I am a new gardener and starting from scratch in my yard, but I have a problem. In my front yard, smack in the middle, is what I can only describe as a weed bush. We have had it pulled out twice and both times the hole filled with dirt, and it grows back. It grows extremely fast. If I mow over it, it already has a foot of growth in a week. I am at a loss as to how to get rid of it. We've tried weed killers, including round up. Any suggetions?