Thatch is the layer of living and dead stems, roots, stolons, and rhizomes between the green blades of grass and the soil surface. 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. 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. Some turfgrass species, do not produce much thatch. Other turfgrass species, such as Kentucky bluegrass, have creeping growth habits and rapidly build thick thatch layers.