To measure the thickness of the thatch layer in your lawn carefully poke a pencil down into it until the tip touches the soil beneath. Grip the pencil with your thumb at the point at the top of the thatch layer, then withdraw the pencil from the soil and measure the space from its tip to your thumb. If the space exceeds ¼ to ½ inch, plan to take some remedial action. Do this in the spring or fall when the live grass is less likely to be stressed by heat and drought and can recover from the dethatching process more quickly.
Tip: Research indicates that thatch naturally decomposes more rapidly when the humidity is high, so southern grasses regularly moistened by sprinkler systems may be slower to develop thatch.