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Thatch in Lawns

Thatch is the accumulation of dead but undecomposed grass stems, crowns, roots and other organic debris at the soil surface. Thatch tends to build up over a period of years to form a mat, sometimes often an inch deep. Heavily fertilized Kentucky bluegrass often develops thatch.

The thatch layer becomes a waterproof blanket on the soil surface. The roots and rhizomes of bluegrass grow in the thatch layer rather than in the soil where they belong.

You can remove thatch from lawns with vertical cutting (dethatching) mowers or power rakes. Rake and remove the material you cut with these machines. May and September are good times to remove thatch.

Research shows thatch buildup is related to earthworms and night crawlers in the lawn. Thatch tends to develop in lawns where there are a few, if any, earthworms.

Lawns do not require dethatching every year. Many lawns never require thatch removal. If a layer of thatch makes it difficult to water through to the soil, you should probably remove the thatch. Topdressing the lawn with one-fourth inch soil speeds natural thatch decomposition.

Additional information on this topic is included in the Extension bulletin H-244, "A Beautiful Lawn Can Be Yours," which is available at your county office of the NDSU Extension Service.

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