The best time to seed a lawn is in the fall or spring, depending on the variety of grass. During those times of the year, the temperatures are mild and grass seed has the best chance of getting a healthy start before freezing winter temperatures set in. However, if you live in a warm climate or are in a hurry, grass can potentially be planted at any time of year when the soil is warm and any weeds on the lawn have been removed.
Late Summer/Early Fall
Mid-August through mid-September is the best time of year to plant cool-season grass seed. Mild temperatures make it easy to keep grass seed moist and grass seedlings do not have to struggle in the hot summer sun. And, since this is the dormant period for most weeds and lawn diseases, your grass seed will have a chance to establish itself before they crop up again in summer. For even more optimal conditions, measure the soil's temperature and only plant grass seed when it falls between 45 and 55 degrees Fahrenheit.
Spring is the best time to plant warm season grass, and only a slightly less optimal time to plant for cool season grass. When planting grass in spring, wait until the soil is between 55 and 60 degrees Fahrenheit. Also make sure to remove any weeds and their root systems before seeding the lawn. Germinating grass seed cannot compete with weeds and you can't spread weed killer or tread on the lawn to hand weed without severely damaging or killing the new lawn.
After You Have Prepared the Soil
While the time of year is an important guideline, the best time to seed a lawn is after the soil has been properly prepared. Before laying the seed, remove any dead grass or other debris from the lawn, loosen the soil, apply a layer of quality top soil and a good starter fertilizer.