Bonsai cultivation and care

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

In this technique, new growing material (typically a bud, branch, or root) is introduced to a prepared area under the bark of the tree. There are two major purposes for grafting in bonsai. First, a number of favorite species do not thrive as bonsai on their natural root stock and their trunks are often grafted onto hardier root stock. Examples include Japanese red maple and Japanese black pine. Second, grafting allows the bonsai artist to add branches (and sometimes roots) where they are needed to improve or complete a bonsai design. There are many applicable grafting techniques, none unique to bonsai, including branch grafting, bud grafting, thread grafting, and others.

Defoliation

Short-term dwarfing of foliage can be accomplished in certain deciduous bonsai by partial or total defoliation of the plant partway through the growing season. Not all species can survive this technique. In defoliating a healthy tree of a suitable species, most or all of the leaves are removed by clipping partway along each leaf's petiole (the thin stem that connects a leaf to its branch). Petioles later dry up and drop off or are manually removed once dry. The tree responds by producing a fresh crop of leaves. The new leaves are generally much smaller than those from the first crop, sometimes as small as half the length and width. If the bonsai is shown at this time, the smaller leaves contribute greatly to the bonsai aesthetic of dwarfing. This change in leaf size is usually not permanent, and the leaves of the following spring will often be the normal size. Defoliation weakens the tree and should not be performed in two consecutive years.

Deadwood

Bonsai growers create or shape dead wood using techniques such as jin and shari to simulate age and maturity in a bonsai. Jin is the term used when the bark from an entire branch is removed to create the impression of a snag of deadwood. Shari denotes stripping bark from areas of the trunk to simulate natural scarring from a broken limb or lightning strike. In addition to stripping bark, deadwood techniques may also involve the use of tools to scar the deadwood or to raise its grain, and the application of chemicals (usually lime sulfur) to bleach and preserve the exposed deadwood.

Care

Small trees grown in containers, like bonsai, require specialized care. Unlike most houseplants, flowering shrubs, and other subjects of container gardening, tree species in the wild generally grow individual roots up to several meters long and root structures encompassing hundreds or thousands of liters of soil. In contrast, a typical bonsai container allows a fraction of a meter for root extension, and holds 2 to 10 liters of soil and root mass. Branch and leaf (or needle) growth in trees is also large-scale in nature. Wild trees typically grow 5 meters or taller when mature, while the largest bonsai rarely exceed 1 meter and most specimens are significantly smaller. These size differences affect maturation, transpiration, nutrition, pest resistance, and many other aspects of tree biology. Maintaining the long-term health of a tree in a container requires a number of specialized care techniques.

Growing environment

Most bonsai species are trees and shrubs that must by nature grow outdoors. They require temperature, humidity, and sunlight conditions approximating their native climate year round. The skill of the grower can help bonsai from outside the local hardiness zone survive and even thrive, but doing so takes careful watering, shielding of selected bonsai from excessive sunlight or wind, and possibly protection from winter conditions (e.g., through the use of cold frames or winter greenhouses).

Common bonsai species (particularly those from the Japanese tradition) are temperate climate trees from hardiness zones 7 to 9, and require moderate temperatures, moderate humidity, and full sun in summer with a dormancy period in winter that may need be near freezing. They do not thrive indoors, where the light is generally too dim, and humidity often too low, for them to grow properly. Only during their dormant period can they safely be brought indoors, and even then the plants require cold temperatures, reduced watering, and lighting that approximates the number of hours the sun is visible. Raising the temperature or providing more hours of light than available from natural daylight can cause the bonsai to break dormancy, which often weakens or kills it.

Even for bonsai specimens that are native to the grower's location, outdoor cultivation requires specific cultivation practices to ensure successful long-term survival of the bonsai. The trees used in bonsai are constrained by the need to grow in a relatively small pot. This state greatly reduces the volume of roots and soil normally available to a freely grown tree, and brings the roots much closer to the surface of the soil than would occur in the wild. Trees in bonsai pots have much less access to water and to nutrients than they do natively, and physically confining roots changes their growth pattern and indirectly the growth pattern of the tree above the soil.

The grower has some control over the following environmental variables, and by controlling them effectively for individual specimens can ensure the health of native species grown as bonsai, and can cultivate some non-native species successfully.

Repotting

Bonsai are repotted and root-pruned at intervals dictated by the vigor and age of each tree. In the case of deciduous trees, this is done as the tree is leaving its dormant period, generally around springtime. Bonsai are often repotted while in development, and less often as they become more mature. This prevents them from becoming pot-bound and encourages the growth of new feeder roots, allowing the tree to absorb moisture more efficiently.

Specimens meant to be developed into bonsai are often placed in "growing boxes", which have a much larger volume of soil per plant than a bonsai pot does. These large boxes allow the roots to grow freely, increasing the vigor of the tree and helping the trunk and branches grow thicker. After using a grow box, the tree may be replanted in a more compact "training box" that helps to create a smaller, denser root mass which can be more easily moved into a final presentation pot.

Tools

Special tools are available for the maintenance of bonsai. The most common tool is the concave cutter (5th from left in picture), a tool designed to prune flush, without leaving a stub. Other tools include branch bending jacks, wire pliers and shears of different proportions for performing detail and rough shaping.

Soil and fertilization

Bonsai soil is usually a loose, fast-draining mix of components, often a base mixture of coarse sand or gravel, fired clay pellets, or expanded shale combined with an organic component such as peat or bark. The inorganic components provide mechanical support for bonsai roots, and—in the case of fired clay materials—also serve to retain moisture. The organic components retain moisture and may release small amounts of nutrients as they decay.

In Japan, bonsai soil mixes based on volcanic clays are common. The volcanic clay has been fired at some point in time to create porous, water-retaining pellets. Varieties such as akadama, or "red ball" soil, and kanuma, a type of yellow pumice used for azaleas and other calcifuges, are used by many bonsai growers. Similar fired clay soil components are extracted or manufactured in other countries around the world, and other soil components like diatomaceous earth can fill a similar purpose in bonsai cultivation.

Opinions about fertilizers and fertilization techniques vary widely among practitioners. Some promote the use of organic fertilizers to augment an essentially inorganic soil mix, while others will use chemical fertilizers freely. Many follow the general rule of little and often, where a dilute fertilizer solution or a small amount of dry fertilizer are applied relatively frequently during the tree's growing season. The flushing effect of regular watering moves unmetabolized fertilizer out of the soil, preventing the potentially toxic build-up of fertilizer ingredients.

Pest management

The common pests afflicting bonsai include insects both above and beneath the soil, and infections, usually fungal. A tree grown as a bonsai is subject to the pests that affect the same species full-grown, and also to pests common to other potted plants. Most pests are species-specific, so a detailed understanding of the specific bonsai species is necessary for identifying and treating most pests. The same materials and techniques used for other affected plants can be applied to the bonsai, with some relatively minor variation. Pesticide chemicals are usually diluted more for bonsai than for a larger plant, as a regular-strength application may overwhelm the smaller bonsai's biological processes.

Outdoors

Bonsai are sometimes marketed or promoted as house plants, but few of the traditional bonsai species can thrive or even survive inside a typical house. Most bonsai are located out of doors. The best guideline to identifying a suitable growing environment for a bonsai is its native hardiness. If the bonsai grower can closely replicate the full year's temperatures, relative humidity, and sunlight, the bonsai should do well. In practice, this means that trees from a hardiness zone closely matching the grower's location will generally be the easiest to grow, and others will require more work or will not be viable at all.

Indoors

Tropical and Mediterranean species typically require consistent temperatures close to room temperature, and with correct lighting and humidity many species can be kept indoors all year. Those from cooler climates may benefit from a winter dormancy period, but temperatures need not be dropped as far as for the temperate climate plants and a north-facing windowsill or open window may provide the right conditions for a few winter months.