Dormant season or winter planting of grasses and legumes can be nearly as successful as planting during the more conventional time of early spring. Dormant plantings succeed as long as your soil is relatively dry and soil temperature is too cold for seeds to germinate. That's the key — too cold to germinate. When these conditions exist, seed just lies in the soil until favorable conditions for germination occur next spring. Then seeds begin to grow as if they had just been planted. Warm-season grasses, like those used in CRP and range plantings, are especially well-suited to dormant planting. They won't germinate until soil temperature exceeds 45 degrees. Since soils generally remain colder than this for most of the winter, dormant plantings of these grasses can be made anytime between late November and March. In addition, the alternate warming and cooling of the soil in spring stimulates a natural process in these seeds to improve their germination.
Cool-season grasses and legumes, however, can germinate at soil temperatures as low as 35 degrees. Our soils often are warmer than 35 degrees for several days in a row during winter, so cool-season grass seeds occasionally germinate and then die when soils freeze again. As a result, dormant planting of cool-season species are successful a little less often than warm-season grasses. If you want to plant grass but never seem to have enough time or the right soil conditions to do a good job in spring, try dormant planting. It can work for you, too.
Source: Dr. Bruce Anderson, Professor of Agronomy, Agronomy & Horticulture, University of Nebraska - Lincoln, Lincoln, NE