Atwood LawnCare

Thatch is a layer of living and dead organic matter that occurs between the surface and the soil. Excessive thatch (over 1/2 inch thick) creates a favorable environment for pests and disease, an unfavorable growing environment for grass roots, and can interfere with some lawn care practices.
The primary component of thatch is turfgrass stems and roots. It accumulates as these plant parts buildup faster than they breakdown. Thatch problems are due to a combination of biological, cultural, and environmental factors. Conditions that favor thatch include heavy, wet soils; alkaline, or high pH soils; and soil compaction.
Cultural practices can have a big impact on thatch. For example, heavy nitrogen fertilizer applications or over-watering frequently contribute to thatch, because they cause the lawn to grow excessively fast. Avoid over-fertilizing and over-watering. Over use of insecticides will also contribute to excess thatch. Do not apply insecticides to your lawn unless you have a pest that is damaging the lawn.
Despite popular belief, short clippings dropped on the lawn after mowing are not the cause of thatch buildup. Clippings are very high in water content and breakdown rapidly when returned to lawns after mowing, assuming lawns are mowed on a regular basis (not removing more than one-third of the leaf blade).
As thatch levels accumulate to greater than 1/2 inch, lawn problems may begin, and the thatch needs to be controlled. Core aerification, followed by topdressing, are methods that will generally correct the reasons thatch is accumulating. Core aeration machines will pull up small soil cores to the surface that is left to act like topdressing. Cores should remain on the surface and allowed to air dry. The holes created help solve problems such as compaction or poor drainage. Aerify in spring or fall, making sure adequate moisture exists in the soil.
Additional topdressing material, such as screened compost, could be added after core aerifying if desired. Topdressing is simply adding a thin layer (1/8 to 1/4 inch) of compatible soil or compost over the thatch, which adds microorganisms to help in breakdown.
Aerifing is an excellent lawn practice with many benefits, as it helps solve soil problems that in turn lead to better root systems and healthier lawns.

This material was condensed from The University of Illinois Extension OnLine Bulletin “Thatch and How to Manage It”. http://www.urbanext.uiuc.edu/lawnchallenge/lesson5.html