crabgrass and new soil
Here is a spot of crabgrass sprayed with quinclorac on my lawn about 4 days ago, July 27th. It is OK to reseed right away--if I wanted.
It is best in the northern states to wait to reseed until about mid-August, or when temps come down below 85. Or...about 8 weeks before frost. Cool night temps will block crabgrass germination and early growth in the fall. Crabgrass is not a problem when temps cool off in the fall.
Grass seed sprouts fast In August (if the soil is moist) because the soil temperature is high.