New root base

I have a Hackberry that has a ugly nabari and want to grow a new root base above it. I heard about a technique that is used and want to know if anyone has tried it. You poke a series of hole around the base of the tree, apply rooting hormone and cover in spagnum moss. Keep most and new roots will grow. Then you cut off the old base. Anyone thats knows about this, please help.

That's called "air layering." Details on how to do it can be googled. It's a fairly common procedure and kind of a crap shoot as to how much improvement you will see -- and it does take several years before the roots you get will be large enough to be shown at the base of your tree.

It's always better to buy (or dig) plants with existing decent bases.