs will keep growing until a hard freeze and even the tops can handle a bit of frost.
Bok Choy / Pak Choi: Bok choy and many other Asian greens, are well suited to fall planting. The so called baby bok choy varieties grow only about 8 inches tall and grow quickly, in about 40 days. They revel in cool weather and fall has the added advantage of less leaf pests. Since they won't bolt to seed as quickly as they might in warming spring weather, you can harvest heads as you need them.
Broccoli: Broccoli, like other cole crops, grows best in cool weather. Fall planting has two big advantages over spring planting. First, although broccoli plants can tolerate frost, tender broccoli seedlings are not as hardy and early spring frosts can shock or kill them. And secondly, since the florets are flower buds, they will open more slowly in cool weather, giving you more time to harvest. Broccoli does take several months to mature, so transplants of a quick grower like Waltham, set out in mid- to late summer, are a good bet for a fall harvest.
Bush Beans: Pole beans need a long growing season. The vines have to grow tall, before they start setting beans. However many bush varieties will start producing in as little as 45 days, making them excellent candidates for a fall crop of beans. Bean plants are too tender to handle frost, but if a frost threatens, you can always toss a row cover over them until the temperatures climb again.
Cabbage: Fall is the perfect growing weather for cabbage. The plants will grow in warm weather, but they need cool weather to form a head. And the best flavor comes from cabbage plants that can grow at a nice steady pace, with plenty of water. Cabbage needs anywhere from 90-120 days to mature, so a fall crop is easiest if you transplant seedlings in mid- to late summer. Most of the vegetables in the cabbage family are hardy enough to handle a light frost. With some protection, you can continue to harvest them well into winter. They won't grow when it's cold, but they'll retain their freshness and even get sweeter.
Carrots: Carrots are not the quickest growers, but some of the smaller varieties like Thumbelina and Paris Market, will mature in about 50 days. Another option is sowing your fall carrots in containers. Long window boxes with a depth of at least 6 inches are great for this. You can keep them close at hand, where they are convenient for watering and safe from four-footed pests.
Cauliflower: Fall is a better time to grow cauliflower than spring. Something about the cool weather keeps the heads tight and tender. If you've experienced heads with a gritty texture, chances are good they were grown in the spring. Set out transplants in mid- to late summer and watch the difference. But remember, cabbage worms can still be a problem, so consider growing them under row covers.
Kale: Kale is probably the easiest cole crop to grow. The seeds germinate in warm or cool soil and it's grown for its leaves, so you don't have to worry about a head forming or the flowers bolting. The advantage of growing it in the fall is the magic that happens to so many vegetables that are grown in cool weather or hit by a light frost; the leaves will still have a great texture and the flavor will sweeten and deepen. Start seeds for fall harvesting in mid- to late summer or transplant in late summer.
Lettuce: Lettuce can be planted pretty much all season. In really hot weather, it can get bitter and bolt to seed. But most varieties take less than 50 days to mature, so you can start planting a seed in mid-August and succession plant into fall. The plants may grow more slowly than in the spring, because the temperature is getting cooler, rather than warmer and because the days are shorter, but the flavor will be sweeter and crisper. Lettuce plants will require some protection against frost. Lettuce is shallow rooted and is a great choice for containers. If you plant your fall lettuce in pots, it's very easy to move it indoors, to protect it from frost. Planted this way, you could have fresh lettuce well into winter.
Peas: Peas are another vegetable that loves to luxuriate in cool weather. Many short varieties will be ready to harvest within 50-60 days. Pea seeds will germinate more quickly in warm weather, but they will also need more water and a little protection from the strong sun, while they are young. But by the time they are ready to start flowering and forming peas, the days and nights should be the perfect temperature in fall, to keep them going.
Radishes: Radishes like to grow quickly and in cool, moist soil. Most of the problems associated with growing radishes in the spring (woody texture, hot and bitter taste, small bulbs...) will be alleviated by showing them in the fall. You might also want to experiment with some of the winter radishes, like Round Black Spanish. They tend to grow more slowly than globe radishes and do not like heat. They prefer being planted in mid-summer and allowed to develop into fall. You can harvest bulbs in late fall to winter or allow them to over-winter in the soil and harvest next spring. They grow very much like turnips.
Spinach: If you've been frustrated by your spinach bolting before it's even large enough to eat, you are going to love growing it in the fall. Spinach only takes about 30-40 days to mature and even less, if you like small, tender leaves. You can get several succession sowings of spinach in throughout the fall. Spinach seeds are also perfect for winter sowing.
Swiss Chard: If you've been harvesting your Swiss chard as cut-and-come-again, you won't even have to reseed it. The plants have probably slowed down a bit during summer, but with a little cool air and water, they will switch back into full production. They may even survive the winter, but harvest them quickly in the spring, before they start to bolt to seed and get tough and bitter.
Turnips and Rutabagas: Both of these root vegetables need a long season, 100 or more days, to mature, but they grow largest, with their best flavor if allowed to mature in the fall. Since they are grown for their roots and their tops are fairly hardy, it doesn't matter too much if they get hit by frosts. They may even taste sweeter. Turnips mature a little faster than rutabagas and don't improve if left in the soil too long, so succession plant and use as they mature. Rutabagas can be left in the ground over winter, if necessary, and harvested in the spring.

10 Biggest Vegetable Gardening Mistakes

Things to Avoid to Have a Successful Vegetable Garden

(From https://www.thespruce.com/biggest-vegetable-gardening-mistakes-1402993)

Like all gardening, growing great vegetables takes experience. It's not particularly hard, but plants can be unpredictable and uncooperative. Here are ten of the most commonly made mistakes and some hard-earned advice on how to avoid them.

Planting Too Early

We're all impatient to get the garden started. It's very tempting to get your hands in the soil and start seeds months before your last frost date. However little seeds quickly become lanky, hungry seedlings. They need a lot more space in your home and they become stressed if they must remain indoors in pots, with limited light.

Even if you start your garden by purchasing seedlings, you have to resist the urge to put them in the ground as soon as you get them home. It may be true that you'll find the biggest selection of varieties if you shop early, but have a plan for hardening them off and for protecting them if a late frost is predicted. Otherwise, you'll be right back at the nursery buying more plants.

Picking a Bad Spot

It's a pleasure to work in a vegetable garden in the crisp days of spring or fall, but if your garden is not handy, eventually you won't be visiting it every day. It only takes a day or two for a zucchini to grow to the size of a bowling pin or a rabbit to break through your fence and finish off your peas.

Two other considerations are siting it by a source of water and in a spot that gets at least 6 hours of sun a day. Vegetables that don't get enough sunlight will struggle all season, set fewer fruits and develop less tasty sugars than well-sited vegetables. And trust me, you are not going to enjoy lugging a hose 100 ft. every time a plant needs watering and water cans are not much better.

Skimping on Soil

You have to cultivate a true love of soil to be a successful gardener. It can't be an afterthought. Starting off with poor soil means you will be fighting against it all season. Good soil harbors all kinds of beneficial creatures that help turn the soil into a repository of accessible nutrients while inhibiting the onset of problems.

Prevent all this goodness from washing away by ending the season with either a green manure, to be tilled in the following season, or a thick layer of shredded leaves or leaf mold. Leaves slowly rot into the soil, adding richness and attracting even more beneficial organisms and earthworms to keep this healthy system functioning. Once you've handled and inhaled the earthiness of rich soil, you may even come to enjoy amending it each season.

Not Harvesting

It sounds counter-intuitive, but many gardeners are hesitant to harvest when things are ready. They worry that there won't be more coming in or they don't want to hurt the plant or sometimes they just want to have the look of an abundant garden. But not harvesting when a vegetable is ready to be picked will actually cause your garden to slow down. A plant won't set more cucumbers or peppers if its branches are already full of them. Herbs, like basil and cilantro, benefit from frequent harvesting. Cutting off the tops of the plants encourages them to branch out and get fuller. Enjoy your vegetables while they are at peak.

Planting Too Much

All gardeners tend to plant more than they can eat, let alone tend, but new vegetable gardeners don't realize how much work it can be. Vegetables don't wait until you have time to take care of them. They will ripen and rot or bolt to seed. Better to start small than to waste both your effort and your vegetables.

There are two basic ways to overdo it: too large a space or too much variety. Too large a space is the most common mistake. For someone brand new at gardening, a 10 x 10 ft. garden is a good way to start out. You can always enlarge it. Feeling overwhelmed or chained to your garden is the number one reason so many new veggie gardens don't keep at it.

Too much variety can also overwhelm you, as you try to make all the plants happy. Better to start with a handful of plants you really enjoy eating or that you can't purchase fresh locally. Learn how to grow them well and then expand your repertoire.

Ignoring Spacing

This is a very closely related problem to starting out too large. Small seedlings properly spaced can leave the garden looking barren. Why leave 3 ft. between tomato plants when you can squeeze them a foot or so closer? Because they are going to grow and fill in quickly and tightly spaced plants don't get the sun and air circulation they need. That leads to diseases and lower yields, not to mention difficulty harvesting.

Vegetables that you will be harvesting as whole plants throughout the season, like lettuce, carrots or beets, can be placed a bit more closely together and thinned as you harvest. But tomatoes, corn, peppers and the like need room to branch out. Use the extra space early in the season to plant a quickly growing crop, like spinach or lettuce. It will be gone by the time the long season plants need the space.

Not Staggering Harvest Times

Your family may eat lettuce every night, but planting a 10 ft. row of lettuce isn't going to keep you in salads all season. You have to do some strategic planning to stagger your harvest times. There are several ways to do this, including succession planting and planting varieties that mature at different times. You'll be glad you took the time to space things out.

Putting Off Maintenance

Weeding, feeding, and watering need to be done on a regular schedule, especially watering. Plants don't like competition for water and nutrients and allowing weeds to fill in will stunt many plants and reduce their yields.

Without regular water and food, plants will stress and shut down. They go into self-preservation mode and refuse to set fruits or simply bolt to seed, to ensure propagation of their species. If you think caring for a pet is time-consuming, you're going to find vegetable plants are true divas.

Not Fencing

Planting vegetables is like ringing the dinner bell. Herbivores, like deer, rabbits, and groundhogs, will clean you out overnight. I can't overstate the need for a sturdy fence. What kind of fence depends on what animal problems you have.

Deer can jump and require either a high fence, electric fencing or one of the clever angled or double fences that makes them unsure about jumping in and being trapped. There are many good sources for setting up a deer fence online.

Burrowing animals, like groundhogs, rabbits, and chipmunks, need both an above ground fence that at least 3 - 4 ft. tall and about a ft. of buried fence. Angle both fences outward from the garden, to deter them even more. Again, there are many sources online that detail the best methods for constructing fencing to foil the animals.

Ignoring Little Problems

Not every problem in the vegetable garden requires a full assault - in fact, most don't. But you need to monitor your plants on a regular basis. If you see yellowing leaves or spots, inspect closer and make a correction before the whole row of plants becomes ill. Insects like to lay their eggs on the undersides of leaves. Check their occasionally. Scrapping off the eggs before they hatch can totally avert the problem.

But don't be so overly cautious that you spray at the first sign of trouble or are tempted to kill off all the insects. There are beneficial insects that are your partners in the garden. They kill off pests or keep plants cross-pollinated. And always be sure of what you are spraying for before your spray anything. Use the appropriate pesticide, start with the least toxic option first and follow the label instructions. More is not better. More could kill your plants.

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 crops—corms, 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.

Herbaceous perennials

Garden perennials include a number of herbaceous species grown for their flowers or occasionally used as vegetative ground covers. Under favourable growing conditions the plants persist and increase year after year. The biggest drawback to perennials as compared with annuals is that they must be maintained throughout the growing season but have only a limited flowering period. Typical perennials are hollyhocks, columbines, bellflowers, chrysanthemums, delphiniums, pinks, coralbells, phlox, poppies, primroses, and speedwells.

Perennials are often produced and sold as a sideline to other nursery activities; some are sold through seed houses. Perennial production could be undertaken on a massive scale, with attendant economies, but the market is neither large enough nor predictable enough (except for the greenhouse growing of such cut flowers as chrysanthemums and carnations) to interest most growers.

Shrubs

Production of ornamental shrubs is the backbone of the nursery trade in Europe and the United States. The nursery business is about equally divided between the production of (1) coniferous evergreens such as yew, juniper, spruce, and pine; (2) broad-leaved evergreens such as rhododendron, camellia, holly, and boxwood; (3) deciduous plants such as forsythia, viburnum, berberis, privet, lilac, and clematis; and (4) roses.

Fields of specialization have evolved within the ornamental shrub industry. Some firms confine activity mostly to production of lining out stock, which must be tended several years before reaching salable size.

The field grower may, in turn, specialize in mass growing for the wholesale trade only. The field plantings are tended until they attain marketable size. Because of the time required to produce a marketable crop and because of rising labour costs, this phase of the nursery industry involves economic hazards. But wholesale growing escapes the high overhead of retail marketing in urban areas, and, although many growers do sell stock at the nursery, they generally avoid the expensive merchandising required of the typical urban-area garden centre. Growers are especially interested in laboursaving technology and are turning to herbicidal control of weeds and shortcut methods for transplanting.

There is a well-established trade in container-grown stock—that is, nursery stock grown in the container in which it is sold. This practice avoids transplanting and allows year-round sales of plant material.

Roses

The production of roses is probably the most specialized of all shrub growing; the grower often deals solely in rose plants. Most are bud-grafted onto rootstocks (typically Rosa multiflora). This is the only way to achieve rapid and economical increase of a new selection to meet market demands. Large-scale production of roses has tended to centre in areas where long growing seasons make rapid production possible.

Because the budding operation calls for skilled hand labour and because field maintenance is expensive, few economies can be practiced in the production of roses. But distribution techniques that do offer certain economies have been developed. These include covering the roses with coated paper or plastic bags instead of damp moss to retain humidity and applying a wax coating to stems of dormant stock to inhibit desiccation.

Trees

Ornamental shade trees are usually grown and marketed in conjunction with shrubs. The 20th-century migration of people in many countries to suburban areas, coupled with the construction of houses on cleared land, has made shade trees an increasingly important part of the nursery trade. As interest in shade and ornamental trees increased, creation of improved cultivars followed. There is still some activity in transplanting native trees from the woodlot, and some are still grown from genetically unselected seed or cuttings; but more and more, like roses and shrubs before them, trees are vegetatively propagated as named cultivars, and many are patented.

The design and planning of landscapes has become a distinct profession that in many cases is only incidentally horticultural. Landscape architecture in its broadest sense is concerned with all aspects of land use. As a horticulturist, the landscape architect uses plants along with other landscape materials—stone, mortar, wood—as elements of landscape design. Unlike the materials of the painter or sculptor, plants are not static but change seasonally and with time. The colour, form, texture, and line of plants are used as design elements in the landscape. Plant materials are also manipulated as functional materials to control erosion, as surface materials, and for enclosures to provide protection from sunlight and wind.

Landscape architecture originated in the design of great estates, and home landscape is still an integral part of landscape architecture. More recently, however, landscape architecture has begun to include larger developments such as urban and town planning, parks both formal and wild, public buildings, industrial landscaping, and highway and roadside development.