What is thatch and why remove it?

All grasses benefit from Verticutting

Thatch is the layer of undecomposed, stolons, roots, and crowns intermingled with soil (picture above). Leaving clippings on the lawn does not cause thatch because clippings are readily broken down by microbes in the soil. Thatch development is greatest in grass that is over fertilized or overwatered. An excessive thatch layer reduces water penetration and can bind up fertilizer or pesticides. In severe cases, roots may be seen actually growing above ground and rooting into the thatch layer. This is a very unhealthy condition and leaves the lawn vulnerable to many stresses.