Thatch is the layer of living and dead stems and roots between the green blades of grass and the soil surface. This residue is bad for your grass, as it keeps water and nutrients from seeping down to grass roots.  A thin layer of thatch (less than 1/2 inch thick) can be beneficial to the lawn because it helps to limit weed germination, reduce water evaporation, and protect from frost damage. However, thick thatch layers can prevent water, air, and nutrients from penetrating the soil, causing reduced root growth and increased potential for drought stress. Thatch also favors fungal growth and can harbor insect pests.  This service is available in spring and fall and can be combined with aeration and reseeding for a much better benefit.