Thnx for the many answers!
@0soyoung I haven't had time yet to read the entire experiment yet, but following your conclusions: August/September repotting would also be possible, but a spring repot is still the safer choice?
I was also under the impression when you repot during spring the roots have more time to develop and prepare for winter/frosts?
I have continued to repot half of these experiment zelkovas in spring and half in Aug/Sep and have no sense of one time is inherently any better than the other. The obvious risk repotting broadleafed trees in summer is desiccation. I indeed saw some persistent loss of turgidity
repotting eastern redbuds in Aug/Sep. However, I did not see any sort of problem with repotting waxy broadleaf species like zelkova and
cork oaks in Aug/Sep. Azaleas, rhododendrons, pyracanthas, quince, hollies (ilex), and roses can certainly be added to this list of 'waxy leafed angiosperms' that can be repotted after the summer solstice.
With the passage of time, I've become convinced that repotting/digging after the summer solstice is safer than spring with many species, particularly flowering ones. I killed a lot of quince trying to repot them after flowering. Repotting before flowering is safer, but it is just too easy to knock off the flower buds! Life is EZPZ repotting them after the summer solstice. But, I am just an amateur gardening hack trying to make bonsai. I've got more than 100 trees in various states of 'development'. I am motivated to spread out the workload and not be overwhelmed by 'repotting season'. I habitually repot my maples in spring 'as buds swell, but it is not a hard rule for me. I tend to repot conifers, azaleas, quince in Aug/Sep. But I also use spring verus Aug/Sep interchangeably. Sometimes I find a pot for a tree and just don't want to wait until spring (or Aug/Sep) - I put it in that new pot in Aug/Sep. (or spring) for instance. I've even gone so far as to half bare root a JBP in spring and again in Aug/Sep (and vice versa) which saved me a year getting them out of nursery soil. I've done likewise with a tsuga Canadensis.
As to winter hardening, this occurs because of the pattern of deepening overnight frosts and daily thaws that are characteristic of fall. Trees reduce the water in living tissues and sugar up to resist freezing. We're also told that roots continue to grow in winter as long as their temperature is above 40F/5C. So, it is not clear to me why newly pruned roots would be any more cold sensitive than unpruned ones, but there are abundant anecdotes from commercial growers about them being more vulnerable to sudden hard freezes - they cite higher survival rates for roots that hadn't been disturbed about 6 weeks or more earlier. It is an interesting thing, but I live in a climate very similar to yours and have no means to investigate it. Were I living in the mountains of Colorado (e.g., Steamboat Springs where the first frost can occur in Aug/Sep), for example, I very well would.