I "saved" a couple bucks by switching to a cheaper (but still a popular name name) fertilizer/weed killer this year. Two weeks ago I put it down, and it fertilized beautifully--lawn is nice and green. problem is, unlike in years past, the clover and crab grass did not turn brown and die. I've learned the hard way not to re-fertilize too soon. Can I put down just a lawn-targeted weed killer?