I would suggest posting this on the Name That Plant Forum if no one answers here. Your description makes it sound like a tree sucker, but not a species I'm familiar with.

Here is a link that might be useful: Name That Plant

Have you had any trees removed withn the last year? That looks very much like some suckers we have coming up-even though we had the stump ground. Guess they didn't do such a good job. As much as I avoid using chemicals, I am going to spray mine. I tried digging, but can't seem to get all.

It looks like a root sucker from an Ampelosis vine.