"Excessive thatch is a common problem in many lawns. Thatch is a layer of organic matter that forms between grass blades and the soil line. The organic matter consists of tightly woven, living and dead grass stems, roots and crowns. These parts of the grass plants are high in lignin, an organic material that breaks down slowly. In most healthy lawns, those parts do break down.
"A little bit of thatch - less than 1/2 inch - is not bad. The thin layer cushions the turf, reduces soil compaction and helps conserve moisture. But if the layer gets over 1/2 inch, then thatch is real trouble."A thick layer of thatch between grass blades and soil blocks the movement of air, water and nutrients to grass roots. Such lawns are sensitive to drought, heat and cold. A shallow-rooted lawn dries out quickly, and because thatch can repel water, getting moisture to the roots can be difficult. And if shallow roots aren't bad enough, thatchy lawns also are more susceptible to insects and disease."