The best time for Illinois residents to plant grass seed is between Aug. 15 and the end of September, according to the University of Illinois Extension website.
Spring is the next best time to plant new grass seed. Grass seed planted in the spring experiences more competition from weeds than that planted in the fall, however.
Spring and fall grass seed planting provide moderate temperatures to avoid heat stress while still offering warm soil and enough moisture for germination to occur. In either case, grass develops a strong root system before having to deal with any temperature extremes.
Plant grass seed in late November for spring germination, a process known as dormant seeding. This option is higher-risk than spring or fall planting because seed may be blown or washed away depending on weather conditions. Ideally, snow covers the seed and keeps it in place until the arrival of warmer spring weather.