It being nearly 4 years later, I have no idea if you found your answer or if you are even still on this forum. I found the post looking for an answer to what a tree that someone asked me about that has similar leaves to a Mimosa but different flowers is. Turns out I believe it is an Acacia Tree. I think what you have is a Honey Locust due to the thorns. I may be wrong on both accounts, but I don't think it is overgrown gripe weed.
HI Dan and welcome! Been such a long time. That plant's been dead for a few years now. I left it outside and the frost killed it. It was more of the climbing habbit, not a tree. Even in a pot and in the apt it was climbing (not like a tree when it doesn't have light). The stems had fine and very many thorns. But very small and turned like a hook. I still don't know what I HAD! WELCOME once again.
Sorry, Where I live there are a number of small trees and shrubs that have that kind of a leaf pattern. I usually identify the mimosa by its flowers which you do not have here. I do have a similar small tree with the thorns that you are showing but never have learned the name of it.
