Jos
From UC Davis
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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, 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. Some turfgrass species, such as tall fescue and perennial ryegrass, do not produce much thatch. Other turfgrass species, such as bermudagrass, bentgrass, and Kentucky bluegrass, have creeping growth habits and rapidly build thick thatch layers.
Tips for preventing thatch build up Follow proper fertilization practices; avoid excessive amounts of nitrogen. Avoid frequent and shallow irrigation on established lawns. Mow properly; remove clippings if too much of the grass is removed at one time. (should never remove more than 1/3 of the length at a time)