Though best done in the spring, most tropical bonsai can be repotted at any time of the year. Hardy bonsai should be repotted during early spring just as they are breaking dormancy. This is almost a necessity if heavy root pruning will be done at the same time.
First, if the tree is in a normal growing pot (not a bonsai pot) remove as much of the soil as possible. Try to disentangle the roots and fan them out as evenly as possible. The first time a tree is put into an actual bonsai pot a little extra effort will go a long way. After the initial planting, most roots close to the trunk should never have to be drastically moved or changed again.
Close to the trunk, remove any large, fat roots that will not bend into a desirable position. The remaining roots can be cut back by about half. It is a good idea to leave some finer roots, which do the majority of the work of water and nutrient uptake.
Place screens over the drainage holes, these will probably need to be wired in place so they donít slide around. Next, fill the pot about half way with a good, fast draining bonsai soil. Run a piece of heavy wire up through both drainage holes (forming a large U) to be used later to secure the tree in place. Make a little hill of soil in the middle of the pot and place the tree into the pot slightly above the eventual soil level and spread the roots out as evenly as possible. Now the wire that was placed through the drainage holes can be fastened over the root ball and tightened until the tree is secure. Finish up the job by adding more soil and any decorations like rocks or moss.
It is important to thoroughly water in your bonsai with a soft stream of water, a water breaker works wonderful. If it is warm enough your bonsai will recuperate much quicker if placed outside in a sheltered, shady location and checked daily for watering.
For established bonsai (those potted in a bonsai pot), repotting is much easier. Most bonsai need to be repotted or root pruned about every two to three years, but younger trees will benefit from yearly repotting. Basically, the majority of the root mass close to the trunk is left undisturbed. Root pruning is necessary when the tree is pot bound and the roots have begun to circle around the bottom and sides of the pot.
To root prune, simply lift the tree out of the pot, lay it on its side, and saw or cut off the bottom third or so of the root ball. Do the same to the all sides of the root ball. The main purpose of this procedure is to allow the tree to develop a finer root system. It also refreshes the soil and allows for easier watering and all in all a healthier living environment for the roots. Without this procedure most trees will not prosper and some will eventually die.