Fall vs. Spring JM Root Pruning Report
So last fall I repotted several large maples. Their roots were essentially braided into knots, they were so rootbound.
I had particularly good luck, which surprised me. I did accidently over-prune a lovely 3' Pink Lace, and I was correct that it wouldn't make it through the (mild) winter. I was sad. I also lost a lovely Hogyoku & Skeeter's Broom.
However, a 6' Karasugawa, a Green Lace, Oregon Sunset, Green Mist, Vitifolium, Red Crusader, Lozita, Wilson's Pink, Hupp's Dwarf, Kito No Ito, Nuresagi, Sanzanami, Red Pygmy, Beni schichihenge, Coral Bark (forget it's name) and several smaller maples did beautifully and are happy, some in the ground now, and some still in pots. A small, maybe 1' Krispa shot up at least another foot this spring.
As more heartily recommended, this spring I tried root pruning a very root bound Kashima, which was happy and looking healthy. It IMMEDIATELY died, although I wasn't aggressive. A Dissectum Nigra did just fine and is still putting out new leaves. For me, at 50/50, spring isn't the time. I'll be doing fall root pruning.
Over our very mild winter, an 8 year old Shania died back to the graft, but is putting out new growth above the graft. A Bloodgood is doing the same thing. Probalby heat stress, since they were in the ground for more than a few years, but we've had such a drought.
About 2 weeks ago, though, my happy, in-ground Karasugawa just died. Death ran up a central lead, all the leaves got soft, then death wandered up and down the external branches. I have no idea why. I was very sad. It was one of my very first trees.
I am also here to tell you that many JMs take far more sun than people want to think. Just like my Hellebores that have never had any shade. Can't keep them down!
I hesitate to plant again in the spot where my Karasugawa died, simply because I'm afraid something happened in the soil.
So here we go. I found using the soil recipes here, pine fines, pearlite, gypsum, etc., have given my trees a real boost in growth.
I discovered I was an idiot to plant hosta and/or Heuchera in the maples' pots with them, even though they were beautiful. Try getting those unknotted from JM roots. OMG.
I have also tested quite a few for full sun exposure, which after this summer will be put into the ground where I can keep them watered, but will have limited shade. Sherwood Flame, Aconitifolium (Oh, it's fall colors!), Japanese Spring, Berrima Bridge, Tana, Johin, Chitosaeyama and a few others
I have a lot of maples that have been in pots for several years, which are definitely overgrown. I'll attack those in the fall.
So there you have it!