Does anyone know what the name of a weed that looks something like a fern and grows very fast?

Question

I recently cut down a tree in my front yard. Since that time, I get these very tall and fast growing weeds. They look something like a fern or black locust leaf. The stalks are very hard. I can mow them down, but they keep coming back. They get pretty tall. Lots of little leaves very close together.

Answer 1

they may be sprouts from the stump

Answer 2

Depending on the tree you cut down, they could be root suckers or maybe bracken fern that's been released by removing the shade. If the new growth comes up in fiddleheads, they're ferns.
If they look like black locust it could be a weedy member of the pea family like a vetch or pea, kudzu or hog peanut (assuming you're in the southeast since you know locust).

Answer 3

They're called suckers...little shoots that grow from the roots of the tree. They call them suckers because, instead of growing with roots of their own, they get their nourishment from the root of the tree and if allowed to grow into a full size tree, they are much weaker, more susceptible to disease, and more likely to fall in a high wind because what is left of the mother tree will die out. Plus they can develop freakish defects.