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.

Causes of thatch
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 may speed it formation.

Damage caused by thatch
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.
•     Binds or ties up pesticides.

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, especially in bluegrass lawns.  Lawns with a thick thatch layer may appear healthy in the spring, and 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.

Preventing thatch
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:
•     Fertilize moderately and regularly to maintain vigor without excessive growth.
•     Cut grass regularly with a mulching mower 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.
•     Collect and remove clippings once a thatch layer has begun to develop to avoid further buildup.
•     Irrigate every seven days, or as needed in dry periods, to encourage deep rooting.
•     Power rake as needed to keep thatch below ½ inch thick.  For bluegrass, early fall is preferred; for zoysia and bermudagrass, midsummer.
•     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.
•     Topdress every one or two years with ¼ inch of weed-free manure or soil, similar in texture to the existing soil, to encourage decay of thatch.
•     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.

Removing thatch
 
Examine the lawn closely regardless of how healthy it appears.  Cut several plugs 2 or 3 inches deep.  Lift and examine the profile.  If thatch is present, it will appear as a distinct horizontal  layer of brown spongy or felt-like material.
When about ½ inch of thatch develops in bluegrass, remove it before the grass is damaged.   Thatch is best removed by power raking, verticutting and coring machines that may be hired or rented.  Coring causes less lawn stress than dethatching with a power rake or vertical mower and provides the additional benefit of reducing soil compaction.

Lawns should be dethatched only when conditions favor rapid turf recovery.  For cool-season lawns, early fall dethatching is most desirable.  Mid-June is preferred for zoysiagrass and bermudagrass but only when lawns are actively growing.  Weak zoysia lawns with heavy thatch (1 inch or more) may not fully recover in one year from intense mechanical dethatching.  Intensive coring causes much less damage to existing turf than power raking or verticutting.

In severe situations, removal of thatch by mechanical means also removes most of the living grass.  In such cases, moderate treatments over two or three years may be more desirable than attempting complete removal in a single operation.

Necessary lawn improvement practices should be done after dethatching or aerifying.  Such operations may be done on the same day, and may include topdressing, reseeding, fertilizing, liming and irrigating.
Brad Fresenburg, Division of Plant Sciences, University of MO. Extension