
Thanks rainbowgardener, and everyone else! I think it's a match for staghorn sumac.
Wikipedia basically says it grows quite aggressively, both by seeds and rhizomes. (That explains why it's growing everywhere.) The flower/fruit is in a big clump which is probably why I never noticed it. (I mentioned that I'm lazy and not very observant

.) The fruit also has a lot of practical purposes, so I think someone in the neighborhood must be purposely growing it, and it wandered into my yard.
Well, it's annoying but not poisonous. Guess I'll just have to keep pulling them.