Dethatching can renew a lawn

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Q. My lawn care company is recommending dethatching for my lawn. I am curious as to how important that is to maintaining a healthy lawn.

A. Thatch is a by-product of the normal growth of turfgrass. It is a tangled mat of organic matter between the crowns of the grass plants and the soil surface, formed as the grass sloughs off dead roots, rhizomes and/or stolons and stems. How quickly it forms and breaks down is governed by the type of grass in your lawn, the pH of the soil and your mowing, watering and fertilization practices.

A 1/2-inch or less of thatch is actually desirable. It acts as a mulch to conserve soil moisture and moderate soil temperature. The organic matter stimulates microbial action, which makes nutrients more available to the turf.

Thatch becomes a problem when it gets more than about a half-inch thick. Once it dries out, it actually repels water, much like dried-out peat moss. It also serves as a breeding ground for insects and disease. Worst of all, if you run into a problem with white grubs, it makes it difficult to get an insecticide down to the turf roots where the grubs are feeding. Instead, the active ingredient of the pesticide binds to the thatch layer, rendering it ineffective.

Kentucky bluegrass and creeping red fescue are the fastest thatch formers typically grown in home lawns. This is because they spread by rhizomes, or underground stems. Perennial ryegrass, turf-type tall fescues and hard fescues are bunch-type grasses. They do not spread by rhizomes, so they build up thatch more slowly.

A soil pH under 6.3 inactivates the microbes responsible for breaking down the thatch layer. It is important to test your soil every few years to make sure the pH remains in the optimum range of 6.5 to 7.0.

Proper mowing is another important way to slow the thatch-formation process. Ideally, you should never remove more than one-third of the blade of grass when you mow. This means that you have to adjust your mowing frequency to how fast the grass is growing.

Also, there is a proportional relationship between the height of the grass and depth of its root system. The longer the grass, the more extensive root system it will have. In spring and through summer, raise the cutting height to 3 inches to encourage an extensive root system. You should cut it a little shorter in fall, with the final cutting before winter down to 11 /2 inches. If you cut your grass short during the heat of summer, it will become stressed and the dead grass and roots will contribute to thatch buildup.

A sure way to create a thatch nightmare for yourself is to overfertilize your lawn. While nitrogen makes a lawn green, it also makes it grow. If you push your lawn with excessive nitrogen, it grows faster and forms thatch faster.

For a typical bluegrass/ryegrass/fescue lawn, 2-3 pounds of nitrogen per thousand square feet over the growing season is sufficient. That should be applied in two to four applications. More than that is asking for trouble. Not only does excessive nitrogen encourage thatch formation, it also makes your lawn more susceptible to certain insect and disease problems, especially during hot, humid weather.

Finally, make sure to water deeply and infrequently. Shallow, frequent watering encourages a shallow root system that is easily drought stressed. When those drought-stressed roots die, they simply add to the thatch layer.

On the other hand, deep watering encourages a deeper, more drought-resistant root system. Deep watering means letting the sprinkler or irrigation system run for two hours or so once or twice a week. This should apply about an inch of water that will wet the soil to a depth of 6-8 inches.

If your lawn has less than a half-inch of thatch, core aeration every couple of years, along with proper mowing, watering and fertilization practices, should be sufficient to keep the thatch layer at the proper level.

If you have more than a half-inch but less than 1 1/2 inches of thatch, running a power dethatcher in fall should remove enough thatch to save your lawn. Run the dethatcher in one direction, then again in the perpendicular direction.

Small lawns can be done with a hand dethatching rake.

A good dethatching job should make you want to cry when you look at your lawn -- it should be torn up pretty thoroughly. That is why dethatching is best done in the fall, so that your lawn has ample time to recover before facing next summer's heat and drought.

Rake away the debris and topdress with a thin layer (1/8 to 1/4 inch) of good quality topsoil or compost. Then overseed your lawn to help it recover.

If you have more than 11/2 inches of thatch, consider starting a new lawn from scratch. The tines on most power dethatchers are not long enough to get through that much thatch and into the soil. Once the new lawn is established, follow good cultural practices to avoid thatch buildup.

garden

Send questions to Sandy Feather by e-mail at slf9@psu.edu or by regular mail c/o Penn State Extension, 400 N. Lexington Ave., Pittsburgh 15208.


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