Dormant overseeding is usually done during the winter (December through February) when it is too cold for germination to take place. Spring seedings done in March can be just as successful as dormant seeding, but spring rains may delay plantings. As with any seeding program, it is vital that good seed-soil contact is achieved. There are several methods that are commonly used in dormant seeding.
One method is to seed when there has been a light snowfall of up to an inch over unfrozen soil. This is shallow enough that bare spots can still be seen. Spread seed by hand on areas that need thickening up. As the snow melts it brings the seed into good contact with the soil where it will germinate in the spring.
Another method is dependent on the surface of the soil being moist followed by some freezing weather. As moist soil freezes and thaws, small pockets are formed on the wet, bare soil which are perfect for catching and holding seed. As the soil dries, the pockets collapse and cover the seed. Slit seeding can also be used as long as the soil is unfrozen.
With any of the above methods, seed germinates in the spring as early as possible. There will be limitations on what herbicides can be used for weed control. Tupersan (siduron) can be used as a crabgrass preventer on new seedings even before they have come up. Also dithiopyr, found in Hi-Yield Turf and Ornamental Weed & Grass Stopper and Bonide Crabgrass & Weed Preventer, can be used on tall fescue, Kentucky bluegrass, and perennial ryegrass two weeks after germination. Dithiopyr is longer lasting and more effective than siduron. Other preemergence herbicides available to homeowners require that the turf be well established before application. (Ward Upham)