I actually wrote this in response to another thread about Japanese Maples but I want it to be a stand alone thread because really is THAT important.
Timing timing timing timing timing.
Make sure you are doing the right thing at the right time of the year and make yourself a written schedule of what you need to do and when it needs to be done. Time flies! With that said you have to prioritize everything so as to not miss a window of opportunity to do something.
Regarding root bound plants, in this case we were talking about root bound Japanese Maples. If they are severely root bound you do need to disturb and possibly cut some of the roots. Since you already this they are probably going to be fine. However, when you have to start cutting the roots vertical to break that girdling habit you have to think about how much you are going to shock the plant.
Cutting the roots is akin to digging a plant out of the ground. It can be a devastating thing for the plant. So when that kind of “surgery” needs to be done, I would much rather see it done after the plant has gone dormant. That way you can cut away with confidence that the plant won’t feel a thing.
It’s really like surgery on a human. Serious injury when a human is awake causes severe shock. However, when sedated and put into a deep sleep (dormancy for humans) the same type of injuries (surgery) can be inflicted without the patient going into shock.
And that’s why I always say, timing is everything in this business.
If you have plants in a grow bed that have to come out, dig them in the early spring and get them all dug before they leaf out. Once they put on leaves, it’s game over until mid November.
If you have plants that have rooted through the pot into the ground, move them and trim off those roots in early spring as well. Doing so during the growing season can shock the plant depending on how many and how large the roots are. But I have had plants wilt down because I moved the pot during the growing season and broke a lot of roots in the process.