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Thatch can be a lawn's worst enemy if not controlled

Thatch is a layer of living and dead roots, crowns and lower shoots that often develops in lawns. It can weaken and even destroy a lawn if not prevented or removed.

Factors favorable to thatch development include excessive growth and conditions unfavorable to the microorganisms that decompose decaying plant parts. Rapid and excessive growth is likely to produce a heavy thatch because plant material is being produced more rapidly than it can be decomposed.

Grass clippings from mowing do not contribute to thatch. However, once a thatch layer has developed, clippings further speed its formation. Thatch buildup varies among lawns. Some lawns never develop a thatch layer, while others become thatch-bound within a few years after being established. The best lawn grasses are those that constantly reproduce new plants to renew the lawn. As old plants age and die they decompose into fine-textured humus that becomes a part of the surface soil.

Once thatch starts to form, conditions develop that may favor even more thatch.

Accumulated thatch:

  • Harbors disease-causing fungi and insects.

  • Prolongs high humidity, which favors disease.

  • Causes shallow root development.

  • Retards movement of air, water and nutrients into the soil.

    These factors contribute to early death of grass plants. Thus, thatch is both a result of unfavorable conditions and a cause of further damaging influences.

    Thatch development may go unnoticed in early stages. Lawns with a thick thatch layer may appear healthy in spring, then suddenly die in large patches during summer heat and drought. As thatch builds up, the roots of new grass plants grow within the thatch layer rather than in the soil. When the lawn is exposed to hot, dry summer weather, the plants are unable to survive. Zoysia and Bermuda grass lawns usually develop thatch layers rapidly, but seldom die suddenly because these warm-season grasses are more tolerant of heat and drought. Severe thatch usually leads to thin, diseased turf. Or very thick layers of thatch may cause uneven and difficult mowing. Thatch may develop over several years before noticeable damage occurs. Good cultural practices, starting when the lawn is new, may not prevent it indefinitely but can retard its formation.

    Desirable cultural practices are:

    1. Fertilize moderately and regularly to maintain vigor without excessive growth.

    2. Cut grass regularly at the recommended height to maintain vigor and to avoid shock. Remove excessive clippings, especially during periods of rapid growth. Clippings may be left to decompose if mowing occurs at regular intervals. No more than one-third of the leaf tissue should be removed with each mowing. Remove clippings that accumulate on the surface. Nutrients are recycled to turf as clippings that filter into the turf canopy decompose.

    3. Collect and remove clippings once a thatch layer has begun to develop to avoid further buildup.

    4. Irrigate every seven days, or as needed in dry periods, to encourage deep rooting.

    5. Power rake as needed to keep thatch below 1/2 inch thick. For fescue, early fall is preferred; for summer grasses, like Zoysia and Bermuda grass, midsummer.

    6. Core aerify to improve penetration of water and fertilizer. Leave soil cores on the surface to dry and crumble before mowing. Mowing the dried soil cores will redistribute the soil microbes that decompose soil and thatch and will aid in reducing thatch.

    7. Topdress every one or two years with 1/4-inch of weed-free manure or soil, similar in texture to the existing soil, to encourage decay of thatch.

    8. Avoid indiscriminate use of pesticides that damage earthworms. Earthworms naturally reduce thatch as they collect it from the surface and mix it deeper into the soil.

    Web posted on Thursday, June 9, 2005

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