Thatch problems Excessive thatch can create an unfavorable growth environment in several ways. First, excessive thatch creates a favorable environment for many turfgrass insect pests and diseases. Turf insect pests such as sod webworms and black cutworms often live in thick thatch. Several turf diseases, for instance summer patch or leaf spot, can cause greater problems in heavily thatched turf. Second, excessive thatch creates an unfavorable growth environment by reducing turfgrass cold, heat, and drought tolerance. Often, in thatchy areas, the majority of turf roots will grow in the thatch layer. Because this layer can change temperature and dry quickly, turf roots grow in an area that is colder, hotter, or drier than the underlying soil. This can result in poor root growth, which can lead to turf deterioration.
Randomly distributed dry spots can create another turf problem caused by excessive thatch. These dry spots can be difficult and expensive to properly irrigate. Furthermore, thatchy turf sometimes "puffs-up" when it dries during hot weather, making it prone to mower scalping and foot printing.
Finally, thatch can keep some pesticides from reaching their intended target, reducing pesticide performance. Certain turf insecticides (i.e., diazinon and chlorpyrifos) are especially prone to being tied up in thatch.

Thatch development A number of biological, cultural, and environmental factors can contribute to thatch production. Biologically, certain turfgrass species produce more thatch than others. Kentucky bluegrass and creeping red fescue spread by rhizomes, creeping bentgrass spreads by stolons, and zoysiagrass spreads by both stolons and rhizomes. These grasses can produce more thatch than perennial ryegrass or tall fescue which have bunch-type growth habits. Furthermore, it is the vigorously growing types within these species that produce the most thatch. For example, `Touchdown,' a vigorous Kentucky bluegrass cultivar, will generally produce more thatch than the less vigorous `Park' (see Table 2).

Table 2. Thatch producing Kentucky bluegrasses.

Culturally, heavy nitrogen fertilization and irrigation can also contribute to thatch buildup because they cause turf to grow vigorously and produce more organic matter. Environmentally, thatch is normally broken down by soil borne fungi and bacteria. Any soil condition that reduces the numbers of these organisms, e.g., a pH that is too acid or alkaline; heavy, wet soils; or the use of certain turf pesticides can slow the breakdown of thatch.
Contrary to popular notion, grass clippings contribute little to thatch buildup. Grass clippings are more than 80% water, and as they dry their weight and size are reduced greatly.

Preventing thatch It is easier, through proper establishment and cultural practices, to prevent thatch than to remove excessive thatch. When establishing turfgrass, select species or cultivars that are appropriate for the intended use, environment, and desired maintenance level. Select and establish turf species or cultivars that produce small quantities of thatch.
When existing turf produces large amounts of thatch, try to slow its production by altering present maintenance practices. Apply balanced fertilizers at rates and timing appropriate for the turfgrass being grown. Avoid excessive nitrogen application. Irrigate deeply and infrequently; avoid light, frequent watering.
Adjusting the soil pH to between 6.0 and 7.0 may allow soil bacteria and fungi populations to increase. Reducing the use of turf pesticides, especially fungicides and insecticides, can also increase microbe populations.

Dealing with thatchy turf Several cultural activities can help to reduce the amount of thatch in turf. One method is to physically remove the thatch with a garden rake or with a mechanical dethatcher, vertical mower, or power rake. Hand raking may not adequately remove large amounts of thatch. Mechanical methods use a series of spinning vertical blades or spring teeth that can be positioned into the thatch, pulling thatch from the turf. NOTE - Use caution when the majority of turfgrass rooting is in thick thatch as a great deal of live turf can be pulled out by mechanical dethatcher, leaving only a small amount of healthy turf.
Another cultural activity that may reduce thatch is topdressing the turf. Topdressing adds a thin layer of compatible soil (1/8" - 1/2") to the top of the turf. It is then raked, broomed, or dragged into the turf, making contact with the thatch. Microorganisms in the soil surrounding thatch will speed its breakdown.
An efficient method of thatch removal combines topdressing and core aerification. Allow the soil cores to air dry on the turf surface before crumbling and spreading them with a hand rake, broom, or by dragging a section of weighted chain link fence. Additionally, coring serves to mix soil into thatch which can also speed thatch breakdown.
Recently, several organic products have been marketed for thatch removal or reduction. Presently, the effectiveness of these products has not been studied at the University of Illinois. Any future tests involving these materials will be made public upon its conclusion.

Timing thatch removal The best time of the year to remove thatch, either by hand or mechanically, is when the turf is actively growing. Remove thatch in cool season grasses (Kentucky bluegrass, creeping red fescue, creeping bentgrass) in late summer/early fall or early spring. Late summer/early fall is preferred because weeds are more likely to invade a turf area and become a problem following an early spring dethatching. Late summer is also a good time to overseed cool season turfgrasses; overseeding can be combined with cultivation activities during that time. Remove thatch from zoysiagrass during late spring/early summer during active growth.