You Are At: AllSands Home > Gardening > Killing lawn thatch
Thatch is a thick, spongy carpet that forms between the soil and living lawn grass. When it dries out, it turns into a waterproof ceiling on your soil, preventing water, air and fertilizer from percolating down. Its dense, tangled layers invite insects and diseases to breed and grow, and it can even cause your lawn grass to turn yellow and die.

Thatch is made up of whole grass plants decaying at different rates on top of your soil. The leaves, cut off by the mower, decompose the quickest. Stems and roots take longer to break down, and the nodes and crowns are last to go.

BUILD-UP

Ideally, grass plants should decompose and fade effortlessly into the soil. If all is well with your lawn, there is only a thin layer of thatch, usually about half an inch. However, if that matted pile is more than three-quarters of an inch, you have a problem.

Thatch piles up when dead grass plants accumulate faster than they can decay. If you fertilize and water your grass too conscientiously, it grows like crazy. Anything you do to stimulate an excessive growth of grass, or inhibit its decomposition, increases the rate of thatch accumulation.

Modern turf grasses are bred for high shoot density, to create a lawn that is thick and carpetlike. Unfortunately, the most beautiful and luxuriant kinds of grass are likely to create thatch. The turf grass varieties that have a tendency to thatch include Kentucky Bluegrass (especially the variety 'Merion'), Creeping Bentgrass ('Toronto'), Bermuda Grass ('Tifgreen') and Colonial Bentgrass. Fescue and Zoysia are species that tend to be especially tough and decay resistant, so the thatch from these grasses is long lasting.

The most satisfactory way to hold thatch in check is to modify your lawn management. You can alter the ways you fertilize, mow and water the lawn. You can aerate it, top dress with soil, or add some commercial microbial inoculants to break down the thatch. If the mat is dense, you can go over your lawn with a cultivator or a de-thatching machine.

FERTILIZATION

A well-fertilized lawn may look green and healthy, but excessive fertilization with nitrogen causes the growth of soft, succulent grass that needs frequent mowing and encourages thatch buildup. It is best to give your lawn light, frequent applications of fertilizer that do not exceed the minimum needs of the plants.

Kentucky Bluegrass and Fescues should be fertilized most in the fall and winter, and not as much during the spring and summer. Bermuda Grass and Zoysia need the treatment in reverse. They should receive the bulk of their fertilizer during the hot weather, not in the fall.

Most grasses have a basic requirement of two pounds of nitrogen per one- thousand square feet, applied every year. A typical lawn fertilizer with a NPK ratio of 10-6-4 (ten parts nitrogen, six of phosphorus and four parts potassium) has only ten-percent nitrogen in the bag. If you buy a hundred-pound bag, you are actually getting ten pounds of nitrogen to spread on the lawn. Therefore, during the course of the season, you would need to apply twenty pounds of that kind of fertilizer on each thousand square feet to meet the basic requirement. Lawn fertilizers with a higher ratio of nitrogen can be applied in proportionately smaller doses -- and should be -- if you want to reduce thatch.

LIME AND WATER

The health of your lawn is also affected by the alkalinity or acidity of your soil as indicated on a pH scale. You can make the soil more alkaline by applying lime, but this has to be done in measured amounts, depending on the type of grass you're growing and the nature of your soil.

You can find out must how much lime to put on your lawn by sending a sample of the soil to your county agricultural extension agent. Be sure to specify the type of grass you have, so the agent can recommend the rate of limestone to put on your lawn. Limestone raises the alkalinity. Once the pH reaches the right level, you can keep it there by applying lime every two or three years.

Ground agricultural limestone is the safest kind to use. Apply it in the fall, so that the rain, snow, freezing and thawing of the soil will help to work the lime into the ground.

In the desert Southwest and other arid parts of the country, many homeowners have a problem with alkaline soil. To correct for alkalinity, apply sulfur, either in the pure form or as a compound, ferrous sulfate. Then water the lawn thoroughly. It takes about twenty pounds of sulfur per one thousand square feet to reduce the pH by one point.

Over watering, like over-fertilizing, can cause excessive growth and, as a result, excessive thatch. Do not water until you have to. However, during the driest periods of the growing season, drench the soil until water seeps down a full six inches. It takes about an inch of hard rain or steady irrigation to wet the lawn this deeply. If you are using a sprinkler, do not make the mistake of turning it on for a few minutes every evening. Shallow watering causes shallow roots to grow. Both disease and crabgrass seem to flourish in grass that only receives a light sprinkling.

AERATION AND TOPDRESSING

Water and nutrients percolate down to the grass roots. If all is well, there is a free exchange of gases between the soil below and the atmosphere above. However, if the ground gets packed down around the grass, that hard compacted layer can barricade gas exchange. Pick up a section of grass and look at it. If the soil around the roots is compacted as little as one-fourth to one-half inch, then growth is being stifled.

To keep this from happening, aerate your soil periodically with a machine that removes plugs of soil from the turf called an aerator or coring machine, or a cultivator or spiker that slices through the thatch. The time to do it is when the lawn is growing vigorously, and it can restore itself from the damage your machine inflicts.

After you have aerated your lawn, you can hasten the decomposition of thatch with topdressing, a thin layer of soil, or soil and sand mixture, spread on top of your lawn. With the soil comes a host of microorganisms that help the thatch decompose. The result will be a rapid breakdown of the thatch into good organic fertilizer. Topdressing has been proven one of the best methods of controlling thatch. Unfortunately, there's a drawback -- it is messy.

MOWING

One of the most important things about controlling thatch is to mow your lawn to the right height and at the right time. The best policy is to mow the lawn frequently with the mower set high. If you cut the grass short, you reduce the leaf surface so the plant has difficulty making enough food for growth. If you do not cut often enough, or you let the grass grow long, it will create dense roots. Since thatch is sixty-percent roots and rhizomes, the denser the growth, the greater the possibility of thatch.

The worst thing you can do to your lawn is to cut it too short. As a rule, grass should be cut so that less than one-third of the whole blade is taken off each mowing. Kentucky Bluegrass and Fine-leaved Fescue should be mowed to a height of two inches. Zoysia and Bermuda grass, which are smaller plants to begin with, can be mowed to three-quarters of an inch.

Mowing frequently at these levels, you do not have to worry about the grass clippings. Let them lie where they fall. After all, the leaves decompose most quickly -- much faster than the crowns and roots -- so they will soon break down and return their nutrients to the soil.

THE DETHATCHING ALTERNATIVE

If that thick, spongy carpet has already settled in about your lawn, you will need more than a rake to take it away. The alternative is a dethatching machine, which can be rented from a hardware store or garden center. There is one kind of machine that makes vertical cuts into the sod, and another that combs out the thatch with tines. Either method has the same result: machines pull organic matter to the surface of the grass. Once this thatchy material has been pulled up, it should be raked away immediately.

The dethatching machine does some ruthless tearing at the roots, so your grass will need some time to recover. Rent the dethatching machine when the grass is growing vigorously and you can look ahead to at least thirty days for the grass to recover. Bluegrass or Fescue lawn should be dethatched only in the fall or spring, never in summer. Thatch in Bermuda grass and Zoysia lawns should be removed in early summer. Apply one pound of nitrogen per one thousand square feet and water deeply soon after dethatching. With newfound air around their roots, your grass will recover quickly.