I bought some Scott's Halt while at HD(was intending to get the Lesco's w/Potash but I have a Koi pond and the label advised against using it near fish). Anyway, when I bought it the weather was dry but since it's rained 2 days straight. How soon after the rain can I put down weed killer/Pre-em? The label says the grass should be dry, but remains effective if it storms after app. If this is the case why does it have to be dry when you first apply it? Just another n00b question. Thanks for your patience.