How often to prune the root

The goal is to keep the bonsai trees small and approximately the same size always, since they are “finished.” This means that their roots should be pruned and replanted back into the same pot with new substrate periodically. Of course, another important aspect of bonsai is to create them from material collected or nursery. In this case, the roots should be substantially reduced, as part of the training process to obtain them in desired final bonsai pot.
The frequency of root pruning is related to the species, the size of the container and the environment. The limit is that a bonsai is the root-pruned and replanted when he shows symptoms of decline and / or chlorosis, or as he begins to push out of the pot.
Some species are simply slow-growing and will require root pruning and transplanting with less frequency. Other species grow at a phenomenal rate and may require root pruning and transplanting more than once per year. Many flowering species are very vigorous and require annual transplant to maintain the vigor with profuse flowers and bear fruit. These include apple, plum and cherry.
In small containers, most plants will colonize the pot within a single growing season. This is especially true of mame (very small bonsai). Almost every nurse should be transplanted every year. Shohin should be transplanted every one to two years depending on species and growing conditions.
Large specimens can be pruned (root) and transplanted every two to ten years depending on the species. Many large pines are comfortable with five to 10 years without root pruning. Cedar and spruce can be treated similarly.
Plants grown under non-ideal conditions, of course, grow more slowly and need root pruning and transplanting with less frequency.