Thatch is a loose, intermingled organic layer of dead and living shoots, stems, and roots that develops between the zone of green vegetation and the soil surface. Thatch build up begins when turf produces organic debris faster than it can be broken down. Those parts of grass plants that are the most resistant to decay — stem nodes, crowns, fibers of vascular tissues, and roots — make up the bulk of thatch. Although you’ll find some leaf clippings in the thatch layer, they usually don’t contribute to thatch build up because soil microorganisms easily break them down.