Michigan State University Extension
Ornamental Plants plus Version 2.0 - 00001565
01/01/98
Thatch is a layer of undecomposed stems and roots that accumulates near the soil surface. Grass clippings do not contribute to thatch accumulation. The rate at which thatch accumulates is determined by the type and vigor of the grass in the lawn. 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 1/2 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 is available to homeowners with underground sprinklers. A 20 minute watering at mid-day, every day, helps control thatch by keeping it wet. Such a watering also is adequate for watering the lawn.
Three other options are open to homeowners without underground sprinklers. They are power rakes, coring and topdressing.
Power rakes, or dethatchers, use power driven tines to tear the thatch out of the lawn. Much of the lawn gets torn out in the process. 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 dethatching injury. Early September is the best time to use this type of thatch removal.
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 may also be combined with coring. The soil introduces micro- organisms that help decay the thatch.