Thoughts on how to train this red maple currently in the ground for a future in its bonsai container ? How long should it stay in the ground and how do I prune?
Leave it in the ground until you’re happy with the size of the trunk. Just let it grow.
Let it grow until you get close to your desired trunk girth.
If the branches are at least 1 foot long, or if you have about 6-10 nodes:
You can prune a node to two (after they have hardened off) from the tip of each branch to encourage more growth.
You can also defoliate the interior foliage if they’re not catching enough sun.
Forgot to mention that you can plant it in a shallow wooden box to develop the nebari or plant in the ground on a tile or wood plank or something. I like using wine barrels, shallow planting. Easier to cover and protect.
Bare root the maple, prune big roots and roots growing inwards, leave ample of feeder roots, spread the roots in a radial manner. You’ll have to tie it down, you can use silicone tubing or wrap the portion of the wire touching the roots with a tubing (aquarium).
If you’re using a wooden plank (dont use treated wood, 3/4 inch cedar about the same size as the canopy), drill 1/4 inch holes all over the plank, about 2-4 inches apart. Layer the planting site ,just under the plank, with 4-6 inches of gravel.
The roots will grow laterally which will develop the nebari.
Make sure your maple’s got a lot of foliage. Do this late winter to early spring, cover the tree with shade cloth or anything to protect it from frost. Cover the root area with sphagnum moss also. I put a big plastic container on mine overnight, then take it off before leaving for work.just until after the last frost.