For cool and most warm-season grasses, fall is the absolute best time of the year to fertilize your lawn. In fact, if you were given the option of applying fertilizer only once a year to a lawn, the best choice would be a fall application. A fall fertilizer promotes increased root growth and results in a healthier, thicker lawn the following spring and no need to apply an early spring fertilizer to a lawn that would normally have excellent growth without additional stimulation from additional fertilization.
A late summer or early fall lawn fertilization helps replenish the nutrient supply that was used up over the summer months. It also gives lawn roots a healthy start for winter dormancy. For warm season grasses such as Bermuda, Bahia and centipede grasses, a fall feeding is recommended, but not for St. Augustine grass.
Caution: Do not apply any type of fertilizer to frozen soil, as it is highly susceptible to runoff and pollutes water in lakes and streams. Nor do you want to apply fertilizer to water logged soils. This not only wastes your money, but it causes environmental damage that is difficult to correct.
Fall fertilization is applied when shoot growth slows and the necessity for lawn mowing lengthens from weekly to an every other week. Cooler temperatures, shorter days, and high light intensity, means that fertilizer applied at this time aids the photosynthetic production of carbohydrates. These carbohydrates are stored for use the following growing season, providing earlier spring green-up and an energy source for turf grasses to recuperate from winter stresses.
Disease and weed problems are usually less severe when fall and late fall fertilization are practiced. Heat and drought tolerance are usually better, thus enhancing summer lawn quality. Finally, the grass plant produces more root mass and a deeper root system, resulting in an overall healthier plant.
It is important that late season fertilization not be confused with dormant and/or winter fertilization. The latter method implies that fertilizer applications are made after the turf has lost most or all of its green color and is not actively growing. This differs notably from the late season concept, which requires that fertilizer be applied before the turf loses its green color in the late fall.