Maximum return can be obtained only from soil with an ample supply of elements necessary for plant growth, combined with sufficient moisture to enable them to be dissolved and absorbed through the plant hairs.

Treatment with farmyard manure or garden compost can supply the majority of these requirements. Because manure and compost are scarce in urban areas it is often necessary to use mineral fertilizers as well as organics. The soil is such a complex substance that all fertilizers must be applied in moderation and in balance with each other according to the deficiencies of the soil and the requirements of the particular crop. Different crops have different fertilizer needs. Manures are generally best dug into the ground in autumn in a temperate climate but also may be used as mulches in spring to control weeds. A mulch is a surface layer of organic matter that helps the several needs of feeding, conserving moisture, and controlling weeds. Black polyethylene sheeting is now widely used for all the mulching functions except feeding.

Watering of newly placed plants and of all plants during periods of drought is an essential gardening chore. Deep and thorough watering—not simply sprinkling the soil surface—can result in greatly improved growth. Water is essential in itself, but it also makes minerals available to plants in solution, the only form usable by plants. About one inch of water applied each week to the soil surface will percolate down about six inches; this is a minimal subsistence amount for many herbaceous garden plants, and small trees and shrubs require more. Proper watering once a week encourages deep penetration of roots, which in turn enables plants to survive dry surface conditions.

Drainage

Drainage is the other important side of water management. All plants need water but the amount needed varies, and if plants are forced to absorb more than they need, a form of drowning occurs. The symptoms are most easily seen in overwatered pot plants but are also visible to an experienced eye in badly drained corners of a garden. Roots require air as well as water and depend on subsurface water to bring the necessary oxygen. In large private gardens and in commercial gardens, buried earthenware piping is commonly used. In smaller gardens drainage can be readily achieved by the use of sumps, that is, holes dug to a depth of about four feet in affected places. The bottom half of the sump is filled with stones, through which excess water drains. Such measures may greatly improve the potential of a garden and the workability of its soil.

Protecting plants

Most plants have a precise level of tolerance to cold, below which they are killed. Many plants from tropical or subtropical regions cannot survive frost and are killed by temperatures below 32 °F (0 °C). These are called frost-tender. Others, called half-hardy, can withstand a few degrees of frost. Fortunately, many of the best garden plants are completely hardy, a quality often encouraged by careful breeding, and will withstand any low temperatures likely to be reached in temperate regions.

Various measures can be taken to give frost protection, from the simple ones appropriate for smaller gardens to the elaborate coverings used to protect valuable horticultural crops. Removing weeds that shade the soil increases the amount of heat stored during the day. Well-drained soil is less susceptible. Any shield against wind in frosty weather enhances survival capability. The simplest form of protection is a wrapping to keep warmer air around the plant. This can be a mulch (leaves, soil, ashes) placed over the crown of a slightly tender plant in winter or a shield of sacking for leaf-shedding plants (not as desirable for evergreens, which utilize their leaves all the year).

Glass structures such as greenhouses or outdoor frames can provide additional protection for tender plants. Such structures can be heated and the temperature regulated by a thermostat to any required degree. Thus, in temperate regions, orchids and other tropical plants can be grown so that they flower throughout the winter, many being forced to flower earlier than their normal season by the higher temperature. Greenhouses are divided by gardeners into four rough categories: (1) The cold house, in which there is no supplementary heating and which is suitable only for plants that will not be killed by a few degrees of frost (such as alpines or potted bulbous plants). The combination of heat from the sun and protection from wind will keep such a house appreciably warmer than the temperature outside. (2) The coolhouse, in which the minimum temperature is kept to 45 °F (7 °C). Most amateurs’ greenhouses fall into this class, and a very large range of plants can be grown in them. (3) The intermediate house, in which the minimum temperature is kept at 55 or 60 °F (13 or 16 °C) and which is suitable for a wide range of orchids. (4) The hothouse, or stove house, in which the minimum temperature is kept above 60 °F (16 °C) and in which tropical plants such as anthuriums and cattleyas (a genus of the orchid family) can be grown.

Training and pruning

Training, the orienting of the plant in space, is achieved by techniques that direct the shape, size, and direction of plant growth. It may be accomplished by use of supports to which plants can be bent, twisted, or fastened. Pruning, the judicious cutting away of plant parts, is performed for other purposes: to contain size, to encourage fruiting in orchard trees, or to improve the appearance of ornamental trees and shrubs. It is one of the most important horticultural arts.

Where trees and shrubs are left to grow naturally, they often become much too large for their space in the garden. Also they may grow lanky and misshapen and have much dead growth. Where a branch or shoot is cut, it will often be induced to make a number of young shoots from below the cut, and these are likely to flower more freely than the older branches. Fruit trees in particular when pruned annually often give fruit of finer quality, larger in size, freer of disease, and of better colour. The two basic pruning cuts are known as heading back and thinning out. Heading back consists of cutting back the terminal portion of a branch to a bud; thinning out is the complete removal of a branch to a lateral or main trunk. Heading back, usually followed by the stimulation of lateral budbreak below the cut, produces a bushy, compact plant, suitable for a hedgerow, and it is often used to rejuvenate shrubs that have become too large or that flower poorly. Thinning out, which encourages longer growth of the remaining terminals by reducing lateral branches, tends to open up the plant, producing a longer plant. In general, pruning, started when the plant is young, obviates the need for drastic and risky remedial pruning later of a large, old, or misshapen bush or tree.

Particular spatial arrangements may increase light utilization, facilitate harvesting or disease control, or improve productivity and quality. Thus, training and pruning form an essential part of fruit growing throughout the life of the plant. Special attention is given in the formative years to obtain desired shape and structure. The key to training is the point on the main stem from which branches form. In the central-leader system of training, the trunk forms a central axis with branches distributed laterally up and down and around the stem. In the open-centre or vase system, the main stem is terminated and growth forced through a number of branches originating close to the upper end of the trunk. An intermediate system is called the modified-leader system. In espalier systems plants are trained to grow flat along a wire or trellis. Properly executed espaliers are extremely attractive as ornamentals. Espaliers in combination with dwarfing rootstocks allow high-density orchards that are very productive on a per-unit-area basis, with the fruit close to the ground for easy harvest. Extensive pruning is required annually to maintain the system.

There are a number of physiological responses to training and pruning. Orientation of the plant may have a marked effect on growth and fruiting. Thus fruit trees planted on an inclined angle become dwarfed and flower earlier; training branches in a horizontal position produces the same effect. This effect is achieved naturally when a heavy fruit load bends a limb down. The main effects of pruning are achieved by altering the root–shoot balance. Thus an explosion of vegetative growth normally occurs after extensive shoot pruning. Severely pruned plants, especially if they are in the juvenile stage of growth, tend to remain vegetative. Similarly the slowdown of vegetative growth by root pruning encourages flowering.

The training of plants to grow in unnatural shapes for ornamental purposes is called topiary. In Roman and Renaissance times, when ingenious topiary was in high fashion, plants were trained to unusual and fantastic shapes such as beasts, ships, and building facades. Though more modestly, hedges and shrubs are still trained to geometric shapes in formal gardens.

Another extreme form of training is the Japanese art of bonsai, the creation of dwarfed potted trees by a combination of pruning (both roots and tops) and restricted nutrition. Living trees more than 100 years old and only a few feet high are grown in special containers arranged to resemble the natural landscape.

Propagation

New plants are produced either from seed or by the techniques of division, taking cuttings, grafting, budding, or layering. For the ordinary gardener, propagation is a relatively simple but interesting process normally used for economic provision of more versions of favourite plants, as part of exchanges with other gardeners, or as a wise precaution against winter losses. (For a fuller description of propagation and breeding processes, see horticulture.)

Propagation by cuttings is the most common practice. Young shoots of the current season are usually the most successful at rooting. Roses are usually propagated by budding, in which a bud from the rose desired is inserted in rootstock (that part of the plant tissue from which a root can form) just above ground level. Fruit trees are usually propagated by layering, in which a young shoot is pegged down in the ground with the end twisted upward almost at right angles; the lower side of the wood just before the twist is wounded so as to induce rooting. When this has taken place, the layer is severed from the parent.