By understanding the characteristics of these perennial grasses, you can choose the best warm-season grass seed to meet your lawn maintenance and enjoyment goals. Warm Season Grasses in Winter. Since most warm-season grasses go dormant and turn brown over the winter, some people choose to overseed with annual or perennial ryegrass. Ryegrass is a cool-season grass that will stay green throughout the cool winter months in the south while the warm-season grasses turn brown. Most warm season grass goes dormant and turns brown with cool temperatures. Warm season grass should be planted in late spring.