Lawn height increases in the summertime, just like the temperature. The lawn’s soil is shaded by higher grass covering it, which helps ward off bothersome weeds like crabgrass and shields the lawn’s roots from the intense heat. For tall fescue and perennial ryegrass, you can mow your cool season grass up to 4 inches high in the summer. Warm season grasses should have a 2 to 2.5 inch rounded end.