Thatch is the accumulation of living and 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 an inch or more deep.

The thatch layer can become a water resistent barrier on the soil surface. Eventually, the roots and rhizomes of bluegrass grow primarily in the thatch layer, rather than in the soil, because insufficient water reaches the soil. Grass anchored in thatch layers are much more vulnuerable to stresses caused by fluctuating temperatures and moisture conditions.

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

Lawns do not require dethatching every year. Many lawns never require thatch removal. One-half to three-quarters inch of thatch is beneficial as it protects the crowns of your plants and helps to hold soil moisture. If a layer of thatch makes it difficult to water through to the soil, you should probably remove the thatch. Aerifying the lawn speeds natural thatch decomposition.

Thatch buildup may be related to earthworms and night crawlers in the lawn. Thatch tends to develop in lawns where there are few, if any, earthworms.

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