Thatch and Its Control

Revision Date: 10/1/1995

Susan Barton, Ornamental Horticulture
Specialist III
Jay Windsor, Agricultural Agent
HYG-41 
 

Thatch is a light brown layer of dead and living shoots, stems and roots. It is located between the green vegetation and soil surface of a lawn. Because these plant tissues are extremely tough, thatch decomposes very slowly. Leaf blade tissue decomposes rapidly, so grass clippings are rarely part of a thatch layer.

Is thatch good or bad? The answer to this question is not a simple yes or no. If a thatch layer is thin (less than 1/2 inch), it can be beneficial.

 

 

 

 

 

 

 

 

 

 

 

Benefits of a thin thatch layer:

  • Acts as a cushion that contributes to the wearability of a lawn and provides a soft, resilient surface for traffic and play.
  • Acts as a mulch that retains soil moisture by reducing bare soil evaporation.
  • Decreases weed seed germination by keeping weed seeds from coming in contact with the soil.
  • Acts as an insulator to reduce temperature extremes. A thick layer of thatch (greater than 1/2 inch) can cause variety of problems that can lead to the deterioration of turf.

Problems caused by a thick layer:

  • Harbors disease organisms and insects.
  • Ties up insecticides, greatly reducing their effectiveness.
  • Pre-emergent herbicides that normally remain on the surface are more mobile in thatch. They move into the thatch layer and can damage established turfgrass roots.
  • Roots confined to the thatch layer are subject to the stresses of temperature extremes and drought.
  • Pre-emergent herbicides that normally remain on the surface are more mobile in thatch. They move into the thatch layer and can damage established turfgrass roots.
  • Dry thatch is difficult to rewet.
  • Nutrients are not available in thick thatch layers. Some forms of nutrients are more volatile in thatch and are lost into the atmosphere. Others pass through the thatch before they can be absorbed by the roots.

Thatch buildup: Thatch develops when the production of new plant tissue is greater than the rate of decomposition. Thus, any factor that influences grass growth rate or organic matter decomposition affects thatch accumulation. These factors include:

  • Species and varieties of turfgrasses which vary in growth rate and decomposition.
  • Presence of an adequate microbial population for decomposition.
  • Environment.
  • Management practices.

 

Turfgrass species and varieties differ in both vigor and method of spread. Species that spread by stolons and rhizomes (many of the warm-season grasses) produce more plant tissue that is difficult to decompose and therefore contribute to thatch. Cultivars of Kentucky bluegrass, tall fescue and creeping bentgrass differ in vigor and in their contribution to thatch.

Thatch Accumulation for Kentucky Bluegrass, Tall Fescue, and Creeping Bentgrass Cultivars.*

* reprinted from June 1985/SEEDS, TREES & TURF

 

Soil microorganisms decompose organic matter in the soil, so any condition which decreases the population of microorganisms contributes to thatch. Pesticides decrease microbial populations. Earthworms are active in organic matter decomposition and some insecticides are lethal to earthworms. Fungicides not only reduce the population of beneficial microorganisms, but they also encourage vigorous turf growth by controlling turfgrass diseases.

Environmental factors can decrease the microbial population and therefore increase thatch. The optimum pH for most soil microorganisms is 5.0 to 7.0. If the pH falls below 6.0, microorganisms are unable to decompose plant tissue.

Compact, poorly drained soils 1ack the oxygen necessary for these microorganisms. Extreme high temperatures and dry conditions also decrease microbial population. Soils with very low organic content can't support an active microorganism population.

High maintenance programs that stimulate rapid growth contribute to thatch buildup. Excess fertilization, irrigation and chemical applications are all contributors. It sounds ironic, but the higher the maintenance level the greater the likelihood of a thatch problem.

It is often assumed that grass clippings contribute to thatch. In fact, these clippings decompose rapidly and do not usually affect thatch accumulation. However, if a thick layer of thatch builds up and clippings do not come in contact with the microorganisms in the soil, they may contribute to the thatch layer.

Thatch control: Thatch builds up slowly, and the problem can only be corrected over a period of time. The best control is to avoid a buildup in the first place. Cut the grass at recommended heights on a regular schedule to eliminate dumping excessive leaf and sheath tissues on the turf at one time. Apply nitrogen in the fall only, and reduce applications if growth is excessive. Add lime to increase soil pH and create a good environment for microbial activity. Mechanical cultivation which brings soil to the surface and reduces compaction also helps control thatch.

If thatch is already a problem, the best solution is core cultivation. Use a machine which removes plugs of soil and brings them to the surface. Follow core cultivation with a topdressing (a thin layer--1/4 inch to 1/2 inch--or a soil mix spread over the lawn surface). Dethatching machines and power rakes will not penetrate a thick thatch layer and therefore do little to correct an existing thatch problem. Remember, the important thing is to bring some soil to the surface and break through the thatch layer at regular intervals throughout the lawn. Do this in the spring or fall when grass is actively growing.

If a thatch layer is several inches thick, complete renovation is the only solution. Use a sod cutter to remove the grass and the thatch, acid soil to bring the level , and reseed. Choose a turgrass species which does not have a tendency to thatch, and follow routine maintenance procedures to prevent future thatch buildup.

Should you be concerned about thatch? If your lawn is intensively maintained, consider how your management practices affect thatch buildup. Careful planning will help you avoid thatch before it becomes a problem. Many lawns maintained at a relatively low level do not develop thatch. Look at your lawn and put thatch control strategies into operation if necessary.



Original Publication Date:

Cooperative Extension Education in Agriculture and Home Economics, University of Delaware, Delaware State University and the United States Department of Agriculture cooperating. Distributed in furtherance of Acts of Congress of May 8 and June 30, 1914. It is the policy of the Delaware Cooperative Extension System that no person shall be subjected to discrimination on the grounds of race, color, sex, disability, age, or national origin.

Disclaimer: Reference to commercial products or trade names does not imply endorsement by University of Delaware Cooperative Extension or bias against those not mentioned.

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