Repotting is generally done in early spring; when the tree is still in dormancy.   This way the somewhat damaging effect of repotting on a tree is reduced to a minimum, the tree does not yet have to sustain a full-growth of foliage.   Repotting in early spring will also ensure that damage done to the root system will be repaired rapidly, when the tree starts the new season’s growth.

Choosing the best time to re-pot your bonsai trees, this is a fairly simple question to answer.   With a few exceptions it is best to re-pot just as the buds of deciduous trees begin to open.   Evergreens should be worked just as the buds begin to unfurl in spring. I have found that, in the U.K, most folk re-pot too early (end of January through February).    As a result the growth rate of the tree is seriously impaired and can take a considerable time to recover.   Potting later can dramatically improve recovery time.

This will depend upon the size of pot the tree is in, and tree species.      Fast growing trees need to be repotted every two years (sometimes even every year), while older, more mature trees need to be repotted every 3 to 5 years.     Do not repot just as a routine activity, instead check on your trees in the early spring by carefully removing the tree from its pot and looking at the root system; with experience you will learn to recognise the signs without even taking it from its pot.

A bonsai needs to be repotted when the roots circle around the root system forcing the tree to seek more space by pushing it upwards and outwards, it is not unknown for strong root growth to crack or break a pot.    When or if the roots are still contained in soil wait another year before checking again.