Repotting

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.
Pre-bonsai material are often placed in "growing boxes" which are made from scraps of fenceboard or wood slats. These large boxes allow the roots to grow more freely and increase the vigor of the tree. The second stage, after using a grow box, has been to replant the tree in a "training box;" this is often smaller and helps to create a smaller dense root mass which can be more easily moved into a final presentation pot.