Thatch is a tightly intermingled layer of living and dead stems, leaves and roots which accumulates between the layer of actively growing grass and the soil underneath. Thatch is a normal component of actively growing turfgrass. As long as the thatch is not too thick, it can increase the resilience of the turf to heavy traffic. If your lawn has a bouncy feel to it when you walk on it, thatch is probably building up.
As excessive thatch layer (more than 1″) can restrict the movement of air, water, fertilizer and other materials to the root system of your grass. This air and water impervious layer causes the grass to restrict root growth development making it more susceptible to drought stress, insect infestations and diseases.