Berks County Cooperative Extension

TURFGRASS - THATCH

What is thatch?

Thatch is a layer of partially decomposed stems, leaf sheaths and roots of grasses that forms between the actively growing grass leaves and the soil.

Thatch formation.

The following factors contribute to thatch formation:

Use of vigorously growing turfgrass species or species with tissue that decays slowly. Some new species of Kentucky bluegrass, for example, developed for high traffic areas, grow more vigorously than common Kentucky bluegrass species and thus are more likely to form thatch. Fine fescue species, whose growth rate is less than Kentucky bluegrass, form thatch because their tissue is more fibrous and decay resistant.
Excessive amounts of nitrogen fertilizer that over stimulate leaf and stem growth. The effect of this factor may be minimized by following recommendations contained in a soil test.
Infrequent mowing or setting the cutting height too high. Both of these practices channel growth into plant stems and other parts that resist decay.
Overwatering. Excessive amounts of moisture fuel unnecessary vegetative growth that contributes to thatch.

Thatch prevention.

Aside from addressing the factors that contribute to thatch formation as part of a thatch prevention program, consideration should be given to optimizing conditions for decomposition of thatch. Thatch is broken down by microorganisms, insects, earthworms and other organisms found in the soil. Maintaining a pH level of 6.5 to 7.0 provides an environment in which these allies in the struggle against thatch will flourish. Eliminating or limiting the use of pesticides and fungicides that may destroy or reduce the number of microorganisms, insects, earthworms and other organisms is also recommended. Chemicals should be used only after a particular problem has been thoroughly diagnosed and the need for application of chemicals has been clearly established.

Aeration is a thatch prevention measure that incidentally provides other benefits to the lawn by relieving soil compaction. Compacted soil impedes the infiltration of water into the soil and the exchange of gasses between the soil and the atmosphere. Compacted soil is associated with accelerated thatch formation. In addition to fostering improved water and air penetration to the soil, aeration operates to strengthen root systems and to improve the environment for thatch decomposing agents such as microorganisms.

Aeration is best accomplished by using a aerifier, a self-propelled machine with hollow tines or spoons that pierce the soil and return cores of soil to the surface. When using this machine, it is best to navigate the lawn in several different directions to maximize its effect. Aerifiers should not be confused with solid tine spikers that actually increase soil compaction.

Aeration should be undertaken in the spring or fall. It should not be performed in the summer when the cool-season grasses typically planted in this region are in a semi-dormant state. The general rule is to aerate only when turfgrass is growing vigorously.

Evaluating a lawn for presence of thatch.

To determine the degree of thatch in a lawn, use a sharp trowel or large knife to carefully remove a two-inch, pie-shaped segment from a little trafficked area of the lawn. Measure the width of the layer between the soil and the green vegetation. (See the image at the top of the page.) If the layer is one-half inch or less, the accumulation most likely can be managed and will not adversely affect the health of the turfgrass. If the layer exceeds one-half inch, intensification of thatch prevention measures or thatch removal should be considered.

Removing thatch.

Thatch removal efforts should be undertaken only when conditions favor rapid recovery of turfgrass. Spring and late-summer/early fall are the best times to remove thatch. While several machines will remove thatch, a vertical mower (also referred to as a power rake) is the preferred device. Unlike rotary lawn mowers which drive a blade that is on a horizontal plane to the surface, vertical mowers propel a series of knife-like tines on a plane that is perpendicular to the surface. The tines should be adjusted so that a bit of soil is brought to the surface together with the thatch.

De-thatching using a properly adjusted vertical mower will produce an astounding quantity of stems and other plant parts which must be removed from the lawn. Before running the vertical mower over a lawn, consideration must be given to disposition of the debris. While the debris may be composted or used as mulch in vegetable or flower gardens, most thatch removal efforts will require that some of the debris be disposed of off site. After the removal process has been completed, a light application of fertilizer and irrigation of the lawn will hasten recovery of the lawn from shock.

So-called "power rake" or similarly styled attachments for rotary mowers are not effective to remove thatch. Renting a vertical mower or arranging with a contractor for thatch removal are recommended. If the services of a contactor are engaged, there should be a clear understanding as to responsibility for debris removal.

Sources: The Penn State Master Gardener Manual
Taylor, Thatch Control in Lawns and Turf, http://www.mes.umn.edu/Documents/D/G/DG1123.html (December 5, 1999)
Document created: 15 December 1999; Revised: 3 March 2001