Fall Is For Planting Grass Seed
Fall is the best time for overseeding. Grass planted in the fall has nine months to grow deep roots and become established before facing summer heat which is very tough on young plants. It is much better able to survive its first year than grass planted in the spring. Also, because in the North many weeds--crabgrass and common chickweed, mayweed, and peppergrass--stop growing and some die out in the fall, they do not compete with the new fall seedlings for space, water and soil nutrients.
Calculate when to sow grass seed in the fall by counting four to six weeks prior to when the first light frost is usually expected in your area. For example, if first frost is usually around October 15th, then September 1st or thereabouts is the best time to sow grass seed. Grass that germinates in early September has roughly three months of growing time before the soil freezes hard in many parts of the North. Its roots develop below the soil surface during this period, even though its foliage has gone dormant.
In The Spring
If you cannot overseed your lawn in the fall, then do so just as early in the spring as the passing winter allows. Try to time it so that you plant before you see leaves on the lilacs. Once lilacs bloom in your area it is getting too late. Any other time during the growing season prior to Labor Day is a poor time to sow grass seed because most varieties take up to 2 weeks to germinate. Only perennial ryegrass germinates quickly enough for seedlings to get a jump on hot weather. You can patch bare spots during the growing season, but your grass choice is limited.
If you are anxious to overseed your lawn without waiting until fall, overseed in the spring with a perennial ryegrass mixture. It has the best chance of surviving the heat of the summer because it germinates quickly. It will renew the color and texture of your existing lawn for the season. Then, if you prefer a lawn of mainly tall fescue or Kentucky bluegrass, plant that in the fall as a second overseeding. The result will be thick, beautiful turf with a variety of grasses.