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.
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.
- Thatchy turfgrass species. Some turfgrass species produce a lot 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.
- Applications of certain pesticides sometimes cause thatch build up. Multiple applications of some fungicides over a 2- to 3-year period 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.