Repotting is one of the most important tasks involved in the successful cultivation of bonsai. It is absolutely vital to the trees survival. Trees growing naturally in the open ground can spread their roots as far as they need to reach water, obtain nutrients from the soil and get the oxygen they need for completing photosynthesis. Once you place a tree in a pot, be it a bonsai or a patio plant, it becomes dependent upon you to supply those needs. The roots will keep growing, it’s what roots do, but in a pot there is restricted room so you must make room by trimming the roots and repotting, making room for them to continue growing.
Unfortunately, it is one of the least studied and understood, and therefore poorly done exercise in many peoples’ bonsai care. It is not merely a matter of placing a tree in a new mix in a different pot. Repotting is done for the long-term health and well-being of the tree. The more root the tree has, the faster the tree can grow and the better it will develop.