Japanese Maple – Swollen Graft Union
Q: I have this really cool Japanese maple but the trunk is very “swollen”. Is this something I should be worried about?
A: As you probably know, most dwarf Japanese maples a grafted onto another vigorous, but not so pretty, Japanese maple rootstock.
You are seeing the graft union. Normally the understock is not as swollen as yours is. As long as the upper part of the tree looks healthy, I would not worry about it.
Since the sizes of the two portions of the trunk are so markedly different, do anything necessary to avoid unbalancing the tree. I think it would be more likely to crack or break off at this mis-matched graft union.