Most homeowners know enough to fertilize lawns in the spring.
But the farther north of the Mason-Dixon line you live, the more important fall fertilizing becomes. Because it helps the grass establish healthy roots that actually keep on growing during all but the coldest periods. In the fall, you need a different fertilizer than in springtime so make sure to hit your lawn with a mix that has 1 part nitrogen to two parts each of phosphorus and potassium. In September, do a major fertilizing. Then follow it up with a booster shot of the same mix shortly before the real cold weather hits your area.