Fall is a great time in our area to enjoy comfortable temperatures and being outdoors in our gardens. Usually during this time of year, you can find some advertisement and promotions of winter fertilizer products for the lawn. The question then comes up, “Am I supposed to be fertilizing my lawn in the fall?” The answer is yes and no, depending on what type of fertilizer you are considering.

In Eastern NC, we mostly have warm-season grasses such as Bermuda, Centipede, Zoysia and St. Augustine grasses. Warm-season grasses are different from cool-season grasses (fescue, bluegrass) in that warm-season grasses actively grow during the summer and go dormant in the winter months, while cool-season grasses are actively growing during the spring and fall months.

Before using a fertilizer, you need to understand some fertilizer basics. Every fertilizer bag will have three numbers on it to indicate the ratio of major nutrients plants need to grow. For example, a 10-10-10 fertilizer has equal parts of Nitrogen (N)-Phosphorus (P)-Potassium (K). Each nutrient helps plants in different ways. Nitrogen (N) promotes green, leafy plant growth. Phosphorus (P) promotes root growth along with flower, fruit and seed production. Potassium (K), also known as potash, improves the overall plant’s health by increasing drought tolerance, winter hardiness and disease resistance.

A general rule for fertilizing lawns is to only use a fertilizer that contains nitrogen (N, first number on a fertilizer bag) when the lawn is actively growing. Since we have warm-season grasses, the time to fertilize with nitrogen is late spring into summer. The nitrogen will encourage green, leafy growth which is what we want from our lawns. You do not want to apply a fertilizer containing Nitrogen after August or before April/May. Fertilizing with nitrogen too late or too early in the year can set up your warm-season lawn for winter injury because the nitrogen will be encouraging the lawn to green-up and grow too early or too late for our climate.

So, why then do you hear advertisements and see lawn fertilizer for sell this time of the year? There are two possible reasons. One reason is that you have to keep in mind we are very close to a transition zone where if you go west or north of our area they would have a climate suited for cool-season grasses. Because we are near the transition zone, you can find advertisements related to cool-season grasses instead of warm-season grasses.

Another reason you may see promotion of fertilizers that winterize your lawn is because they are fertilizers that do not contain nitrogen but instead contain potassium. Early fall is a good time to apply a potassium fertilizer to encourage a healthier lawn that is more winter hardy. A soil test would be the first step to finding out if your soil is deficient in potassium. Soil test kits are available at the Wayne County Cooperative Extension office. If your soil test results show a deficiency in potassium, the recommendation is to apply a high potassium fertilizer such as 1.6 pounds of muriate of potash (0-0-60) or 2 pounds of potassium sulfate (0-0-50) per 1,000 square feet. For the best results, potassium fertilizers need to be applied at least six weeks before the first expected frost. The average first fall frost date is October 30^th for Wayne County, meaning mid- to late September is a good time to apply a potassium fertilizer.

Going back to the original question now is a good time to fertilize with a potassium type fertilizer (last number on a fertilizer bag), if your soil is deficient in potassium, to improve the winter hardiness of your lawn. However, be sure to closely check the three numbers on a fertilizer bag before purchasing to be sure it does not contain nitrogen (first number on a fertilizer bag) to avoid encouraging growth at the wrong time or year which would result in winter injury and lead to future lawn problems and poor growth.

Jessica Strickland is an Agriculture Extension Agent, specializing in horticulture for North Carolina Cooperative Extension in Wayne County.