Because the best time to sow cool weather grass seed is the fall, the time to renovate a lawn in the North and those areas of the South where it is possible to grow cool weather grasses is just around Labor Day. Varieties of Kentucky bluegrass and tall fescue germinate slowly and seedlings need several months to establish strong roots before facing summer heat. The trick is to time the renovation project so that young grass seedlings can develop fairly good root systems before hard frost arrives in your area.