WW-01123 Reviewed 2009
Figure 1. Thatch is the layer of undecomposed and partially decomposed plant material tightly interwoven with living tissue between the soil surface and green vegetation.
Excessive thatch - more than 1/2 inch - has become a problem in many highly maintained Minnesota lawns. Thatch is a tightly interwoven layer of living and dead tissue existing between the green vegetation and soil surface. It is composed primarily of products from stems, leaf sheaths, and roots that are fairly resistant to decay. Although a little thatch improves the wear tolerance of a lawn, excessive thatch harbors disease organisms and insects making the lawn more susceptible to damage from disease and drought.
In many lawns, organic matter is produced faster than it can decompose and thatch gradually develops. Several factors determine the rate of thatch development by affecting how fast organic matter is produced or by affecting its rate of decomposition.
These factors increase thatch development in grass by increasing production of stems, leaf sheaths, and decay-resistant tissue:
Generally, the more aggressively grass is growing, the faster it will develop a thatch layer. Newer, improved varieties of Kentucky bluegrass that have been developed for vigorous growth and better recovery on athletic fields and high-quality home lawns develop thatch more quickly than the less vigorously growing common types of Kentucky bluegrass. Fine fescue lawns, though less vigorous in growth than Kentucky bluegrass, can form dense thatch because the plant tissues are more fibrous and resistant to decomposition.
Adequate levels of plant nutrients are essential for healthy lawns; however, excessive amounts, particularly nitrogen, stimulate too much production of stems and leaves increasing thatch development. Infrequent mowing also increases thatch development because more of the plant growth goes into producing stems and other parts of the plant that are resistant to decay. The primary time for stem growth is spring and early summer. Reduce spring applications of nitrogen so as not to encourage additional stem growth.
Other factors increase thatch development by limiting the rate at which organic matter is decomposed. Decomposition is principally accomplished through the activity of micro-organisms, earthworms, and insects. Acidic soil conditions or poor soil aeration reduce the activity of most organisms involved in the breakdown of organic matter. Using insecticides or fungicides may also reduce the amount and activity of organisms in the thatch.
Assessing Thatch in a Lawn
Figure 2. An aerifier takes plugs out of the ground, leaving holes approximately one-half inch in diameter and up to three inches deep.
To determine the amount of thatch accumulation, remove a two-inch deep, pie-shaped wedge from the lawn and measure the amount of thatch between the soil surface and green vegetation. If the layer is one-half inch or less, it usually is not a problem. If, however, the layer exceeds one-half inch, a program of thatch management should be started.
Thatch control may include both prevention and removal. Preventing excessive thatch should concern all homeowners and turf managers interested in maintaining a high-quality lawn. A thatch removal program should be considered any time thatch has accumulated in a layer more than one-half inch thick.
While you can remove thatch in spring, fall removal is preferred because it results in fewer weed seeds sprouting. Remove thatch when two or three weeks of reasonable growing conditions still remain before weather turns too hot in summer or six weeks before soil freezes in autumn - usually the end of September or early October in the Twin Cities area.
Preventing Thatch Accumulation
Fertilization - Some fertilization is required to maintain a healthy lawn and foster thatch decomposition. Fertilize only enough to maintain the lawn’s desired color and growth. Fall fertilization (mid to late October) is preferred to spring fertilization because the resulting growth is not as rapid and lush. Have a soil test run to determine the proper amount of nitrogen, phosphorus, and potassium to apply.
Aerification - Compacted soils and soils with poor drainage tend to accumulate thatch faster than well-drained soils. Aerification promotes better moisture and air penetration into compacted soils. It helps establish a deeper and healthier root system and also stimulates the microbial activity involved in decomposing the thatch layer. To be effective, the aerifier must have hollow tines or spoons that bring the cores of soil to the surface.
It takes about six weeks for grass to fill in the holes left by aerating. To hasten the process, fertilize the lawn one week prior to aerification. The best time to aerate is between late August and early October, depending on how far north you live.
Spring or summer aerification can also be successful, especially when followed with regular, frequent watering. In spring, wait until you’ve mowed the lawn twice before aerifying. Then, unless you are reseeding the lawn, follow up with an application of pre-emergence herbicide after aerifying to help prevent annual weeds from sprouting.
It’s best to run a core aerifier over the lawn several times in different directions to break up compacted soil as much as possible. Then allow the cores of soil to dry partially before raking them back into the lawn - or just let them sit on the surface and crumble apart over time. This "top dresses" the grass with soil containing desired micro-organisms that will work at decomposing thatch.
Mowing - If lawns are mowed regularly so that no more than one-third of the leaf height is removed with each mowing, there is no need to bag the clippings. Small grass clippings filter down into the grass and decompose rapidly, recycling nutrients back into the lawn. However, the clippings must be uniformly distributed and not deposited in windrows.
Figure 3. A vertical mower or power rake has blades that cut into the turf and bring much of the thatch to the surface where it can be raked or vacuumed off.
While it is not necessary to use a mulching mower in order to let clippings fall back to the lawn, mulching mowers do distribute clippings far more evenly than standard mowers. Using a mulching mower does not, however, lengthen the interval between times that grass should be mowed. Mowing frequency is determined by the growth rate of the grass. If the desired height is two inches, grass should be cut when it is no more than three inches tall - regardless of the mower used.
Pesticides - Avoid using pesticides as much as possible. Many pesticides affect the microbial and earthworm populations that are involved in decomposing the thatch layer. Pesticides should only be used when a pest problem has been clearly identified and the pesticide is necessary and known to be effective.
Biological removal of thatch, using liquid or dry dethatching materials, has thus far proven inconsistent.
Aerification may be warranted prior to vertical mowing, when thatch buildup is severe. (See discussion above.)
Mechanical removal of thatch can be best achieved with a vertical mower (also called a power rake). These machines have steel knife-like or spring-like tines that rotate perpendicular to the ground surface. To be effective they should be set so the tines bring a small amount of soil to the surface with the thatch debris.
Vertical mowing results in a large volume of debris that must be removed from the lawn (as much as 80 bushels from a good-sized lawn). Disposal of this debris sometimes presents a problem, though you should be able to take it to a county compost site.
The debris actually makes an excellent soil conditioner when composted, and can be incorporated into garden soil or used as a mulch for gardens, woody plants, and flower beds. It will contribute organic matter and nutrients and improve the structure of heavy soils.
The best time for vertical mowing is usually between late August and early October, depending on your location, because grass is growing vigorously then and should recover quickly from any damage. In addition, few weed seeds germinate at that time. A light application of fertilizer (1/2 to 3/4 pounds actual nitrogen per thousand square feet) and regular watering will speed the lawn’s recovery after vertical mowing.
Because of infrequent use, it is more practical to rent aerifiers and vertical mowers than to buy them. They are available at many rental dealers as well as garden centers and suburban hardware stores. Some professional lawn care companies also can be hired to aerify or vertical mow home lawns. "Power rake" attachments to rotary mowers are not effective in removing thatch and are not recommended because they can be quite destructive to existing grass.
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