That bottom growth is a sign that the maple is not too healthy. Stems die, that's not a big deal. Bark splits naturally.

Here is a link that might be useful: Propagating Perennials

Would you please enlighten me regarding your statement? When I first started growing JMs, I pruned the bottom growth, that is until I read that the bottom growth adds to the beauty of the tree. Did you mean the "appearance" of my Maple leads you to believe it's not healthy?

In my pix this is "new" growth and the leaves are just now unfurling.

JM are fussy that if they get too much sun, they only need a half a day, the bark could split as well as cause other problems. All sucker growth should be removed because it takes energy away from the top the of the plant, the rootstock will take over, and yes the plant can be root bound, but only you will know, but i'd wait until the fall. I always selectively prune JM, crossing branches, etc. and not all buds form but because JM are generally smaller in size they are more noticeable. You should also fertilize in the fall slighly. I noticed yesterday on my grafts the flower heads which most people hardly never notice.

Here is a link that might be useful: Propagating Perennials