Thatch is the intermingled layer of living and dead stems, roots, rhizomes, and stolons that develops between the live green vegetation of your lawn and the soil surface. The thatch layer is composed of plant parts at various stages of decay by soil microorganisms. Grasses that spread by rhizomes (underground plant stems that form new plants at nodes) and stolons (above ground horizontally growing plant stems) are prone to forming thatch layers. Most lawn turfgrasses used in the southern United States are perennial grasses that spread by both rhizomes and stolons, and form thatch. Having a moderate thatch layer (between ¼ and ½ inch thickness) is beneficial because it makes turf more traffic tolerant, decreases compaction, protects grass crowns (growing points) from temperature extremes, and acts as a mulch to decrease water loss from the soil surface. Thatch decay also releases beneficial nutrients to the turf environment. Excessive thatch layers of more than ½ inch can lead to serious problems. Grass species vary rates of thatch production and some new grass varieties have very vigorous growth characteristics that quickly lead to thatch accumulation.
Why Does Thatch Accumulate?
Thatch accumulation occurs when the production of plant parts (stems, rhizomes, stolons) is faster than the rate of thatch decomposition by microorganisms. There is a balance between thatch production and decomposition that can be achieved through proper lawn maintenance techniques. Thatch production is greatly affected by fertilization and mowing. Excessive fertilization (especially using a soluble nitrogen source such as urea or ammonium sulfate) leads to quick production of plant parts that adds to thatch. Follow the AggieTurf fertility and mowing recommendations for proper management. The health of microorganisms and their activities can be aided by cultural practices. Factors influencing microorganism activity are soil pH and adequate, but not excessive soil moisture. Soil pH that is too acidic (below 5.5) or too basic (above 8.5) reduces microorganism activity. Most soil microbes are aerobic and soil that is too wet from poor drainage or excessive irrigation leads to microorganism death from anaerobic (without oxygen) conditions. The key is to create a good environment for soil microbial activity, have adequate lawn quality and growth using fertility recommendations, and employ some mechanical practices to remove excessive thatch and improve soil drainage.
Problems include reduced water, nutrient and air infiltration into the soil (which leads to reduced rooting and decreased drought resistance), and increased disease and insect damage. A lawn with an excessive thatch layer declines in quality and becomes much harder to manage. The turf becomes less responsive to fertilizer applications and much more water will be required to wet the soil profile through a thick thatch layer.
How to Check for Thatch?
A lawn that has too much thatch will feel spongy and tend to scalp when mowed. To estimate thatch thickness, use a soil probe, spade, or knife to remove a small section of sod. Measure the thatch layer with a ruler in several places around the yard. If the estimated thatch thickness is greater than ½ inch, plan to use mechanical dethatching practices in the near future.
How to Control Thatch
Thatch control is a preventive and curative process. Thatch prevention can be achieved through proper fertilization and mowing. Avoid applying nitrogen at a rate exceeding one pound per thousand square feet. Maintain adequate, but not excessive soil moisture to encourage thatch decomposition (deep and infrequent watering - 1/2 inch every four days when dry). Use composted or organic materials for topdressing sparingly and lightly. Adding highly organic materials adds to the already decomposing organic thatch layer and can increase thatch problems.
Dethatching and core aerification are curative practices designed to remove excess thatch that has built up in a lawn. Machines specifically designed for thatch removal (vertical mowers, core aerifiers, and power rakes) can be found at tool and equipment rental companies. The best time for thatch removal is in the early spring. Be sure blades penetrate through the thatch layer to the soil surface when using a vertical mowing or power rake. Vertical mowing should also be done in two or three directions.
Core aerification is the process of pulling soil plugs (2 to 3 inches long and ½ to ¾ inch in diameter) out of the ground. Ideally, eight or nine plugs should be removed per square foot to significantly enhance water and air movement in the soil. This may require making several passes over the lawn. Plugs may be left on the lawn or removed.
Reducing Soil Compaction
Soils that have excessive amounts of fine silt, compacted by heavy traffic, and sodic soils that have excessive amounts of sodium ions are usually compacted. Compaction is enhanced in wet areas that receive lots of traffic. Highway rollers wet soils immediately prior to rolling over them in order to get the highest degree of compaction and stability possible. The soil loses structure and forms an impenetrable layer on the surface. In a home lawn or park there is almost always a place where kids or animals have worn a path in the yard and the grass does not grow anymore. If soils get compacted and the infiltration of water and air into the soil is impeded, the lawn will probably begin to decline. Very hard, empty spots will develop in those areas of compaction and compaction resistant weeds (like Goosegrass and prostrate Knotweed) will become more prominent.
How to Control Soil Compaction
Soil compaction mostly develops in high traffic areas, but is greatly enhanced by excessive soil moisture. That is why AggieTurf recommends deep and infrequent watering schedules. In some areas, compaction cannot be avoided and steps must be taken to periodically alleviate the problems. Core aerification, where you remove soil cores (at least 8 to 9 per square foot), is the best method for alleviating soil compaction. You can rent an aerifier from a tool and equipment rental company.
Soils that have high levels of sodium loose structure and have poor water infiltration and air movement. They become rock hard. Sodium mostlys accumulates from irrigation water. Build-up occurs when irrigation water evaporates and leaves sodium and other ions behind on the soil surface.
Sodium can be removed from soils by adding Gypsum and deep, infrequent irrigation practices. Gypsum replaces sodium ions with calcium ions and the sodium ions can be leached through the soil profile. The soil regains its structure and loosens up. Water and air infiltration begin again. Combining Gypsum application (40 to 90 pounds per thousand square feet) as needed with core aerification is the best recommended treatment.