Thatch is a dense, spongy collection of living and dead grass stems and roots lying between the soil surface and green grass leaves in established lawns. A thatch layer greater than inch thick makes watering difficult, since thatch dries out quickly and is difficult to rewet. It also restricts the movement of pesticides, thus reducing their effectiveness. Nutrients and water cannot be properly absorbed by the grass roots that tend to grow into this area.
Grasses differ in their tendency to produce thatch.
Tall fescue and perennial ryegrass have a low tendency for producing thatch. Heavy thatch builders include hybrid bermuda cultivars and Kentucky bluegrass, mostly due to their vigorous growth rates. Slower growing grasses, such as zoysia, produce thatch because their fibrous tissues are very slow to break down and easily accumulate on the soil surface. Apply the correct amount of fertilizer and water to the lawn. Avoid excess nitrogen, and do not exceed the fertilizer recommendations for your particular type of grass.