The types of turf grasses best suited to Illinois, where temperatures can get hot in summer and quite cold in winter, are called cool-season grasses. These grasses are adapted to cool climates and concentrate most of their growth in spring and fall, when temperatures are low and frequent rains provide them with plenty of moisture. When temperatures climb and rains are less frequent in the middle of summer, cool-season grasses become dormant.
Commonly used cool-season turf grasses include Kentucky bluegrass (Poa pratensis) and tall fescue (Festuca arundinacea), which are both hardy in U.S. Department of Agriculture plant hardiness zones 2 through 7. Hardiness zones in Illinois range from zone 7a in the extreme southern tip of the state to USDA zone 5a in the state's northwest corner. Most of the state's southern half falls into USDA zones 6a and 6b, and most of its northern half falls into USDA zone 5b. Thanks to its location on the Lake Michigan shore, the Chicago area falls into USDA zone 6a.
The best time to plant grass seed in Illinois is late in summer, after temperatures have peaked and the relatively wet fall season is about to begin. Seeding at that time reduces drought and heat stress that can kill grass seedlings, but it also gives the young plants enough time to develop adequate roots systems to get them through winter.
In northern Illinois, including the Chicago area, the ideal planting time is between the middle of August and the middle of September. In southern Illinois, any time in the month of September is a good time to seed.
Spring and Early-Summer Planting
Lawns seeded in spring or early summer have less chance of succeeding than those seeded in late summer. Not only will new seedlings have to endure the worst of the summer heat and drought, they will face competition from weeds that germinate in the middle of summer. If you have no choice but to seed in spring, then do it as early as possible, generally no later than April.
When late-summer seeding isn't possible, another option is winter seeding, sometimes called dormant seeding. In this method, grass seed is sown between early winter and very early spring, when the soil is too cold to trigger seed germination. The seed remains dormant just until temperatures begin to climb in spring, giving grass seedlings the best possible head start on the growing season.
In Illinois, the window for winter seeding lies between late November and March. The technique is tricky, however, because mid-winter warm periods, particularly in the southern part of the state, can cause seed to germinate, and a subsequent return to cold temperatures may kill the seedlings that developed.