What is thatch and what contributes to thatch buildup?

Thatch buildup in fescue lawns is the accumulation of grass clippings that have not been bagged or raked and disposed of. Under appropriate conditions, grass clippings can support and protect the turfgrass plant. However, if there is too much buildup of clippings, it can adversely affect the growth and survival of the turf since it may act as a barrier to prevent water, light, nutrients, fertilizer and pesticides from reaching the soil layer.

Lawns which have a thick thatch layer are also prone to more disease and insect problems. Insects and disease organisms are protected over winter in the thatch layer. If the thatch layer becomes too thick, it binds up pesticides which might otherwise kill destructive insects and pathogens.