I don't know of anything you can broadcast that won't also kill the grass. Might have to go the pigweed route... pull them up. I had a bunch in a fence row last year. Sprayed all kinds of stuff on it, from 24D to Roundup to Gramoxone. Everything I used would knock the leaves off, but after a few months they would come back to life. Once winter showed up, you know 80 degrees, we pulled them up. A few tried to come back from old roots, but they were easy to kill with the fence row concoction this spring when very small. Wish I knew of something, because pulling weeds/bushes sucks.