Bonsai cultivation and care involves the long-term cultivation of small trees in containers, called bonsai in the Japanese tradition of this art form. Similar practices exist in other Japanese art forms and in other cultures, including saikei (Japanese), penjing (Chinese), and hòn non bộ (Vietnamese). Trees are difficult to cultivate in containers, which restrict root growth, nutrition uptake, and resources for transpiration (primarily soil moisture). In addition to the root constraints of containers, bonsai trunks, branches, and foliage are extensively shaped and manipulated to meet aesthetic goals. Specialized tools and techniques are used to protect the health and vigor of the subject tree. Over time, the artistic manipulation of small trees in containers has led to a number of cultivation and care approaches that successfully meet the practical and the artistic requirements of bonsai and similar traditions.
The term bonsai is generally used in English as an umbrella term for all miniature trees in containers or pots. In this article bonsai should be understood to include any container-grown tree that is regularly styled or shaped, not just one being maintained in the Japanese bonsai tradition.
Bonsai can be created from nearly any perennial woody-stemmed tree or shrub species which produces true branches and remains small through pot confinement with crown and root pruning. Some species are popular as bonsai material because they have characteristics, such as small leaves or needles, that make them appropriate for the compact visual scope of bonsai. Bonsai cultivation techniques are different from other tree cultivation techniques in allowing mature (though miniature) trees to grow in small containers, to survive with extremely restricted root and canopy structures, and to support comprehensive, repeated styling manipulations.
Sources of Bonsai Material
All bonsai start with a specimen of source material, a plant that the grower wishes to train into bonsai form. Bonsai practice is an unusual form of plant cultivation in that growth from seeds is rarely used to obtain source material. To display the characteristic aged appearance of a bonsai within a reasonable time, the source plant is often partially grown or mature stock. A specimen may be selected specifically for bonsai aesthetic characteristics it already possesses, such as great natural age for a specimen collected in the wild, or a tapered, scar-free trunk from a nursery specimen. Alternatively, it may be selected for non-aesthetic reasons, such as known hardiness for the grower's local climate or low cost (as in the case of collected materials).
Propagation
While any form of plant propagation could generate bonsai material, a few techniques are favored because they can quickly produce a relatively mature trunk with well-placed branches.
Cuttings. In taking a cutting, part of a growing plant is cut off and placed in a growing medium to develop roots. If the part that is cut off is fairly thick, like a mature branch, it can be grown into an aged-looking bonsai more quickly than can a seed. Thinner and younger cuttings tend to strike roots more easily than thicker or more mature ones. In bonsai propagation, cuttings usually provide source material to be grown for some time before training.
Layering. Layering is a technique in which rooting is encouraged from part of a plant, usually a branch, while it is still attached to the parent plant. After rooting, the branch is removed from the parent and grown as an independent entity. For bonsai, both ground layering and air layering can create a potential bonsai, by transforming a mature branch into the trunk of a new tree. The point at which rooting is encouraged can be close to the location of side branches, so the resulting rooted tree can immediately have a thick trunk and low branches, characteristics that complement bonsai aesthetics.
Commercial bonsai growers Commercial bonsai growers may use any of the other means of obtaining starter bonsai material, from seed propagation to collecting expeditions, but they generally sell mature specimens that display bonsai aesthetic qualities already. The grower trains the source specimens to a greater or lesser extent before sale, and the trees may be ready for display as soon as they are bought. Those who purchase commercially grown bonsai face some challenges, however, particularly of buying from another country. If the purchaser's local climate does not closely match the climate in which the bonsai was created, the plant will have difficulties surviving and thriving. As well, importing living plant material from a foreign source is often closely controlled by import regulations and may require a license or other special import arrangement on the buyer's part. If a local commercial bonsai grower does not exist, buying from a distant one may be unsatisfactory.
Nursery Stock A plant nursery is an agricultural operation where (non-bonsai) plants are propagated and grown to usable size. Nursery stock may be available directly from the nursery, or may be sold in a garden centre or similar resale establishment. Nursery stock is usually young but fully viable, and is often potted with sufficient soil to allow plants to survive a season or two before being transplanted into a more permanent location. Because the nursery tree is already pot-conditioned, it can be worked on as a bonsai immediately. The large number of plants that can be viewed in a single visit to a nursery or garden centre allows the buyer to identify plants with better-than-average bonsai characteristics. According to Peter Adams, a nursery visit "offers the opportunity to choose an instant trunk". One issue with nursery stock is that many specimens are shaped into popular forms, such as the standard or half-standard forms, with several feet of clear trunk rising from the roots. Without branches low on the trunk, it is difficult for a source specimen to be trained as bonsai.
Collecting Collecting bonsai consists of finding suitable bonsai material in its natural environment, successfully moving it, and replanting it in a container for development as bonsai. Collecting may involve wild materials from naturally treed areas, or cultivated specimens found growing in private yards and gardens. For example, mature landscape plants being discarded from a building site can provide excellent material for bonsai. Hedgerow trees, grown for many years but continually trimmed to hedge height, provide heavy, gnarled trunks for bonsai collectors. In locations close to a tree line (the line beyond which trees do not grow, whether due to altitude, temperature, soil moisture, or other conditions), aged and naturally dwarfed survivors can be found.
The main benefit of collecting bonsai specimens is that collected materials can be mature, and will display the natural marks and forms of age, which makes them more suitable for bonsai development than the young plants obtained through nurseries. Low cost is another potential benefit, with a tree harvest license often being more economical than the purchase of nursery trees. Some of the difficulties of collecting include finding suitable specimens, getting permission to remove them, and the challenges of keeping a mature tree alive while transplanting it to a bonsai pot.
Styling Techniques
Bonsai are carefully styled to maintain miniaturization, to suggest age, and to meet the artist's aesthetic goals. Tree styling also occurs in a larger scale in other practices like topiary and niwaki. In bonsai, however, the artist has close control over every feature of the tree, because it is small and (in its container) easily moved and worked on. The greater scale of full-sized trees means that styling them may be restricted to pruning and shaping the exterior volume once per growing season, never pruning within the canopy nor bending and forming individual branches. In contrast, in a bonsai being prepared for display, each leaf or needle may be subject to decision regarding pruning or retention, and every branch and twig may be formed and wired into place each year. Given these differences in scope and purpose, bonsai styling uses a number of styling techniques either unique to bonsai or (if used in other forms of plant cultivation) applied in ways particularly suitable to meet the goals of bonsai development.
Leaf Trimming This technique involves selective removal of leaves (for most varieties of deciduous tree) or needles (for coniferous trees and some others) from a bonsai's branches. A common aesthetic technique in bonsai design is to expose the tree's branches below groups of leaves or needles (sometimes called "pads") by removing downward-growing material. In many species, particularly coniferous ones, this means that leaves or needles projecting below their branches must be trimmed off. For some coniferous varieties, such as spruce, branches carry needles from the trunk to the tip and many of these needles may be trimmed to expose the branch shape and bark. Needle and bud trimming can also be used in coniferous trees to force back-budding on old wood, which may not occur naturally in many conifers. Along with pruning, leaf trimming is the most common activity used for bonsai development and maintenance, and the one that occurs most frequently during the year.
Pruning The small size of the tree and some dwarfing of foliage result from pruning the trunk, branches, and roots. Pruning is often the first step in transforming a collected plant specimen into a candidate for bonsai. The top part of the trunk may be removed to make the tree more compact. Major and minor branches that conflict with the designer's plan will be removed completely, and others may be shortened to fit within the planned design. Pruning later in the bonsai's life is generally less severe, and may be done for purposes like increasing branch ramification or encouraging growth of non-pruned branches. Although pruning is an important and common bonsai practice, it must be done with care, as improper pruning can weaken or kill trees. Careful pruning throughout the tree's life is necessary, however, to maintain a bonsai's basic design, which can otherwise disappear behind the uncontrolled natural growth of branches and leaves.
Wiring
Wrapping copper or aluminium wire around branches and trunks allows the bonsai designer to create the desired general form and make detailed branch and leaf placements. When wire is used on new branches or shoots, it holds the branches in place until they lignify (convert into wood). The time required is usually 6–9 months or one growing season for deciduous trees, but can be several years for conifers like pines and spruce, which maintain their branch flexibility through multiple growing seasons. Wires are also used to connect a branch to another object (e.g., another branch, the pot itself) so that tightening the wire applies force to the branch. Some species do not lignify strongly, and some specimens' branches are too stiff or brittle to be bent easily. These cases are not conducive to wiring, and shaping them is accomplished primarily through pruning.
Clamping For larger specimens, or species with stiffer wood, bonsai artists also use mechanical devices for shaping trunks and branches. The most common are screw-based clamps, which can straighten or bend a part of the bonsai using much greater force than wiring can supply. To prevent damage to the tree, the clamps are tightened a little at a time and make their changes over a period of months or years.
Grafting