Managing Thatch in Lawns

Thatch is a loose, intermingled organic layer of dead and living shoots, stems, and roots that develops between the zone of green vegetation and the soil surface. Thatch build up begins when turf produces organic debris faster than it can be broken down. Those parts of grass plants that are the most resistant to decay -- stem nodes, crowns, fibers of vascular tissues, and roots -- make up the bulk of thatch. Although you'll find some leaf clippings in the thatch layer, they usually don't contribute to thatch build up because soil microorganisms easily break them down.

Figure 1. Thatch development in Kentucky bluegrass (Photo:Peter Landschoot, Penn State).

About 25 percent of thatch is made up of a compound called lignin. Lignin is resistant to decay by microorganisms and one of the main reasons why thatch builds up faster than it breaks down. The remainder of thatch is primarily cellulose and hemicellulose compounds that decompose more easily. Turfgrasses that generate more lignin-containing tissues are generally high thatch producers.

The Pros and Cons of Thatch

Thatch can have either beneficial or detrimental effects on turf depending on the amount present. For example, a thin layer of thatch in lawns (½ inch or less) provides insulation against temperature extremes and fluctuations in soil moisture. Some thatch on sports turf (about ½ inch) provides much needed resiliency, softens players' impact on the surface, and improves footing. However, if your lawn has more than an inch of thatch, turf problems will likely result.

Extensive root development often occurs in thick thatch layers. Because thatch can heat up and dry out quickly, these root systems are vulnerable to desiccation. In contrast, wet thatch holds excess water during rainy periods, resulting in reduced oxygen to turf roots.

Mower scalping is another significant problem in turf with thick thatch layers. Scalping results when mower wheels sink into thatch allowing the deck to ride on the turf, effectively lowering the height of cut. Also, the crowns of grass plants growing in the thatch layer tend to be elevated above the soil, making them more susceptible to scalping.

Excess thatch can increase pest problems by harboring large populations of disease-causing organisms and insects. Some fungicides and insecticides are bound in thatch, thereby reducing effectiveness and preventing movement into the soil.

The Culprits behind Thatch Build up

• Thatch-producing turfgrass species. Some turfgrass species produce large amounts of stem tissue as rhizomes or stolons. Consequently, these species form more thatch than others. The thatch forming cool-season grasses are Kentucky bluegrass, creeping red fescue, and creeping bentgrass. Perennial ryegrass and tall fescue are primarily bunch-type grasses and do not produce significant thatch build up.
• Acid soils and reduced microbe activity. Thatch accumulates when soil conditions don't sustain high populations of thatch-decomposing organisms. Acid soils (pH 5.5 or lower) generally inhibit these microorganisms, resulting in rapid thatch build up. Soils that contain large amounts of clay or sand may contain low populations of soil microorganisms that decompose thatch. Compacted soils and soils with poor structure also contain low microbial activity and are subject to thatch build up.
• Pesticides. Applications of certain pesticides sometimes result in thatch build up. Applications of certain fungicides over multiple years can increase thatch compared to untreated areas. These fungicides promote thatch by increasing the rate of turf root and rhizome production rather than by suppressing soil microorganisms. It is unlikely that significant thatch build up results from occasional applications of fungicides. Certain insecticides influence thatch development through their affects on earthworms. As earthworms burrow, they mix soil and castings with thatch, thereby stimulating microbial activity and breakdown of the thatch. Studies have shown that applications of certain insecticides can significantly reduce earthworm populations resulting in reduced thatch breakdown.
• Aggressive fertilization. Thatch can accumulate quickly following aggressive nitrogen fertilizer programs. Nitrogen promotes thatch development because it increases production of root and stem tissues. The ability of some nitrogen sources to acidify soils also may reduce the rate of thatch breakdown.

Managing Thatch

A good thatch management program requires preventing thatch build up as well as physical removal. A preventative program centers on appropriate species and cultivar selection, liming, core aeration, and proper pesticide and fertilizer use. Physical removal can be accomplished with vertical mowers or other thatch removal equipment.

Thatch prevention begins with your choice of grasses. Although it may be tempting to seed new lawns with very aggressive, high density Kentucky bluegrass cultivars, they may produce more thatch than less aggressive cultivars. Very aggressive bluegrass cultivars are better for use in seeding heavily trafficked sports turf or in blends with less aggressive cultivars. Mixing some perennial ryegrass with Kentucky bluegrass or using perennial ryegrass or tall fescue alone will cut down on thatch removal operations.

Take soil tests periodically to monitor nutrient and pH levels. Most soil test labs provide lime requirement recommendations to raise soil pH to levels that support good turf growth and microbial populations for thatch decomposition.

Core aeration is an important preventative maintenance practice for thatch control. Regular core aeration alleviates compaction and improves air movement into soils, thereby increasing microbial activity and breakdown of thatch. This practice also physically removes some thatch.

Fertilization should be adequate for good turf growth, density, and green-up; but it should not be so heavy that thatch builds up at a rapid rate. Also, only make fungicide and insecticide applications when you need to control pests. Using pesticides that do not severely reduce earthworm populations will improve the chances for a faster rate of thatch breakdown.

Although some homeowners and turf managers believe clippings contribute to thatch, little evidence supports this theory. However, if a thick thatch layer already exists, clippings may be visible in the upper thatch layer. This may be due to the inability of decomposing microbes to persist in the upper thatch layer where clippings accumulate.

Typically, when thatch layers reach an inch or more, preventative practices won't provide adequate control. In such cases, thatch must be physically removed. Use vertical mowers with blades that are perpendicular to the turf surface (Fig. 2). As the unit moves over the turf, the rotating blades slice into the thatch, bringing up organic debris. You can adjust the blades on vertical mowers to different depths, allowing you to regulate how much thatch you remove.

Figure 2. Vertical mowers with blades that are perpendicular to the turf surface are suitable for removing thatch. (Photos: Peter Landschoot, Penn State)

You also can use power rakes for thatch removal. Power rakes have vertically rotating wire tines that remove thatch. These are better suited to smaller jobs because they do not have the power and durability of vertical mowers.

Before you begin a thatch-removal operation, try to obtain some experience with the vertical mower in an area that is not in a high visibility area. Removing more thatch than desired can severely thin the stand. You can remove more or less thatch depending on the spacing between blades, the depth setting of blades, and the power source. In some cases, it may be necessary to set the blades high at first and only remove a small amount of thatch in the first pass. This is a safer approach because it allows you to assess the amount of thatch remaining and the degree of turf thinning before making a second pass.

The amount of thatch you remove in a single operation depends on several factors: the depth of the thatch, whether you plan to seed the site after removing thatch, and the turf's condition. If the turf has a heavy accumulation of thatch (two inches or more), it is almost impossible to reduce it to a desirable level without severely thinning the turf. In these situations, most of the root system is growing in the thatch layer. As you remove the thatch, you also remove many live turfgrass plants. Thus, you need to make provisions to seed the thinned turf.

Don't remove thatch when turf is weak or under heat or drought stress. Doing so increases the chance of injury and decreases recovery potential. Remove thatch from cool-season turfgrass stands during late summer/early fall. Temperatures are usually cooler at this time, reducing the chances of serious turf injury. You can perform late summer/early fall thatch removal in conjunction with renovation practices such as aeration, fertilization, and seeding. Also, weed encroachment is not as likely to occur with late summer/fall thatch removal as it is with spring removal.

Prepared by Peter Landschoot, Professor of Turfgrass Science.