Thatch is a layer of dead organic matter below the green growing part of the grass and above the soil. In excess it can form a dense, brown spongy layer that impedes water and nutrient movement into the soil. Thatch accumulation on lawns is a natural process that occurs as stems and roots die and are slowly broken down by soil organisms. Small layers of thatch less than one-half inch are acceptable. When managed properly, lawns may not accumulate excessive amounts of thatch. The problem comes when certain management practices, especially fertilization, are done incorrectly causing an increase in organic matter production. Some management practices, such as irrigation, if mismanaged can negatively influence the soil environment where the organisms that break down thatch live. Differences in growth habit among grass species also affect the rate of thatch accumulation. Spreading type grasses like Kentucky bluegrass are more prone to thatch accumulation because of their vigorous rhizomes, whereas bunch-type grass like perennial ryegrass and tall fescue are not as prone to thatch accumulation.
Contrary to popular belief, grass clippings do not contribute to thatch accumulation and should be returned to the lawn. See the bulletin, Don’t Bag It! for more information onl what to do with lawn grass clippings.
Excessive thatch (more than one-half inch) is detrimental to the health of a lawn for many reasons. Thatch has poor water holding capacity, does not buffer temperatures well and impedes water and nutrients from entering the soil. A thick thatch layer soon becomes the growing medium for roots and growing points of the grass instead of the soil. When this happens, grass is much more prone to heat and drought stress as well as more susceptible to disease and insect damage.