Winterizer Fertilizer for St. Augustine: When, What, and How Much

My pick is Nitro-Phos Fall Special 8-12-16. If you can't find it, Fertilome winterizer is the fallback I'd grab.

What a winterizer actually does

"Winterizer" is a marketing word, not a formula. Some bags sold under that name are loaded with nitrogen, which is the opposite of what St. Augustine needs going into a Gulf Coast winter. The fall feeding has a different job than the spring and summer ones. You're not trying to push new leaves. You're trying to send the grass into dormancy with strong roots and good cold and disease tolerance.

That's where potassium comes in. Texas A&M AgriLife turf specialists point to potassium as an important nutrient for stress tolerance, and recommend a late-season product with some nitrogen and some potassium rather than a heavy nitrogen feeding. The LSU AgCenter goes further for the Gulf South: a fall winterizer for warm-season grass should have a very small first number and the highest third number, and winterizers with a lot of nitrogen are a poor fit for our area.

Why heavy nitrogen in fall backfires

• It feeds large patch. Large patch fungus gets active as soil cools into the 50s and 60s °F, and soft, fresh growth from a big nitrogen feeding is exactly what it attacks. See large patch fungus.
• It makes the grass less cold hardy. AgriLife warns that too much nitrogen too late in the year keeps the grass growing leafy, tender tissue that's more vulnerable to frost and can lead to winter kill.
• It feeds the wrong plants. Nitrogen that the grass can't use as it slows down helps winter weeds like henbit and Poa annua instead. See winter weeds.

What to look for on the bag

Read the three numbers as nitrogen, phosphorus, and potassium (N-P-K). For a St. Augustine winterizer:

• First number low. Single digits is a good sign.
• Last number highest, or at least well above the first.
• Middle number based on your soil. Many Houston-area soils already test high in phosphorus. A soil test is the only way to know. I use the MySoil kit for this.

The fall product in the Turf Method program is Nitro-Phos Fall Special 8-12-16. It's 8 percent nitrogen, 12 percent phosphate, and 16 percent potash, made for St. Augustine and Bermuda in the South. If your store doesn't carry it, Fertilome's winterizer is a reasonable fallback. Check the bag for the same pattern: low first number, highest last number. If the numbers on any bag confuse you, here's how to read a fertilizer bag.

When to apply it in the Houston area

Timing matters more than brand. Texas A&M AgriLife's rule is that the last nitrogen application should go down at least 4 to 6 weeks before your county's historic first frost, while the grass is still actively growing and can take it up.

For Houston, AgriLife's frost and freeze guide lists the average first freeze as December 9. Some spots in Fort Bend County run a little earlier and some later, depending on how open and low-lying they are. Working back from that puts the window at the first week or two of November.

In the Turf Method program, the fall feeding goes down in early November, right inside that window, while the grass is still actively growing. A few things to adjust for:

• Colder yard, go earlier. If you're north or west of Houston, or in a low spot that frosts first, get it down in the first week of November.
• Still growing is the test. If you're still mowing every week or two and the lawn is green, it can use the feeding. If it has already gone tan and stopped growing, you've missed it for this year. Save the bag for next fall.
• Watch the soil, not just the calendar. A soil thermometer shows you how fast things are cooling off. Texas A&M AgriLife notes that warm-season grass roots can stay active until the soil cools to about 50 °F, so there's still root growth to feed in fall.

How much to put down

Use the rate on the bag for St. Augustine. For Nitro-Phos Fall Special, that's 5 pounds per 1,000 square feet, so a 40 pound bag covers about 6,000 square feet.

At that rate you're putting down about 0.4 pounds of nitrogen per 1,000 square feet. For comparison, AgriLife's general guidance is no more than 1 pound of quick-release nitrogen per 1,000 square feet in a single application, and it notes that 0.5 pounds can help avoid burning or injury. A fall blend stays under both, which is the point. Most of the work in that bag is the potassium.

I weigh what goes in the spreader on an Etekcity digital scale so the amount matches the label instead of a guess with a scoop.

How to apply it

1. Measure your lawn. You need square footage to know how much product to use. You can measure it in Google Earth.
2. Weigh out the product. Square footage divided by 1,000, times the label rate.
3. Spread in two directions. Set the spreader to about half the rate and make two passes at right angles. It evens out coverage and avoids stripes.
4. Sweep it off hard surfaces. Granules left on driveways and sidewalks wash into storm drains, and some blends with iron can stain concrete.
5. Water it in. About a quarter to half inch, in the early morning. Don't water at night in fall. Wet grass overnight is what large patch wants.

What happens after

Once the winterizer is down, the feeding schedule is done for the year. Keep mowing at 3.5 to 4 inches as long as the grass is growing (see mowing height), cut watering back, and keep leaves from matting on the lawn. When the lawn goes dormant and turns tan, that's normal. Don't try to green it up with more fertilizer. The next feeding comes after spring green-up. The full fall and winter checklist is in fall and winter care.

Common questions

What's the best winterizer for St. Augustine?

The one I use is Nitro-Phos Fall Special 8-12-16. It's low in nitrogen and higher in potassium, which is what St. Augustine needs going into a Gulf Coast winter. If you can't find it, Fertilome's winterizer is my fallback. Whatever you buy, check that the first number is low and the last number is the highest.

Should I use a winterizer on St. Augustine?

Yes, as long as it's the right kind. For St. Augustine you want a fall fertilizer that's low in nitrogen and higher in potassium, applied while the grass is still growing. Skip winterizers with a big first number, since heavy nitrogen in fall feeds large patch and leaves soft growth exposed to cold.

When should I apply winterizer in Houston?

In the first week or two of November, while the lawn is still green and growing. Texas A&M AgriLife says the last nitrogen should go down at least 4 to 6 weeks before your area's historic first frost. Houston's average first freeze is around December 9, so early November is the target here.

What numbers should a St. Augustine winterizer have?

A low first number (nitrogen) and a higher last number (potassium). Nitro-Phos Fall Special 8-12-16 is one example. Let a soil test tell you whether you need the middle number (phosphorus) at all.

Can I fertilize St. Augustine in December or January?

No. Once the lawn is dormant it can't use the fertilizer, and the nitrogen mostly feeds winter weeds or washes away. Wait for spring green-up for the next feeding. See when to fertilize in spring.