It won't be that effective this early, as other people have said. It'll kill annual weeds and grasses, but the tougher stuff will survive. You need to get cracking once it dies back, and either dig it or put down black plastic. I've seen plots which were sprayed and then left several times; the perennial weeds pull through, and without the competition, they flourish as never before. The last state of those plots was worse than the first.