Excellent field work @PeterNC and @billm nice results on that maple. Love the bark. I need to airlayer a lot of my field grown trees and am workign to try and figure out how to prevent that. I’m hoping the ground growing bags allow me to dig up easier and root prune more frequently (maybe every 3 years?) to prevent the need for airlayer.
@Tiibee - The best time to dig is simliar to the best time to repot. So variety depenent, but spring right before or as buds begin to swell has given me a very high success rate (except the two times I was an idiot and washed the roots of 10 year old pines and killed them… whooops).
Even if some leaves have pushed I’ve had success. The biggest concern, like most early repots, is a late frost or freeze. So protect like you would any repot and you should be good to go.
I’m my experience (with ground growing maples at least) if you want the radial root spread the best thing to do is air-layer before you put it in the ground. The more roots you have at the start the less likely you will get dominant roots (this may also be species specific?).
I also screw a board under the trees and arrange the roots with nails before planting
These are my seedlings that haven’t been air-layered:
And these were airlayered before being put in the ground:
Love those roots. Legit amazing. I’m doing this exactly now. I have some I’ve been growing radial roots in a pot for two years then putting into the ground.
That’s them coming OUT of the ground, right?
How many years were they in the ground?
Do you have pictures of them going in the ground (size of trunk and/or root pictures would be awesome).
yes I have pictures from when I airlayed them if you don’t mind me posting them on your thread?
I took 6 airlayers off this Japanese Maple in April 2015:
They where planed straight into terracotta pots without disturbing the root much at all. Wires attached directly to the pot and trunk (4 points)
I then repotted them again in March 2016 once buds started to move. I cut the roots back, Cut the stub right back to the sheen and got rid of as much sphagnum moss around the roots as I could. I then screwed a board to the bottom of the tree, Untangled and combed the roots out flat (no nails used at this stage). Then back into the terracotta pot (bigger pot if need be). Tip: at this stage, make sure you screw the board to the tree at the angle you want the tree to come out of the soil
March 2017 When I repotted this time around I arranged the roots with nails and planted it in the ground:
April 2019 I dug them back up to see their progress. I cut back and re-arranged roots again with nails and planted them back in the ground. I wont dig them back up for another 3,4 or 5 years I think.
before cutback and arranging:
After
back in the ground:
Looks like some well thought out, carefully crafted work @Drew. I’m sure with this attention to detail you will have some fine trees in the fullness of time. 
Be good to see what they look like in 4 or 5 years when I dig them back up
@Drew Are you planning to cut the roots with a spade/shovel every year or two to prevent a few of them from taking off?
I agree the key to getting good roots in the ground is to get good roots in a pot first. I do that, but not the extent you have done. It looks like I need to up my game.
No I don’t think so, cutting the roots every year will defeat the purpose of growing it in the ground I would have thought as it will kill the trees momentum. The vigour of the roots was pretty evenly spread across the whole nebari between 2017 and 2019 so I think it will be ok untouched for a few years as I also want to build the trunk… we’ll see