Thatch and Mat: Problems of Turfgrass

Curtis E. Swift, Ph.D.
Colorado State University Extension

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What is Thatch?

Thatch is an intermingled organic layer of dead and living shoots, stems and roots that develops between the green vegetation and the soil surface. Thatch has a high lignin content and resists microbial breakdown.

What is the difference between thatch and mat

Mat is thatch intermixed with mineral matter. Thatch may be transformed into a mat as a result of top dressings or when turf grown on a flood plain becomes covered with silt due to flooding. Like thatch, mat develops between the green vegetation and the soil surface. Mat develops on putting greens and other areas of turf that have been top dressed. Golfers often describe the feel of mat as a `cushion'.

Does thatch or mat cause problems?

• Thatch tends to shed water, preventing its infiltration and creating localized dry spots. This can be a serious problem especially on slopes and south and west exposures.
• Thatch and mat can minimize the movement of air and fertilizers into the soil layer weakening the turf and making it more susceptible to insect and disease problems.
• Many turfgrass disease-causing pathogens are weak parasites that exist in thatch and mat zones until environmental conditions are optimum for the rapid infection of the turf. Due to the alternate wetting and drying of thatch and mat, conditions are often ideal for these disease-causing organisms to produce spores and other infectious bodies and build up in the layer of thatch or mat. Diseases enhanced by thatch include the Helminthosporium diseases, Typhula blight, stripe smut, dollar spot and Rhizoctonia brown patch.
• Thatch and mat can provide an ideal environment for turf damaging insects such as white grubs and billbugs that feed beneath these layers and sod webworms that nest in the thatch layer. Thatch is also reported to attract chinch bugs. Thatch and mat may tie up certain insecticides, herbicides and fungicides rendering them ineffective.

Insects that Inhabit Thatch
Hairy Chinch Bug Blissus leucopterus hirtus Montandon	Southern Chinch Bug B. insularis Barber
Bluegrass webworm Parapediasia teterrella (Zinck.)	Corn root webworm Crambus caliginosellus Chem.
Larger sod webworm Pediasis trisecta (Wlk.)	Pretty crambus Microcrambus elegans (Clem.)
Silver barred lawn moth Crambus sperryellus Klots	Striped sod webworm Fissicrambus mutabilis (Clem.)
Black cutworm Agrotis ipsilon (Hufnagel)	Variegated cutworm Peridroma saucia (Hubmer)
Bronze cutworm Nephelodes minians Guenee	Armyworm Pseudaletia unipuncta (Haworth)
Fall Armyworm Spodoptera frugiperda (J.E. Smith)	Lawn Armyworm S. mauritia (Boisduval)
Fiery Skipper Hylephila phyleus (Drury)	Bluegrass weevils Hyperodes spp.

Thatch and mat increase the turfgrass's susceptibility to heat, cold and drought stress by encouraging the growth of the grass crowns, rhizomes and roots in these layers losing the protection of the soil. The more thatch and mat, the greater the tendency for scalping the turf.

Are there any benefits from thatch or mat?

A moderate level of one-quarter to one-half inch thickness has the following advantages:

• the grass is more tolerant of wear
• the soil is less susceptible to compaction due to the cushioning effect
• the cushioning effect of the thatch or mat layer results in increased safety to sports players
• the layer of thatch acts as a mulch preventing accelerated drying of the soil surface

Causes of Thatch and Mat

• Turfgrass species that develop thatch and mat are sod formers with a rapid growth rate. Bermuda grass, bentgrass and Kentucky bluegrass are the predominant thatch and mat formers in the Tri River Area. Thatch accumulates because the growth of the crowns and lateral stems exceeds their decomposition. Mat results from the mixing of soil to the thatch layer as discussed previously.

• Other factors that influence thatch and mat buildup include:
  • aggressive grass species such as the Kentucky bluegrass cultivars Baron, Birka, Bristol, Bonnieblue, Cheri, Galaxy, Glade, Nugget, Sydsport, Touchdown, and Victa.
  • soils with a pH above 7.2 or below 6
  • fine-textured (heavy) and salty soils such as the Billings silty-clay loam soil most of the Tri River Area is blessed with
  • excessive irrigation. This allows for shoot growth but prevents the development of those soil microbes responsible for the decomposition of thatch and mat
  • the use of fungicides and other pesticides that kill or impede the growth of soil microbes and earthworms
  • high nitrogen levels

Do grass clippings contribute to thatch?

In a word - NO!

Research shows that turf clippings are from 85 to 95 percent water. During the summer months these leaves decompose quickly leaving nitrogen and other beneficial nutrients for the turf. Clippings remaining on the lawn in the fall may not decompose until warmer weather returns in the spring.

Plant parts differ in cell wall content, with roots, rhizomes and stolons containing the greatest levels of lignin and are resistant to decomposition. Leaf blades contain cellulose and hemi-cellulose that are readily broken down by microbes. Grass clippings contain little lignin.

Grass clipping should be left on the lawn because:

• nitrogen and other nutrients in clippings are recycled into the lawn. An additional 1 to 2 pounds of nitrogen per 1000 square feet (.5 to 1 kg per 100 square meters), as well as supplemental potassium, will usually need to be added each season where the clippings are removed.
• the decomposition of clippings encourage beneficial earthworms and microbes responsible for the breakdown of thatch and mat
• bagging of clipping will be reduced or eliminated - clippings may be collected on occasion to add to your compost pile
• the volume of yard waste that typically ends up at the local landfill is reduced by 25% by leaving grass clippings on the lawn. This helps preserve vital landfill space

The removal of grass clippings to help control crown- or root-rot diseases is not effective, as collecting clippings has little effect on inoculum levels of such disease organisms.

How do I know if my lawn has a thatch or mat problem?

• The thickness of the thatch layer can increase quite quickly beyond the 1/2 inch thickness, making it difficult to control later on. Therefore it is important to check the thickness of thatch occasionally to determine the need for corrective procedures.
• Cut a plug of grass and soil several inches deep from the lawn, pull it out and examine it. A dense thatch or mat layer is easy to see between the crown of the grass plants and the top of the soil. Measure the accumulation from several areas in the turf to determine the extend of the problem. If this layer exceeds one-half inch, you should consider measures to reduce this layer.

What can be done about thatch?

The following steps can be taken to help reduce thatch accumulation. Keep in mind that reducing thatch is a long-term process and must be accomplished over several years. Thatch and mat prevention and correction is not an occasional practice. It requires proper turf management to correct the balance between accumulation and decomposition of thatch.

• Follow proper cultural practices to include:
  • proper mowing frequency and height of cut
  • apply fertilizers at a rate and frequency designed to provide the nutritional needs of the grass without stimulating excessive growth
  • Avoid the use of pesticides, especially insecticides that harm earthworms and fungicides that will reduce soil microbe populations.
• encourage earthworm activity
• if applying a soil as a top dressing, be sure it is comparable to the underlying soil
• aerate the lawn with a device that pulls a plug of thatch and soil two to three inches long (the longer, the better!). A single aeration using a machine equipped with 1/2 inch diameter tines will remove about 10% of the thatch from a lawn if enough passes are made over the lawn to result in an average two (2) inch spacing between holes. The improved air, water and fertilizer penetration will benefit the soil microbes. Aerating also mixes microbes from underlying soil into the thatch or mat layer speeding up the decomposition process.
  • The cores can be allowed to disintegrate and filter back into the lawn. Dragging a piece of chain link fence over the lawn or running over the cores with a rotary lawn mower will speed up the decomposition of the cores. The cores can also be vacuumed up and composted for use later as a mulch.
  • Aerating should be avoided in the hot summer months. Spring and fall are the preferred times to aerate a lawn.
• studies have demonstrated that different cultivars of Kentucky bluegrass differ significantly in their tendency to develop thatch. The use of a blend of cultivars could be used as a means for reducing thatch accumulation when a lawn is to be reseeded.

Should you dethatch your lawn?

One researcher answers this question by asking how much weight you gained during the winter. Vargas suggests that if you gained a lot of weight, the exercise of running the power rake might be good for you. But, if you are out of shape, it might kill you. He continues by explaining that this process does little but remove winter-killed grass that will breakdown anyway, but you will not remove the thatch or correct a thatch problem.

Gardeners who hand rake or power rake in the spring normally only collect the grass clippings that did not decompose the previous fall. A power rake that is set to cut deep enough to remove thatch is often quite injurious and disruptive to the turf. Core aerating while removing less thatch causes less problems.

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References Used:

Ali, A.D. & C.L. Elmore. 1989. Turfgrass Pests. Extension, University of California, Division of Agriculture and Natural Resources.
Koski, A. 1995. Turfgrass Management - Master Gardener Training. Colorado State University Extension.
Beard, J.B. 1986. Thatch. Grounds Maintenance, November, pages 36-40
Couch, H.B. 1995. Diseases of Turfgrasses: Third Edition. Krieger Publishing Company, Malabar, Florida.
Erusha, K.S. & T. P. Riordan. Thatch Prevention and Control. 1990. NebGuide G85-751, Extension, University of Nebraska-Lincoln
Koski, A. and C. Wilson. E.A.S.Y.* Lawn Mowing: Environmental Action Starts in your Yard. Colorado State University Extension and United States Environmental Protection Agency, Region VIII, Denver Colorado.
Shearman, R.C., A.H. Bruneau, E.J. Kinbacher, and T.P. Riordan.Thatch Accumulation in Kentucky Bluegrass Cultivars and Blends. 1983. HortScience 18(1):97-99.
Shurtleff, M.C., T.W. Fermanian & R. Randell. 1987. Controlling Turfgrass Pests. Prentice-Hall, Inc. Englewood Cliffs, N.J.
Tashiro, H. 1987. Turfgrass Insects of the United State and Canada. Cornell University Press.
Vargas, J.M., Jr. 1994. Management of Turfgrass Diseases; Second Edition. Lewis Publishers

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Placed on the Internet August 26, 1996
Updated May 25, 2009