When Should You Fertilize Your Lawn for Fall? Understanding the Numbers on the Bag

If you're standing in the lawn-and-garden section looking at bags of fertilizer, you may have wondered what all those numbers mean.

10-10-10.
15-0-15.
24-0-10.
21-3-6.

What do those numbers mean, and when is the right time to put fertilizer on your lawn?

As summer winds down, these are good questions to ask—especially if you want your lawn looking good going into fall and coming back strong next spring.

First, Know What Kind of Grass You Have

Before buying fertilizer, find out whether you have a warm-season or cool-season grass.

Warm-season grasses such as Bermuda, St. Augustine, and zoysia grow primarily during the warm months and go dormant when temperatures drop.

Cool-season grasses such as tall fescue and Kentucky bluegrass have a different growth pattern and are generally fertilized on a different schedule.

So don't assume that a "fall fertilizer" sold at the store is automatically right for your lawn.

What Do the Three Numbers on the Bag Mean?

The three numbers are the fertilizer's N-P-K analysis.

For example, a fertilizer labeled 24-0-10 contains:

• 24% Nitrogen (N)

• 0% Phosphate (P₂O₅)

• 10% Potash (K₂O)

The numbers are percentages by weight.

Nitrogen — The First Number

Nitrogen is the nutrient lawns generally use in the greatest quantity.

It promotes green color and leafy growth.

That's why nitrogen is usually the first number people pay attention to when choosing lawn fertilizer.

But more isn't necessarily better. Too much nitrogen can cause excessive, rapid growth and create additional mowing and other problems.

Phosphorus — The Second Number

The second number represents phosphate.

Phosphorus is important for plant growth, including root development, but established lawns often don't need additional phosphorus unless a soil test indicates a deficiency.

Adding phosphorus when your soil already has enough can simply waste money and contribute to environmental problems.

Potassium — The Third Number

The third number represents potash, which is the fertilizer industry's way of expressing potassium.

Potassium plays important roles in overall plant function and stress tolerance.

Again, though, the best way to know whether your lawn needs additional potassium is through a soil test.

You Don't Always Need All Three

This is one of the most useful things to understand about fertilizer.

You don't necessarily need a fertilizer that contains nitrogen, phosphorus, and potassium every time you fertilize.

A soil test can tell you what nutrients your soil actually needs. Texas A&M AgriLife Extension recommends using soil-test information to determine phosphorus and potassium needs rather than automatically applying them.

If your soil already has adequate phosphorus and potassium, a fertilizer with nitrogen and little or no phosphorus may make more sense.

That's not only better for the lawn—it can save you money.

When Should You Apply Fall Fertilizer?

Here's where things get interesting.

For warm-season grasses, you don't want to fertilize simply because the calendar says August or September.

The grass needs to be actively growing and able to use the fertilizer. Fertilizing during severe heat or drought stress isn't a good idea.

Texas A&M recommends making the final nitrogen application no later than about six weeks before the expected first autumn frost, with the exact cutoff varying considerably across Texas. For example, its guidance lists October 1 for areas including Dallas and October 15 for Austin, Houston, San Antonio, and Waco.

In Oklahoma, the recommendation for warm-season grasses is generally earlier, with the final nitrogen application no later than September 15.

So if your lawn is suffering through extreme late-summer heat and drought, wait until conditions improve rather than rushing to fertilize.

How Much Nitrogen Does Your Lawn Need?

This is where reading the fertilizer bag becomes useful.

Suppose you have a fertilizer labeled:

16-4-8

The first number tells you that the fertilizer is 16% nitrogen.

If you wanted to apply 1 pound of actual nitrogen per 1,000 square feet:

1 ÷ 0.16 = 6.25 pounds of fertilizer

So you would need approximately 6.25 pounds of 16-4-8 fertilizer per 1,000 square feet to provide 1 pound of actual nitrogen.

If you wanted only ½ pound of nitrogen:

0.5 ÷ 0.16 = 3.125 pounds

That's why two bags that look very different can actually provide similar amounts of nitrogen—you have to look at the numbers.

Texas A&M gives the same calculation method in its lawn-fertilization guidance.

Don't Fertilize a Heat-Stressed Lawn Just Because Fall Is Coming

This may be the most important point for gardeners dealing with a hot August.

If your lawn is brown or struggling because it isn't getting enough water, fertilizer isn't necessarily the answer.

For example, Texas A&M's Bermuda lawn guidance specifically advises against fertilizing during hot, dry periods when moisture is limiting. Nutrients are taken up most efficiently when the grass is actively growing and not under heat or drought stress.

Sometimes the best thing you can do for the lawn is wait.

A Soil Test Can Save You Money

If you've never had your lawn soil tested, consider doing it before buying fertilizer.

A soil test can tell you the soil's pH and help determine whether phosphorus and potassium are actually needed.

That can keep you from buying fertilizer simply because the bag has an attractive picture of a beautiful green lawn on the front.

The best fertilizer is the one your lawn actually needs.

What About "Winterizer" Fertilizer?

You'll see products marketed as "winterizer" or "fall fertilizer."

Don't assume that the word on the front of the bag tells you everything you need to know.

Turn the bag over and look at the N-P-K numbers.

Then ask:

What kind of grass do I have?

Is it actively growing?

What does my soil need?

Am I still far enough ahead of the first frost for the fertilizer to be useful?

Those questions are more important than the word "winterizer" printed on the package.

A Simple Rule for the Budget Gardener

Before buying another bag of fertilizer, remember:

Know your grass.
Know your soil.
Read the numbers.
Don't fertilize just because the bag tells you to.

Understanding N-P-K can help you make a better decision and avoid spending money on nutrients your lawn doesn't need.

And sometimes, saving money on fertilizer is just as much a part of budget gardening as saving money on plants and seeds.

Happy gardening!