Thatch is an Invisible Menace Threatening Many Lawns
Thatch is a tightly matted layer of living and dead grass stems, crowns and roots which accumulates between the zone of green foliage and the soil. A small amount of thatch is acceptable, providing a cushion and insulating the soil. But when thatch builds beyond about 1/2", the disadvantages can make themselves known very quickly.
Thatch creates serious turfgrass problems
Among the problems you can expect from excessive thatch are: a) increased disease and insect populations; b) localized dry spots; c) chlorosis; d) increased scalping; and e) decreased heat, cold and drought hardiness.
Increases of leaf spot, dollar spot and brown patch activity have all been connected to heavy thatch. Chinch bugs and many other damaging insects find thatch an almost ideal living and feeding area.
Not only does thatch increase the likelihood of these pests, but the effectiveness of many fungicides and insecticides is severely reduced by excess thatch. Many pest controls are captured in the thatch itself and are unable to reach the soil and root zone. When this occurs, the thatch must be opened, removed, or cultivated before effective control of damaging disease or insects can be achieved.
A thatched turf also becomes hydrophobic, which means that once the thatch dries, water cannot penetrate to the soil area beneath. This condition leads to dry spots which simply shed water. Hillsides are especially hard hit by this problem as thatch acts just like a thatched roof and prevents water, fertilizer or pest controls from reaching the soil.
Increased scalping due to excessive amounts of spongy thatch can be seen in hot months, and chlorosis (or yellowing) of the turf is also common when thatch has been allowed to become too heavy.
Finally, in a thatch situation, the crowns, rhizomes (surface roots), and stolons are elevated above the soil surface and exposed to much greater temperature extremes than when they are firmly planted in the earth. This results in reduced water absorption and increased drought and temperature stress. A heavily thatched lawn uses water less efficiently and shows harmful reactions much faster to changes in temperature and to a lack of water.
Causes of thatch
There are many factors which can contribute to thatch buildup. Very aggressive, fast-growing grass varieties stimulate thatch accumulations, as do acidic conditions, poor aeration, excessive nitrogen fertility, and infrequent or excessively high mowing.
Anything that interferes with the microbial activity needed for thatch decomposition increases thatch. Acidic conditions, dryness, and low oxygen levels all impair microorganism activity and have a negative effect on thatch levels. When infrequent mowing removes more of the plant than just the grass blades, stems and other more fibrous parts are returned to the thatch zone. This results in heavier thatch, because these plant parts are more resistant to decay.
Prevention and cure
One of the keys to good turf management is to monitor the thatch level and take preventive measures before the thatch builds to a point where complete removal is the only option.
A vertical cross section of the sod should be cut to measure the actual thatch thickness. Usually, a thatch layer of up to 1/2" is acceptable. Ideally, the cycle of thatch buildup and breakdown would maintain a small amount of thatch for soil insulation and to provide a cushion on the turf. Soils and turf types vary greatly in their thatching habits, and the proper balance of cultural practices for thatch control must be applied to each individual property.
Thatch builds up faster on fine-grained, poorly drained soils (like clay) than on those that are open and well-drained. Many of the microorganisms responsible for thatch decay require air, which is less available in heavy and compacted soils. Heavy soils should be aerated or otherwise cultivated to improve thatch decomposition.
Checking the pH of the soil is important. Well-managed turfgrass tends to become acidic, which slows thatch breakdown. And often, the thatch layer itself is much more acidic than the soil. The best microbial action is achieved when the pH is near neutral. Because of this, frequent, light applications of lime are sometimes beneficial for keeping thatch under control.
Top dressing is the process of spreading a thin layer of soil over the thatch. This is a very effective means of thatch control, but not generally practical on any but small areas.
Turf cultivation provides the best form of thatch control
Mechanical cultivation is the best way to control and cure thatch problems. Cultivation of turf has special requirements. Unlike a garden or field cultivation, turf must not be disturbed beyond its ability to quickly recover.
Turf cultivation can be done by means of vertical mowing (or verti-cutting), which slices through and removes a portion of the thatch. This process usually brings some soil to the surface and mixes it with the remaining thatch to speed thatch breakdown. Vertical mowing also opens the thatch to allow penetration of water, air and fertilizer to the root zone.
Another method of cultivation is core aeration. This system is usually less expensive and less disruptive to the existing turf than vertical mowing. The cores pulled from the soil become a top dressing as they “melt” down. The aeration pockets create growth zones for the root system while allowing improved penetration of air, water, fertilizer and pest controls into the soil.
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