In areas with mild to moderate winters, cool-season grasses grow best in spring, fall and winter. They grow poorly with a straggly appearance and are subject to disease in areas with summer temperatures above 80 degrees Fahrenheit. You should not fertilize them in hot summer months. Cool-season grasses include Kentucky bluegrass (Poa pratensis), tall fescue (Festuca arundinacea), red fescue (Festuca rubra) and perennial ryegrass (Lolium perenne). Cool-season grasses will not grow in areas warmer than USDA zone 7.