Pruning of the roots is best done at the time of re-potting. Usually early spring but can be slightly later if new shoots haven’tbegun to develop.
The idea for root pruning is to cut the roots back to allow a fresh supply of soil between the pot and the root. Root pruning is also to encourage the finer, feeder roots; It is these feeder roots that more easily absorb the nutrients from the growing medium.
If a tree never gets root pruned it will become pot-bound and this is never a good situation for any plant. There will not be enough space in the pot for a sufficient amount of soil to keep the plant alive.
How to Root Prune
Lift the bonsai from the pot and shake out the old soil. You can use a blunt stick to remove soil from in between the roots so that the roots don’t get damaged. As your collection grows you may wish to invest in your hobby and buy a root rake or root hook.
Cut the longer, thicker roots first using sharp bonsai shears. Cut these roots further back than the thinner roots. Then cut the thinner roots and finally the finer roots can be trimmed.
Cutting the thicker roots further back gives more space for the finer roots which is what we want for a healthy bonsai.
The roots should be sprayed with water after root pruning is finished so that they don’t dry out to quickly while you prepare the pot ready for replanting.