Thatch is the dense layer of decomposing grass, roots, and other organic debris that accumulates on the soil surface. A thin layer of thatch is actually beneficial for turf, and is a natural occurrence in an actively growing lawn. A thin layer of thatch (1/2") provides essential nutrients and promotes turf resiliency.
When the thatch layer exceeds (1/2") it increases turf grass diseases, reduces drought tolerance, and reduces tolerance to excessive hot weather. Thatch buildup also minimizes air movement, water penetration, and effectiveness of turf fertilization. Symptoms of excessive thatch may occur if the lawn appears to be dry even after adequate watering.
Core aeration will help alleviate the problems associated with thatch build up.