Today there is a thread on BonsaiTalk about when is the best time to chop a Japanese maple. There are lots of observations and opinions and most of them are right! HERE is the thread, but I have reproduced my response below.
Wow, lots of opinions and observations here. I may be able to help make some sense of the seemingly conflicting information. First, let me state that just about everyone is right, but what you are failing to recognize is the full context in which pruning, chopping, and repotting is taking place.
First, Jason is spot on. These guys at Oregon bonsai have dug hundreds of huge trees, pruned them back and got the root balls so small on first digging that they fit perfectly in a 16x 16 x 4 inch Anderson mesh pot. They hardly ever lose one. I bought 20 of them two years ago, had them delivered in early March, potted them up immediately and every single one lived, 100% survival rate, even moving from the greenhouse climate of Oregon to the semi arid oven of summer in N. California. These trees have four to six caliper inch trunks. We are talking serious maples. However, these were dwarf trees, so even with all this pruning back, there were still plenty of existing buds to open as soon as it got warm. Not one of them was a naked trunk.
Does this mean that you can treat potted maples like this and expect the same kind of success? No. Healthy, vigorous growing trees in the ground have gigantically thick vascular systems compared to pot grown trees and a tremendous amount of carbohydrates stored in both above and below ground tissue. So, even with all this wood removed, there is still plenty of stored energy for extensive growth in the spring.
Disease. Japanese maples maples are inordinately susceptible to a whole host of water mold and fungal diseases, and the nastiest is verticillium wilt. These water molds and other fungi and bacterial diseases are most active in spring. I would never chop or prune a maple as the buds are swelling, or after since in my area that is just asking for trouble. Most of these diseases can enter the pruning wounds. Now, you may not have these pathogens in your area due to your climate, and if not, then no problem. But if they are present and you get a lot of cold wet spring weather, you may be giving them the kiss of death. I think this is why we are getting conflicting information, but in general, I think it is BETTER to avoid chopping and pruning once the sap begins moving and you getting close to bud break, just to be on the safe side. Some of these water molds are spread by rain splash on soil, so if the tree is in the ground and you prune in spring, it is an open invitation to infect the wound. Plants in pots and on benches would not be affected as much, especially in the kind of soils we use.
Late spring pruning. For potted maples, I prefer to prune hard or even trunk chop in early May after we have begun to get warm dry weather and the first set of leaves have hardened off. Maples respond very quickly to both top and bottom pruning at that time. So you can both root prune, top prune and repot at the same time, and expect a quick recovery by mid summer. I did this last spring on about 200 two gallon size cutting grown Japanese maples and losses were very low. I think there were less than 5 that didn't make it, and these trees were in pretty bad shape to start, having been neglected for a number of years. Now, in general, I try not to chop back to a naked stump when doing this operation. If you leave even one branch, the chances of success are much higher because recovery starts immediately. This occurs because you don't remove ALL of the photosynthetic food making ability of the tree. Leaving a branch and some leaves greatly increases your survival rate. Now this May, I will go back and chop them again since there is a lot more lower branches and foliage that will be left after chopping, the trees are stronger, and hitting them again will encourage even more bud back. There is no need to do everything at once. If you can't wait a year, you are probably in the wrong hobby. I will not repot them since they will not be anywhere near rootbound and leaving the roots intact will fuel explosvie growth after pruning.
Fall and early winter pruning should be just fine, although I don't do it then. I had hoped to do some this year but didn't get around it. however, I thought this through, and probably the best time from a disease and physiological standpoint would be to prune a couple weeks after leaf drop. Earlier than this, you may have fungal activity at the elevated temperatures and the tissues are loaded with sap and sugars. Once full dormancy sets in, but before really cold temperatures hit, there should be enough activity to quickly wall off any pruning cuts and disease activity will be nil. Later than this, the cuts probably won't wall off as quickly, but fungal activity would still be low unless you are in a mild climate (zone 9). Fall repotting this late is NOT recommended since there is insufficient time for recovery, and any new roots will just freeze and weaken the plant.
So, you see, you just can't make blanket statements in a vacuum and expect them to be universal generalizations. Life just isn't that simple.