[header] How to repot bonsai trees [title] Determine when your bonsai needs to be re-potted. [step] The primary reason for re-potting a bonsai tree is when its root system begins to choke itself off. To determine whether or not this is happening, gently lift the entire tree from its pot. 
If the roots have begun circling around themselves, you need to re-pot. Eventually the roots will grow thick enough to displace all of the soil within the root system, and the tree will starve.