It looks like a mimosa. The reason I am posting is because we had 2 mature Mimosas that we cut down 20 years ago. I am still getting babies even after all this time. They were especially prolific in the beginning. Hope someone else will post. Carol

could be coming from a root still in the ground...they are impossible to get rid of.

Thanks for firming up my initial impression. Looked into Albizia julibrissin seeds after hearing of Caroltropicals' experience and found they're known to remain viable in dry storage for nearly 150 years.... so imagine a mere 10 or 20 in well draining soil is not unreasonable. The spot is in a corner of a long alley that has always collected a lot of windblown tree debris, such as leaves...... or seedpods. Am guessing I disturbed a seedbank when I removed the mulch and turned the soil in a multi-season effort to eradicate running bamboo that finally found its way in from the adjacent yard.

Thanks again for the inputs.

Yes, disturbance can bring all kinds of seeds to the surface. Many are programmed to wait for just such an opportunity. Sorry about the bamboo. That's one I hope I never have occasion to battle.