This is best done in the dormant season when the plant is not growing. So, repot spring-flowering trees in autumn, deciduous trees in autumn or early spring, and conifers any time except midsummer and mid-winter.

Young trees, obviously, need repotting more frequently than old ones as they grow more quickly. To see if a tree needs repotting look at the bottom drainage holes, if more than two or three roots are poking out then it needs repotting. (If, on the other hand, none are visible some time after repotting check the plant, it is a sign that the roots are unhealthy.)

Let the soil dry out before repotting to make it easier to remove. Loosen the soil from around the roots, then repot with dry soil as this will not clog and prevent air circulating around the roots. Water well, and then leave the tree in a protected place - in the garden if there is no danger of frost for a few days to recover and settle in.

In areas where winters are severe, bonsai need protection not from the cold so much as from high winds that cause the plants to die from parching. Where winters are mild, routine care and watering are all that are necessary.

In extremely cold areas, the bonsai can be wintered in the ground. The pots are dug into the soil up to their rims, mulch is piled about half-way up the trunk, and a packing case is placed over the entire plant. This solution is particularly
helpful to owners of only a few trees.