Ronald Strahan, Koske, Thomas J.
The term "winterize" when used in warm-season grass lawn care is simply stated as slow down growth and beef up plant levels of the nutrient potassium (K). This is an attempt to toughen plant tissues for the winter freezes much as you would add antifreeze to your car’s radiator.
Research has shown that higher levels of plant K enhance tolerance to a broad range of environmental stresses, including cold damage. Therefore, a good strategy is to maintain high levels of soil K throughout the growing season. When soil test levels of K have reached a high level, maintain that by applying about 2/3 part of K2O fertilizer for every one part of nitrogen (N) supplied. This is not an exact calculation but good rule of thumb. The third analysis number on the bag of fertilizer is the percent by weight of potassium K2O equivalent also called "potash."
You can't just go by the term WINTERIZER on the fertilizer bag because many winterizer formulations sold in Louisiana are for cool-season grasses in the northern states. So, as usual, it's "buyer beware." An informed consumer will be looking for a southern type winterizer for theIr warm-season grasses to avoid the plant-softening effect of using a northern, high-N formulation.
Phosphorus (P), the middle number (as percent P2O5) on a fertilizer bag analysis, does not usually come into concern here as long as soil levels are not very low. Extra fall P would just promote winter weed seed germination. An exception is if you will be overseeding for winter color; then you may apply P for better ryegrass seedling development.
With adequate K available all season, special winterizing would be unnecessary, and the turf would be in the best shape to handle environmental and pest problems all season long. Just make sure your last application of N is lower (maybe half rate or less) when applied after late summer and before mid-fall. After mid-fall, do not apply N to a warm-season lawn.
One last warning -- too much of a good thing is often bad. Excessively high rate of K fertilizer may lead to foliage burn (salt burn) or can lead to competitive inhibition of other soil-nutrient uptake. This is especially known to affect magnesium (Mg) uptake, which can appear as a light green lawn color.