10:03 Jul 22, 2009 

Thatch can be described as a tightly intermingled layer of partially decomposed stems, roots, and some leaves of grasses which develops beneath the actively growing green vegetation at the soil surface. Stems and roots of turfgrasses contribute primarily to thatch accumulations, while turfgrass leaves contribute little.

http://cru.cahe.wsu.edu/CEPublications/eb1117/eb1117.html