That's true, and the same holds for mulch. But once things start sprouting, they need the old growth cleared away or it will choke them. If they grow up through the tangle, then you can't trim the old without cutting into the new. 0_o

My prairie garden only gets cut in strips or patches, so it's kind of a thicket of weedy wildflowers. The wildflower garden and the others, I try to clean off in late winter or early spring, so they grow better.