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 on 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.
Following proper guidelines for mowing, fertilization and irrigation will help keep thatch accumulation to a minimum. Mow at the proper mowing heights and follow the 1/3 rule to keep the grass from becoming stressed. Fertilization to avoid excessive growth also is important in preventing thatch buildup. Never apply more than 1 lb of nitrogen per 1000 ft² at any one time, especially in the spring when the grass is growing vigorously. Irrigate to encourage deep rooting will also keep thatch to a minimum. Remember to water ‘deep and infrequent’ for best results.
Power Raking is commonly used to remove excessive layers of thatch, but do so with caution. Deep power raking can cause excessive damage to a lawn and even remove large amounts of living grass. Frequent, shallow power raking may be more beneficial.
Core Cultivation/Aeration is the preferred method for managing thatch. Aeration involves using a machine with hollow tines that penetrate the lawn and remove soil cores. The benefits of core cultivation include relief of compaction, improved air movement into the soil, improved water infiltration and improved root growth. Additionally, the soil cores, if left on the surface, will mix with and help break down thatch. Make sure the ground is moist before aerating to ensure maximum depth penetration of the tines. Dragging the cores with a piece of chain-link fence helps break and mix them into the lawn. Soil cores can also be broken up with a rotary mower. Over time and with irrigation, the cores will wash into the lawn.
When to Aerate and How Often? Most home lawns should be core cultivated at least once each year. The best time of the year is either in the fall or the spring when soil temperatures are ideal for root growth. Fall is the preferred time since the aerification holes will not be exposed to excessively hot temperatures during the summer and any weed seeds that were exposed with the soil cores are less likely to compete with the grass. Do not core cultivate during the summer due to excessive heat and drying.
The University of Idaho publication, Thatch Prevention and Control in Home Lawns, contains additional information and can be downloaded in PDF format.