Thatch is the build-up of undecomposed, fibrous roots, stolons, and rhizomes just above the soil surface. It is not build-up of undecayed grass clippings, nor is it caused from the clippings left on the lawn. Thatch results from an improper balance of plant growth and decomposition of old grass parts. grass clippings decay rapidly, but old stems and stolons break down slowly. Heavy thatch prevents air, water and fertilizers from penetrating the soil. Lawns with heavy thatch problems are difficult to keep wet because they dry out quickly. During drought and heat these lawns will suffer the most.
To determine if you have a thatch problem, use a spade and remove a six inch deep section of your lawn. Distinguish the thatch layer from the soil and living plant material. Thatch will look like a spongy layer of peat moss, but will have live grass roots penetrating it. Measure the depth of the thatch. One half inch or less is beneficail, because it cushions the turf from wear and tear, insulates the soil from heat and cold, and holds in moisture. Thatch that is over one half inch prevents air, water and fertilizer from penetrating the soil. The grass roots may not actually grow much past the thatch layer, so they don’t penetrate the soil to reach nutrients and moisture.