William E. Pound
John R. Street
Thatch is a tightly intermingled layer of living and dead stems, roots, rhizomes, plant crowns and other plant parts which develop between the layer of green vegetation and the soil surface. This thatch layer is plant residue in various stages of decay. Turfgrasses vary in the rate in which they develop thatch layers. The "improved" cultivars of Kentucky bluegrass and Red Fescues are likely to develop a thatch problem under high maintenance programs. Perennial Ryegrass and Tall Fescue seldom exhibit excessive thatch accumulation. Where Creeping Bentgrass is cultured as a home lawn turfgrass, thatch control/prevention measures are usually also necessary.
In addition to the species of turfgrass in the lawn, the level of management practiced can impact the rate of thatch accumulation. Thatch accumulation occurs when the rate of thatch production is faster than the rate of decay. Some factors that encourage rapid build-up of thatch are excessively high mowing (greater than 3.0 inches), heavy fertilization programs, vigorous turfgrass cultivars, etc. Factors which slow decomposition include excessive soil acidity, heavy clay soils with poor drainage, reduced soil insect activity and reduced levels of soil microflora (fungi and bacteria).
High maintenance lawns with vigorous turfgrass cultivars and heavy fertilization programs are likely to develop excessive layers of thatch. Thatch build-up is often identified as an "ailment of good lawns." An excessive thatch accumulation (greater than 1/2 inch in depth) creates conditions that may result in the deterioration of the lawn. This layer leads to reduced water and fertilizer infiltration into the soil resulting in less resistance to drought, reduced rooting of the turfgrass plants into the soil, increased sensitivity to temperature and moisture extremes, interference with the performance of pesticides and makes the lawn more susceptible to insect and disease problems.
The overall quality of the lawn declines when thatch build-up occurs. The lawn will be unresponsive to fertilizer applications, irrigation, etc. and is easily damaged by stress conditions.
Excessive fertilization programs can increase thatch accumulation by
producing vegetative material faster than the plant material
decomposes. Frequent applications of high rates of quickly available
forms of nitrogen can create this condition. Grass clippings returned
to the lawn do not significantly contribute to thatch layers.
Clippings are 80-85 percent water and the remaining dry matter is
easily digested by soil insects and soil microflora. The roots,
stems, rhizomes and crown tissue contain lignin. This molecule is not
easily digested and plant parts containing lignin are the most likely
the be found in the thatch material.
As previously stated, thatch accumulation results when the plant material is produced faster than it is decomposed. In many lawns the build-up is the result of slow/restricted decomposition. The decay of thatch is slow in soils which are very acid (low pH) or alkaline (high pH) and on poorly drained soils. Presumably due to the reduced number or activity of the insects or soil microflora in these soils. Research has shown earthworm activity is important in reducing thatch. Earthworms feed on thatch as well as other forms of organic matter found in the soil. Any management practice which may harm the native earthworm population in the lawn should be avoided, particularly the excessive or indiscriminate use of certain pesticides.
Thatch control is both preventive and curative in nature. Thatch prevention can be managed by proper use of fertilizers and pesticides and the implementation of proper cultural practices. Maintaining a soil pH between 6.0-7.0 encourages microbial degradation and earthworm activity. Selecting turfgrass species which do not commonly form thatch will also prevent/reduce the rate of thatch development. Mowing grass regularly at proper heights (generally 2.0 to 2.5 inches) can also help slow thatch build-up. The turfgrass clippings only need to be removed when grass is wet or extra long and a layer of clippings remains on the surface.
Topdressing, the process in which a thin layer of soil (1/8 inch) is added onto the turf, is an another preventive approach which will help prevent thatch build-up. This light coating of soil helps improve the environment at the soil surface and facilitates microbial activity and thatch prevention. This soil should be similar in texture to the original soil to prevent drainage or other problems due to incompatible soils. Peat moss or other high organic materials should not be used, as these products will add to thatch deposition. Topdressing should not be added on top of an already existing thatch layer. A serious layering problem will result, further complicating turfgrass culture.
Curative control measures should be implemented once thatch accumulation has begun but before the layer exceeds 0.50 inch in thickness. Cultivation practices that address existing thatch layers include dethatchers (i.e. power rakes, lawn combers, vertical mowers), mower blade attachments and core aerification. For many years, dethatching was recommended as a way to remove the thatch layer. This method physically removes the thatch and is most effective if the existing layer is less than 0.50 inch in depth. This operation should be done during a cool season of the year when several weeks of good growth and recovery can be anticipated following the dethatching. Experience has shown that early fall is the best time for removing thatch. Fewer weed problems occur and two growing seasons (fall and spring) follow before the lawn encounters summer stress. Very early spring is the next best time. Machines for removing thatch can be rented at most tool and equipment rental companies. In recent years, various mower blade attachments have been advertised by retailers. These blades come equipped with steel prongs which physically tear the mat and thatch layer during the mowing operation. Even though a considerable volume of material is pulled to the surface, these attachments usually have little impact on the total quantity of plant debris in the thatch layer. Some damage to the desirable turfgrass should be anticipated with these attachments and, therefore, should only be used in the spring and/or fall during periods of favorable growing conditions.
The last option, which research has shown to be the best approach to thatch control, is core aerification. Core aerification, also referred to as aerification, is the process where hollow tines are used to remove plugs of thatch and soil from the lawn and deposit them on the surface. Once the plugs of soil are deposited on the surface, rainfall or irrigation will incorporate the soil into the thatch layer. This soil addition to the thatch layer will improve the environment in this area resulting in increased microbial activity and thatch breakdown.
The process of core aerification pulls plugs which are usually 2 to 3
inches in length with a diameter of 0.50 to 0.75 inch. Ideally, at
least 8 to 9 plugs should be pulled per square foot. With some
aerifiers, this may mean making more than one pass over the lawn to
achieve the desired number of holes. The plugs should remain on the
lawn and be allowed to disperse into the canopy. Aerification can be
performed in the spring (April or May) or in the fall (September or
October). Fall is usually the preferred time as soil moisture is
adequate for tine penetration, temperatures are more conducive to
recovery and root growth is favored during the fall period. Unless
excessive thatch layers are present, one aerification per year is
Various products containing enzymes, yeast, bacteria, etc. are currently on the market and are advertised for thatch control. Research has failed to show the use of these products to significantly reduce thatch accumulations and are currently not recommended.
All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status.
Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.
TDD No. 800-589-8292 (Ohio only) or 614-292-6181