They say to apply during a rainy day or cloudy. I'd say the best time would be to apply after a core aeration. I would think it would be easier for them to get into the ground faster, but hey its just a theory. Next year I"m gona be offering much more nematode release's compared to this year. This year it seems that I have absolutly no grubs at all. Kinda weird I think but then again its not the end of august or begining of september so we'll see.