Thatch is the accumulation of partially decomposed rhizomes, shoots and roots that develop in a layer above the soil line. Under appropriate conditions, it can support and protect the lawn. However, too thick a thatch layer can adversely affect the growth and survival of the lawn since it may act as a barrier to prevent water, light, nutrients, fertilizer and pesticides from reaching the soil layer.
Thatch accumulation may be attributed to numerous improper maintenance practices such as over-fertilization, improper irrigation practices and improper mowing habits. 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.