Do not cut off too much roots, do not cut off foliage. After care it needs much less than in spring. This is one of the best things about it. The days get shorter, temperatures get down slowly, it rains more, humidity in the air is higher, much more due overnight. Water thoroughly after repotting - everything, also the foliage must be dripping wet. Then water once every day thoroughly, start feeding immediately, tree should not be in full sun all day, rather half shade, sun in the morning and shade in the afternoon is best. You must use modern substrate - absolutely no soil. You must leave the foliage on. Do not cut anything until October. Water every day all over the tree. Only on very hot days water twice. If you don't have much to do you can mist the tree several times a day. Frost free winter protection is a must.
Why does this work so well and better than in spring? Because the tree has perfect conditions now to grow roots until the end of October. This is why now! Earlier is better than later to have more time for root growth. This is eight weeks and sufficient.
Roots only grow now if there is plenty of foliage that creates energy for that. If you have not cut any branches the terminal buds will send signals to the roots with hormones called auxin that signal to grow. If that signal is not there the roots will hesitate.
The tree must get frost free over-wintering. In next spring it will start as if nothing happened.
If you repot in spring there is no foliage yet to help the roots to grow, the tree must do this out of stored energy. The energy was stored for foliage growth and not for root growth and the tree will be weakened. The foliage in spring will grow into an environment which gets worse every day, days longer, temperatures higher, humidity lower every day. The foliage will find it hard to grow well. It will quickly be too much to be supported by the existing roots - especially if you have cut off many. You have to cut short very soon after the shoots have stretched in order to avoid too much foliage mass which cannot be supported by the roots. The tree is weakened all the time by these measurement. It has a tough first summer after repotting. After two years the same again and people wonder why their trees are not doing well,m why thy do not get anywhere and stagnate.
Larry Morton, Modern Bonsai Practice: 501 Principles of Good Bonsai Horticulture.
