University of Vermont Extension 
Department of Plant and Soil Science

Fall News Article


Contact: Dr. Leonard Perry, Extension Professor
University of Vermont

If you want to have an attractive lawn, you must control lawn thatch. What is thatch, you ask? Thatch is a layer of undecomposed stems and roots that accumulates near the soil surface.

Grass clippings do not contribute to thatch accumulation. The type and vigor of the grass in the lawn determine the rate at which thatch accumulates. A thatch-prone bluegrass sod, that is given lots of water and fertilizer, forms thatch more rapidly than other grasses given less care.

Thatch is a normal part of any lawn and only becomes harmful when the thatch layer is thicker than one-half inch. When thatch becomes excessive, the lawn may root into the thatch rather than the soil. Thatch does not hold moisture so lawns rooted into thatch will not tolerate dry weather or cold temperatures. In fact, a very "thatchy" lawn can be rolled up just like a rug when it dies out.

Thatch management can take several forms. The easiest management technique involves use of a thatch hand rake with knifelike teeth. Three other options are vertical mowers, coring, and topdressing.

A vertical mower is a specialized machine, which often can be rented, that uses power driven tines to bring the thatch to the surface. While this is the most common way to dethatch, it is probably the least desirable. Sections of very "thatchy" lawns may need to be reseeded due to dethatching injury. Early September is the best time to use this type of thatch removal, with early spring next best.

A more desirable alternative is coring. The limitation here is the availability of the coring machines. Coring machines remove cores of soil and sod. The hole allows air and moisture to penetrate the thatch and help in its decomposition.

A thin layer of soil can be applied over the lawn to help decompose the thatch layer. This topdressing also may be combined with coring. The soil introduces microorganisms that help decay the thatch.

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