Bonsai-care - Repotting
Bonsai need repotting every 2-3 years. Cutting back the rootsystem at the same is vital for a healthy bonsai. It motivates the tree to create a rootsystem compensating the loss of its large natural roots. If you do not buy your bonsai from a knowledgable dealership repot it as soon as possible. Cheap Bonsai from China usually come in poor soil that does not suit conditions for a happy tree in other parts of the world. Consider buying soil (e.g. Akadama) from a specialst or mix your own. It must drain well. A lot of Bonsai die because the soil is always wet. As small tree still is a tree, not a water plant (neither a cactus)!
Indoor bonsai can be repotted all year round, outdoor bonsai preferably during springtime.