Thatch is not caused by leaving grass clippings on the lawn. Instead, thatch is an indication that dead plant stems and roots are not being broken down as quickly as new growth is taking place.

Dethatching involves the removal of the matted layer of dead and decaying plant material between growing grass and the soil, known as thatch.

If your lawn is thick, rich and deep green, and you can't see bare ground between the plants but instead see matted, fibrous material, it's time to dethatch before new growth becomes inhibited.

If your lawn is off-color and thinning, and you can't see bare ground between the plants but instead see matted, fibrous material, it's time to dethatch. (If you can see bare ground, your lawn needs aeration.)