Every fall the garden centers around Tallahassee stack up bags labeled "winterizer," and every fall we see lawns in Jefferson County that were fed with them in October and came out of winter worse than the ones that got nothing. The idea behind a fall feeding is right. The nutrient most of those bags lean on is wrong.
What potassium does for turf
Potassium is the K in N-P-K, the third number on the bag. It does not make grass greener or push growth the way nitrogen does, which is why it gets ignored. What it does is regulate how the plant handles water and stress. Adequate potassium makes turf more tolerant of cold, drought and disease pressure, and it thickens cell walls so the plant holds up better under traffic.
For a warm-season lawn heading into a Big Bend winter, with hard freezes almost guaranteed and a wet, disease-prone November likely, that is exactly the list of things you want.
Why late nitrogen backfires here
Nitrogen tells the grass to grow. In October in Monticello, the days are shortening and soil temperatures are falling, and the grass is trying to slow down and store energy in its roots and stolons for dormancy. A shot of nitrogen interrupts that. It pushes soft, tender top growth right when the first frost is a few weeks off, and that soft growth is what freezes. It also feeds large patch, which is the fungal disease that shows up in centipede and St. Augustine every fall when nights cool and the ground stays wet.
On centipede the damage compounds. Centipede does not want much nitrogen at any time of year, and a fall application is the most likely one to trigger the slow collapse people call centipede decline. Our guide to why weed-and-feed fails on centipede covers the same trap from another direction.
What the last feeding should look like
The last application before dormancy should be low or zero nitrogen and high potassium. Look at the bag: something like 0-0-50 or a low first number with a high third number. Sulfate of potash is the common source and is easy on turf. Muriate of potash works but has a higher salt index, so keep the rate modest on sandy ground.
A soil test from your county extension office tells you whether potassium is actually low, and on the sandy soils south of town it very often is. On the Red Hills clay north of Monticello it holds better, and the rate can be lighter.
Timing for the Big Bend
- Last nitrogen: mid to late August for centipede, early September at the latest for St. Augustine and Bermuda. After that, no more N until spring green-up.
- Potassium application: September through early October, while the grass is still actively growing and can take it up. Applying it after the lawn has gone dormant does very little.
- Water it in with about a quarter inch, and do not apply it right before a forecast downpour on sand.
Our month-by-month fertilizing calendar puts these dates alongside the rest of the year.
Grass-by-grass notes
Centipede
The most sensitive to late nitrogen and the one that benefits most from getting the fall feeding right. Potassium only. Keep the total annual nitrogen low regardless.
St. Augustine
Fall potassium helps with cold tolerance, which St. Augustine lacks. It also reduces large patch severity in a wet November. Keep the last nitrogen to early September.
Bermuda and Zoysia
Both handle cold better than St. Augustine but still benefit from a potassium finish. Bermuda that has been fed heavily all summer is especially prone to freeze damage if the feeding does not taper.
Reading the winterizer bag
Many products sold as winterizer in Florida are built for cool-season lawns up north, where a fall nitrogen feeding makes sense because the grass is actively growing. Turn the bag over. If the first number is high and the third is low, it is the wrong product for a warm-season lawn going dormant. Our guide to reading a fertilizer label walks through the numbers.
FAQ
Can I use potassium in spring too?
Yes. A balanced feeding in spring should include it. The fall application is about the ratio, not about potassium being a fall-only nutrient.
Will potassium green up a pale lawn?
Not directly. If the lawn is pale in fall, that is normal as it slows down. Iron gives color without growth if you want it.
What about phosphorus?
Most Big Bend soils have enough, and it should only be applied when a soil test calls for it.
Do you handle fertilization as part of a plan?
We build lawn care plans around your grass and your soil. See our services.
If you'd like your lawn to go into winter with the odds in its favor, get in touch. We set up seasonal programs for yards across Monticello and the surrounding towns.