Winterizing lawns
Contributed by
Clemson University

By August, lawn winterizing products are fairly easy to find in the lawn and garden section of retail stores. This may lead one to think winterizing is a common practice. There are, however, misconceptions associated with this practice that the following information should help to clarify.

Cool & warm season

turfgrass

First of all, both cool and warm season turfgrasses grow in South Carolina, and most of these products do not differentiate between these two broad categories. Fescue is a cool season grass and grows best in cool temperatures. In the fall, you should fertilize fescue lawns with a lawn fertilizer according to soil test results. On the other hand, in South Carolina, warm-season grasses such as centipedegrass, St. Augustinegrass, Bermudagrass, and zoysiagrass begin to enter dormancy during the late fall. It's these grasses to which the term winterizer applies.

Nitrogen fertilizer

Many products marketed as winterizers contain a high percentage of nitrogen fertilizer. If nitrogen is applied in the fall, it should be done at least four weeks before the first average frost date. For centipede lawns, extend this time frame to eight weeks. Furthermore, the application needs to be light to avoid damaging the turf. A light application means one half pound of actual nitrogen per 1,000 square feet of lawn.

To determine how much of any fertilizer to use to apply a set amount of nitrogen, multiply the number of pounds of nitrogen you want to apply by 100. Divide the result by the percentage of nitrogen in the product, as shown on the label. In a fertilizer analysis, the percentage of nitrogen is represented by the first number. In the above case, one half times 100 equals 50. For a 5-5-25 fertilizer, for example, divide 5 into 50. The result, 10, is the number of pounds of 5-5-25 that contains one half pound of actual nitrogen.

When applied too late and too heavily to warm season turfgrass, nitrogen fertilizer will promote shoot growth at the same time the plant's metabolism is slowing. This results in a depletion of carbohydrates and stress on the plant. The new, tender shoots are also less tolerant of cold temperatures. Furthermore, the additional nitrogen will be available to cool season weeds and may increase the incidence of large patch disease, which is very prevalent in the fall.

Potassium fertilizer

Potassium plays the key role in winterizing because it has been shown to enhance cold tolerance of turfgrasses. If a soil test indicates that your soil is low in potassium, it can be applied at a rate of one pound of potash per 1000 square feet of lawn. Materials available include fertilizers low in nitrogen such as a 5-5-25 (use 4 lbs per 1000 square feet) or muriate of potash (0-0-60, use 1.6 pounds per 1000 square feet), or potassium sulfate (0-0-50, use 2 pounds per 1000 square feet). However, if your lawn has been fertilized throughout the summer with fertilizers containing potassium, it's unlikely a fall application would be helpful.

Be aware that an excessively high rate of potassium fertilizer can cause foliar burn or may cause a competitive inhibition of other nutrient uptake. Excessive potassium is especially known to affect magnesium uptake, which will result in a lighter green turf color. Always apply granular fertilizers onto dry foliage to reduce the likelihood of salt burn.

Cultural practices

Besides applying potassium, there are cultural practices that help warm season turfgrasses maintain optimum health through the winter. One suggestion is to increase sunlight to the turf. Turf in shaded areas stays cold longer and produces fewer carbohydrates due to reduced photosynthesis. Carbohydrates are needed for increased cold tolerance. Another suggestion is to loosen compacted soils through aeration or a gypsum application. Compacted soils are colder than well drained areas.

Also, keep fallen leaves off the lawn, as this will reduce the turf's ability to photosynthesize for carbohydrate production. And lastly, raising the mowing height in late summer will not only promote deep rooting but will leave more leaf tissue which will produce more carbohydrates. An increased height will additionally reduce weed seed germination in the fall before the turf goes dormant.

In summary, a lawn that has been well maintained through the summer is prepared for winter. If you are concerned, follow the recommendations above and submit a soil sample to your local Clemson Extension office. Among other helpful information, the report will include the potassium level of your soil.