Repotting of bonsai plants is usually needed when soil insects damage the plants, or when soil is in poor condition. Sometimes, however, a soil condition can be corrected without repotting and disturbing the roots of the plant. This is done by adding new soil around the outer surface, or by removing plugs of soil and replacing them with a free- draining soil mix.
The health of trees grown as bonsai depends largely on the care of changing the soil in the pots and the proper pruning of surface roots.
A healthy bonsai puts out new surface roots every year. The growth of these roots makes it difficult for vital water and air to penetrate the soil. The surface roots will be nourished but the main root near the trunk will die. Therefore, periodically cut back the main root and thin out the surface roots.
A tree's rate of growth determines the frequency of repotting. Pines and spruces, for example, need repotting only once every 3 to 5 years; flowering and fruiting trees, every year or--depending on the variety-- every second year. Repot quick-growing species, such as willow and crape myrtle, at least twice a year. These intervals apply to healthy trees that have received proper care.
Repot your plants in the early spring when the first new buds appear. A secondary season occurs in late summer or early autumn when, for a short time, the roots check their growth. It is dangerous to repot in late spring and early summer when the leaves are just open and still tender.
When the tree is in a dormant state it is unable to establish itself in the new soil and root diseases are likely. For this reason, bonsai must never be repotted in winter, except when kept in greenhouse culture.