...because you need a bonsai
With limited space in a bonsai pot, regular attention is needed to ensure the tree is correctly watered. Sun, heat and wind exposure can dry bonsai trees to the point of drought in a short period of time. While some species can handle periods of relative dryness, others require near-constant moisture. Watering too frequently, or allowing the soil to remain soggy, promotes fungal infections and root rot. Free draining soil is used to prevent waterlogging. Deciduous trees are more at risk of dehydration and will wilt as the soil dries out. Evergreen trees, which tend to cope with dry conditions better, do not display signs of the problem until after damage has occurred.
Bonsai are repotted and root-pruned at intervals dictated by the vigour 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.
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.
Location and Overwintering
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. The best guideline to identifying a suitable location 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 outdoors, and others will require more work or will not be viable at all.
Most bonsai species are outdoor trees and shrubs by nature, and they require temperature, humidity, and sunlight conditions approximating their native climate year round. The skill of the gardener can help plants from outside the local hardiness zone to 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 boxes 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 in the dormant period can they safely be brought indoors, and even then the plants require cold temperatures 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.
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.