You can put the preen down anytime but you want all the weeds gone when you do it. If you rake hoe or till the ground after the preen it doesn't work anymore?? read instructions. You can see the ground in my garden and we haven't weeded since first day. Around here if you want Sweet corn you go see a farmer they will give you sweet corn so you don't grow it. They grow Hybred seed corn around here and they don't want to risk cross pollination.