Warm season grasses grow primarily in the South and turn brown in the fall after the first frost. The best time to fertilize warm season grass (such as Bermuda, Carpetgrass and St. Augustine) is summer.
Cool season grasses, which include bluegrass, ryegrass and fine fescue, grow in the northern parts of the U.S., and stay green year-round. For optimum health, these grasses need fertilization in early spring and possibly twice in the fall — once in mid-September with a high nitrogen fertilizer to strengthen blade growth and again in November with a high phosphorus fertilizer to promote root growth.