so you got thatch?

Thatch and How to Manage It (from the University of Illinois Extension)

Thatch is a dense layer of living and dead organic matter on the soil surface.

Thatch in lawns is often misunderstood; both its cause and control. Some lawns have serious thatch problems while others do not. Thatch is a layer of living and dead organic matter that occurs between the green matter and the soil surface. 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. Cultural practices can have a big impact on thatch. For example, heavy nitrogen fertilizer applications or overwatering frequently contribute to thatch, because they cause the lawn to grow excessively fast. Avoid overfertilizing and overwatering. 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).

Excessive thatch accumulation has lead to lawn rooting into thatch rather than soil.

As thatch levels accumulate to greater than 1/2 inch, lawn problems may begin, and the thatch needs to be controlled. Thatch may be torn out with a dethatcher or vertical mower, but will most likely return unless the cause is corrected. Mechanical dethatching is also very destructive to the lawn because roots are in thatch instead of soil, so plants tear out easily. Overseeding is usually required afterwards. For this reason, it's best to tear out thatch in late August for optimum reseeding timing.

Core aerating helps degrade thatch and also helps solve some of the causes of thatch.

Core aerification, followed by topdressing are two methods that will generally correct the reasons thatch is accumulating. Core aerifying machines will pull up small soil cores to the surface that are left there to act like topdressing. The holes created help solve problems such as compaction or poor drainage. Topdressing is simply adding a thin layer(1/8 to 1/4 inch) of compatible soil over the thatch, which adds microorganisms to help in breakdown.

Aerifying equipment may be rented or services are available to do it for hire. Aerifing is an excellent lawn practice with many benefits, as it helps solve soil problems that in turn leads to better root systems and healthier lawns. Aerify in spring or fall, making sure adequate moisture exists in the soil. Make two trips over the lawn, the second perpendicular to the first. An average of 15 to 20 aeration holes per square foot is suggested. Cores should remain on the surface and allowed to air dry. These cores act as topdressing that helps degrade thatch. Additional topdressing material could be added after core aerifying if desired.