Many homeowners believe that the best time to fertilize lawns is in spring — when their warm-season grass begins to green up or when their cool-season grass starts actively growing again — and then throughout summer. And, that’s true — for warm-season grasses. However, the best time to fertilize a cool-season lawn, such as fescue or bluegrass, is in fall.
During the heat and humidity of summer, cool-season grasses often enter a period of dormancy, where growth slows considerably. When temperatures start to cool down in fall, cool-season grasses resume active growth, and they typically need fertilizer to rebuild their strength before winter arrives. In fact, the recommendation for cool-season grasses in our region is to make two fertilizer applications in fall — one in September and another one in November.
Many fall fertilizers available around this time at home-improvement stores and garden centers are advertised as “winterizers.” A winterizer may contain some nitrogen (N, the first number listed on the fertilizer label) and phosphorus (P, the second number), but it is typically high in potassium (K, the third number on the label). Potassium helps the grass at the cellular level — it both helps the plant strengthen and harden before the cold of winter arrives. Not only is potassium important for cool-season lawns, but it should also be the last application of fertilizer in fall (typically four to six weeks before the first expected frost) for any of the warm-season grass types.
The best way to determine your lawn’s actual nutrient needs is by a soil test. Boxes and instructions can be picked up at your local Extension Office. Test results will tell you what nutrients your soil needs, and they will make recommendations for both lime and fertilizer for the type of grass in your lawn.
Below are general guidelines for fall fertilization of the different grass types.
 
Tall Fescue/Bluegrass
In the absence of a soil test, use a complete nitrogen-phosphorus-potassium (N-P-K) turf-grade fertilizer with a 3-1-2 or 4-1-2 ratio (such as 12-4-8 or 16-4-8). Fertilize with 1 pound of actual nitrogen (N) per 1,000 square feet in September and again in November (about the time the grass is green but not actively growing).
To determine the amount of product required to apply 1 pound of nitrogen per 1,000 square feet, divide 100 by the FIRST number on the fertilizer bag. For example, for a 12-4-8 fertilizer, divide 100 by 12. The result is 8.33 pounds of product to be applied per 1,000 square feet in order to supply 1 pound of nitrogen.
 
Bermudagrass
Four to six weeks before the first expected frost, apply no more than 1/2 pound of nitrogen per 1,000 square feet. Use a low-nitrogen, high-potassium fertilizer such as a 5-10-30, or supplement a nitrogen fertilizer source with 1 pound of potash (K2O) using 1.6 pounds of muriate of potash (0-0-60), 2 pounds of potassium sulfate (0-0-50) or 5 pounds of Sul-Po-Mag (0-0-22) per 1,000 square feet.
 
Zoysiagrass
Do not apply nitrogen to zoysiagrass after September 1. Instead, fertilize with 1 pound of potash (K2O) using 1.6 pounds of muriate of potash (0-0-60), 2 pounds of potassium sulfate (0-0-50) or 50 pounds of Sul-Po-Mag (0-0-22) per 1,000 square feet.
 
Centipedegrass
Four to six weeks before the first expected frost, fertilize with 1 pound of potash (K2O) per 1,000 square feet using 1.6 pounds of muriate of potash (0-0-60) or 2 pounds of potassium sulfate (0-0-50).
DO NOT lime centipedegrass unless recommended by a soil test.
 
St. Augustinegrass
Do not fertilize St. Augustinegrass after August 31.