It is advisable to wait until later in winter or into early spring. It is safer to do root pruning after the late summer spurt of growth on the tree has hardened. Fast growing varieties such as willows or privet may appear to be dormant during the winter months but their root systems continue to grow. Since fall leads into winter dormancy it may be a good time to plan the replenishment of your potting soil inventory . Repotting in the spring can be a time consuming activity and a ready supply of an all purpose bonsai soil will be helpful.
Bonsai soil is equivalent to plant food.
It's quality directly affects the growth and health of the trees. It should have a fresh smell and good color and appearance. A proper mix should have large pore spaces, good drainage, nutrition absorbing capacity and be well ventilated to permit the growth of beneficial bacteria which processes the plant nutrients. Depending upon the species of the trees, a good soil mix should vary from slightly acid to slightly alkaline - with a PH factor from 6.0 to 7.5. This condition is favorable to most growing trees.