Thatch is that brownish layer of organic debris (old stems) that sits between the soil and the grass leaves. Having some thatch on your lawn is good, too much is bad. A healthy lawn will have thatch, it helps hold in moisture and keeps the crown safe from excessive wear. However a layer of thatch that’s ½ inch or more high can actually keep moisture from reaching the roots. It prevents the water from penetrating deeply into the soil, and you end up with a shallow root system because the only water available is near the top of the soil. Shallow roots equal grass that is susceptible to disease and drought.