I have a black walnut establishing itself in a hanging basket pot on top of a 5 gallon bucket so it can get all of the water it wants. Is this a good, or bad idea? Why?
Juglans nigra forms a large tap root:
Young black walnuts are very "carrot" rooted; their entire root system is comprised mainly of one large tap root. There is no root ball, no large fibrous mass of roots.
So you'll likely have best results if you only keep it in that shallow pot the first year. The longer it's kept in shallow soil, the more likely the root will get distorted. That will hinder growth, even if you transplant later.