Choosing the best time to seed your lawn depends on what type of grass you intend to plant and the climate conditions where you live. Generally speaking, the best time of year to seed your new lawn is the early fall for cool-season grasses, and the late spring or early summer for warm-season grasses.
Popular cool-season grasses include rye, Kentucky bluegrass, fine fescue and tall fescue. These grasses are typically planted in the upper half of the country, and can remain green year-round. Early fall is considered the ideal planting time, although a second seeding in the spring may alleviate problem areas that are not completely covered by grass that was planted in the initial fall seeding.
Fall is considered "the perfect time to start a new lawn," in the words of This Old House. Temperatures are beginning to cool, lessening the chances of your seeds drying out, but there's still sufficient sun for your seeds to germinate. It's best to plant seeds in the daytime, when the temperature is between 60 and 75 degrees and the soil temperature is between 50 and 65 degrees. If it's too hot, your seeds may dry out, unless you carefully monitor irrigation; if it's too cold, your seeds may not properly germinate.
Warm-season grasses such as Bermuda, buffalo grass, Bahia grass and centipede grass are typically planted in the southern part of the country. These grasses are best seeded in the late spring or early summer, because the combination of rain and steadily increasing sunshine and temperatures can lead to faster germination and ground coverage.
The ideal time to plant is when daytime temperatures are upwards of 80 degrees, with a soil temperature of about 70 degrees. Warm-season grasses also may be seeded in the fall, but no later than 60 to 90 days before the first frost so that the grass has enough time to mature and survive winter dormancy.
The Seeding Process
The first step in seeding a lawn is to remove all remnants of the old lawn, as well as any lingering rocks and roots. Next, turn the soil with a rotary tiller and mix in sand and compost -- figure on using about three cubic yards for every 1,000 square feet.
Before you plant the seeds, test the pH level of the soil with a soil test kit, which measures the soil for acidity and alkalinity. Grass grows best right in the middle; if your soil has too much acid, a problem common in cooler, wetter climates, add lime; if it is too alkaline, add some peat moss.
Mix in some starter fertilizer and then rake the soil to a level surface; then plant the grass seed using a spreader. Rake again when you have finished planting the seeds, and water thoroughly and often.