Controlling thatch

Thatch is a layer of organic material, or "residue," found directly above the soil surface in turf. Within a thatch layer, living roots and shoots intermingle with non-living fibrous components, which provides some measure of stability. This characteristic differentiates thatch from other organic residues in turf, which tend to be more loosely arranged.

Excessive thatch is undesirable because it may promote disease and insect problems, reduce tolerance to cold, heat and drought stresses, and cause an array of other problems--including scalping, puffiness, localized dry spot and chlorosis from nutritional deficiencies. Modest accumulations, however, may improve traffic tolerance and soil stability. Let's look at some of the effects of thatch--desirable and undesirable--as well as methods for controlling it.

Comparing thatch and soil * Effects on plant growth. Thatch influences the distribution of plant organs within the turf-soil profile. Compare the thatch-free and thatchy Kentucky bluegrass turfs in Figures 1 and 2 (page Golf 56). In the thatch-free turf, the crowns at the base of aerial shoots are situated at or slightly below the soil surface. Roots emerging from the lower portions of the crowns grow directly into the soil. Rhizomes developing from axillary buds along the sides of the crowns also grow directly into the soil and emerge to form daughter plants some distance from the mother plant. Thus, the soil completely supports both root and lateral shoot growth.

In thatchy turf, emerging roots and rhizomes grow, at least initially, into the organic residues comprising the thatch. If a substantial thatch accumulation exists, these organs may be almost entirely confined to the thatch layer, with little growth into soil.

* Effects on soil physical qualities. Studies with Kentucky bluegrass turf comparing the soil underlying thatch with surface soil from adjacent thatch-free plots showed that the presence of thatch was associated with higher soil bulk densities in the underlying soil (see Figure 3, page Golf 56). This was attributed to the absence of root and rhizome growth in the soil and the suppression of earthworms by pesticides. (Pesticides were deliberately applied to reduce earthworm populations, which is what promoted the development of the thatch layer.)

The more-compacted soil from the thatchy turf also showed significant reductions in water infiltration, hydraulic conductivity, stored water, organic matter and shrinkage on drying compared with the less-compacted soil from the thatch-free turf. Thus, thatch formation may, in some instances, be associated with undesirable physical properties of the underlying soil. This is presumably due to a lack of soil pores that would be created by root and rhizome growth (as well as suppression of earthworms).

* Effects on water movement. Studies comparing thatch and a silt-loam soil extracted from thatchy and thatch-free Kentucky bluegrass turfs, respectively, showed that the thatch had more pores, especially large pores, and retained less water than the silt loam. This is inconsistent with the common perception that thatch retains water for prolonged periods after rainfall or irrigation. The explanation lies in the difference between small pores within the fibrous particles comprising the thatch (intra-fiber pores) and the larger macropores between these pieces (inter-fiber pores). Because much of the total porosity of thatch is derived from large inter-fiber pores, water should drain quickly if it has somewhere to go.

If the underlying soil is highly compacted, however, it will be slow to accept water. The result is a temporary water table in the thatch layer. Interfaces of differently textured materials, such as a sand layer next to a clay layer, tend to inhibit water movement. In the case of thatch, a coarser-textured material (thatch) is situated atop a finer-textured material (a compacted soil). Thus, much of the water that remains in a thatch layer for prolonged periods following rainfall or irrigation is a result of water having difficulty crossing the thatch-soil interface, not the moisture-retention properties of thatch, per se.

Infiltration and evapotranspiration eventually will dry out a wet thatch layer, albeit slowly. When this happens, grass plants growing in the thatch may experience moisture stress and show wilt symptoms despite the high moisture content of the underlying soil. This reflects a similar problem, but in reverse: water does not easily move from the finer-textured soil up to the coarser-textured thatch, where most of the plant's roots are growing.

* Fertility. Studies comparing thatch and silt-loam soil extracted from thatchy and thatch-free Kentucky bluegrass turfs, respectively, showed that the thatch had lost more nitrogen from leaching and volatilization.

Cation-exchange capacity (CEC) measurements of thatch have generated some confusion. In one study, ground-up thatch samples showed relatively high CECs compared with similar amounts (by weight) of selected silt-loam soils. However, when the CECs were compared on a volume (bulk density) basis, the thatch compared unfavorably with soil.

* Effects on pesticides. Other undesirable features of excessive thatch accumulation include increased phytotoxicity and reduced efficacy from selected pesticides. For example, a prior application of paraquat for turf renovation dramatically reduced the stand of overseeded perennial ryegrass due to the residual toxicity within the thatch. When soil was incorporated into the thatch, however, the paraquat residue was quickly adsorbed onto clay particles and the ryegrass germinated normally.

Greater turfgrass injury from the pre-emergence herbicides benefin and oxadiazon has been observed in thatchy Kentucky bluegrass turf several months following application under heat- and drought-stress conditions.

The degradation of DCPA and benefin was significantly faster in thatch than in soil, suggesting that these herbicides would require higher rates to maintain concentrations at effective levels in thatchy turf.

Thatch formation Close observations of thatch-free Kentucky bluegrass turf during the summer months revealed the appearance of a thin thatch-like layer composed of aboveground adventitious (prop) roots, leaf-sheath residues and other plant debris. This "transitory" thatch disappeared during the fall months.

The disappearance of this thatch appeared to be due to increased earthworm activity, while its formation was associated with reduced earthworm levels. This suggests that the presence of thatch reflects an imbalance between the processes that produce turfgrass biomass and those associated with its decomposition. As I discussed above, we were able to induce thatch formation in otherwise thatch-free turf by inhibiting earthworm activity, which is consistent with this concept.

In another series of investigations, I found evidence to suggest that the decomposition of organic residues may also involve microorganisms, because several indicators of microbial activity were lower in soil underlying thatch than in soil from the thatch-free turfs.

Turfgrass thatch includes stem tissues and fibers, and leaf remnants, which are largely confined to the surface layers. Organic residues may undergo a higher rate of decomposition in the lower layers near the soil surface, consistent with studies that have shown that the percent lignin in thatch increases with depth. Lignin is more resistant to decomposition than cellulose and other components, so it is expected that where the rate of decomposition is higher, lignin will constitute a greater proportion of the thatch.

Thatch control Methods for controlling thatch take one of two approaches: either restore the balance between thatch production and decomposition; or compensate for any imbalance between these processes. The former would certainly include avoiding the use of thatch-inducing pesticides, while the latter includes various cultivation and top-dressing operations.

* Cultivation. Cultivation practices include vertical mowing and coring (hollow-tine cultivation, or HTC).

Vertical mowers use vertically oriented knives mounted on a rapidly rotating vertical shaft to cut into turf. You can use different penetration depths to achieve different objectives. Setting the knives deep enough to penetrate the thatch layer will remove some of the accumulated thatch. Setting the knives to reach below the thatch layer will bring up some of the underlying soil and intermix it with the remaining thatch, which will aid decomposition. Intensive vertical mowing can be injurious, especially in shallow-rooted turfs. Therefore, perform no more vertical mowing than is necessary to achieve your objective.

Coring removes a small "plug" of thatch and some of the underlying soil. If you remove the cores from the turf, coring may have little effect on thatch levels. If you incorporate the cores into the turf, however, the soil from the cores may intermingle with the thatch, improving its qualities for turfgrass growth. Soil incorporation increases the bulk density (BD) and the cation-exchange capacity (expressed on a volume basis as CEC-BD) of the thatch. As a result, the "transformed" thatch is less susceptible to compression under traffic and its water- and nutrient-retention capacities are greater.

* Topdressing. An alternative method for incorporating soil into thatch is topdressing. Its primary advantage is that you can select a topdressing material with precisely the qualities you want. As with the incorporation of native soil (such as after HTC), the purpose of topdressing is twofold: to improve moisture and nutrient retention of the thatch and to accelerate its biodegradation.

For intensively cultured turfs, the trend in topdressing has been toward lighter, more frequent applications. This, in turn, has encouraged the selection of pure-sand media because of the ease with which sand can be distributed at light rates. However, sand produces different effects in thatch than topdressing with heavier, clay-containing soils. Measurements of sand-topdressed creeping bentgrass turf showed that dilution of organic residues is at least as effective as accelerated biodegradation in controlling the problems associated with excessive thatch. The frequency of topdressing did not influence the amount of organic matter detected; however, more frequent topdressing resulted in greater uniformity.

In a frequent sand-topdressing program, topdressing intensity can dramatically influence the quality of the thatch. Let's compare three intensities: minimum, optimum and excessive (see Figure 4, at left). At a minimum intensity, the amount of sand applied is just sufficient to fill in the pores between organic strands in the thatch, so that there is no significant increase in volume. This results in an organic-rich medium that may somewhat limit aeration. At an optimum intensity, the amount of sand applied is sufficient to substantially expand the volume of the resulting medium and to provide a proper balance between aeration, resiliency, and moisture and nutrient retention. Where the sand topdressing intensity is excessive, the organic-matter content of the resulting medium is so dilute that resiliency, and moisture and nutrient retention are similar to that of pure sand.

Turf managers must address excessive thatch to maximize turf performance. Removing thatch with vertical mowing can reduce an existing problem, but you need to take steps to prevent its recurrence. Aeration and deep vertical mowing are two such steps. Topdressing-mostly limited to golf courses but gaining some acceptance with other types of turf-also is beneficial.

Dr. A.J. Turgeon is professor of turfgrass management at The Pennsylvania State University (University Park, Pa.).

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