In most areas of the United States where winter temperatures occasionally dip to or below freezing, the predominant grasses grown for lawns are cool-season types, such as Kentucky bluegrass, fescue and perennial ryegrass. Relatively frost-free areas in the far south use warm season grasses, such as St. Augustine, Bermuda and zoysia.
Cool season varieties of grass seed planted in early to mid spring can take advantage of spring rains and a month or more of cool weather in which to grow a strong root system before the heat of summer arrives. Spring-planted grass that is sown too early while the soil is still wet will most likely not germinate and will instead rot in the ground.
Warm season grasses germinate and grow best in the hot weather of early summer. Sow them during June and July but supply ample moisture to the germinating seeds and young seedlings. Ornamental grass varieties are also warm-season grasses and do well when started from seed in very late spring after the weather has warmed up.
The best time to plant cool-season grass seed is early to mid autumn. The soil is still warm from summer and there are less weeds to compete with young grass plants. Plant in September in the far northern areas and as late as October in transition zones across the mid section of the United States. The above ground portion of grass plants will stop growing after exposure to about 10 days of temperatures below 50 degrees F. The root system will continue to grow and develop until the ground freezes, even after the top growth has gone dormant for the winter, according to Cornell University Extension.