Thatch is a layer of dead grass, leaves, and roots that accumulates on the top of the soil and the bottom of live grass. If kept under half an inch, thatch helps promote a healthy lawn. At this reasonable level, it may operate as a layer of protection for grass, increase moisture retention, and subtract from a lawn’s health by stopping nutrients from reaching the roots, attracting insects and other problems, and making it less robust under tough circumstances.
When circumstances are favorable for development, the optimum times to dethatch are in the spring and fall.