A dead, organic layer of stems, leaves and other matter, that accumulates on the surface of the soil below the grass line. Thatch is a helpful element in lawn care it if it is a thin layer, which can limit weed growth and protect some grasses from early frosts. Too much thatch however - over ½ inch thick - encourages insect infestation, an unpleasant growing environment for grasses, and some lawn diseases. Core aeration is the proper lawn treatment for reducing thatch.