The best time to seed cool-season grasses such as tall fescue and Kentucky bluegrass is September because the turf has more time to mature before spring crabgrass germination and the heat stress of summer. Dormant seeding of turfgrass is sometimes used to help fill in bare spots of lawns that weren’t overseeded in the fall. Dormant overseeding is done during the winter (December – February) when it is much too cold for germination.