Thatch is an intermingled layer of dead and living shoots, stems, and roots at the soil’s surface. Thatch is a nuisance because it builds up faster than it breaks down. If your lawn contains a layer of thatch thicker than one inch it can cause the following issues:
Dry out the turf's root system. Thatch heats up and dries out quickly which dries out the turf's root system.
Reduce oxygen to turf roots. During rainy periods, thatch holds water reducing oxygen to turf roots.
Create pest problems. Excess thatch harbors disease-causing organisms and insects which can lead to pest problems