Cool-season lawns: why then
The cool-season “winterizer” is a real nitrogen feeding 2–4 weeks before the first fall frost, while the lawn is still green and photosynthesizing — it converts to root reserves and carries green-up into spring. It is the second-most-important feeding of the year after the early-fall one.
Warm-season lawns: why then
Do not winterize a warm-season lawn with nitrogen — late N produces tender growth that winter burns. If a soil test shows need, an early-fall potassium application (timed with the last-fertilization window, 6–8 weeks before frost) improves cold hardiness. Otherwise “winterizing” a southern lawn mostly means raising the mower a notch and keeping leaves off dormant turf.