Fall is the best time to plant grass seed because the temperatures are still warm enough for the seeds to germinate, yet the cool nighttime air spurs the grass to grow dense roots. Most experts recommend seeding new lawns or over seeding existing lawns within two weeks of Labor Day. Seeding in the fall can make a lawn up to 49 percent thicker in the coming spring.
In the fall, the roots of grass plants begin storing nutrients for use in the spring. While the grass is preparing for the winter dormancy period, a light application of fertilizer can help ensure the grass has adequate energy reserves to grow quickly when spring arrives. Some landscapers recommend fertilizing the lawn at the same time that the over seeding takes place and again in late November. The falling leaves of autumn also serve to nourish lawns; mulch the leaves on the lawn rather than raking them.
Another reason that fall is the best season for seeding grass, is that it requires less water. In the heat of the summer, the soil tends to dry out very quickly, requiring excessive watering. If necessary, grass seed can be planted in the spring, but it is not as effective a time as the fall.