A Threat to Turf: Turf Thatch

Moderation in all things’ is a familiar expression; and when it comes to thatch, the same principle applies. A small thatch layer in a lawn can be beneficial because it improves a turf’s resiliency, but once thatch begins to build up, the disadvantages quickly outweigh the advantages.

“Thatch is an organic material that does not decompose,” explained Bob Brophy, president of Turfco, Minneapolis, Minn. “Once a lawn develops a thatch layer it becomes the perfect living environment for bugs and diseases to kill the grass.”

These are the exact reasons why it is so critical that contractors understand how to proactively prevent thatch and know what means are available in order to control this lawn menace.

THATCH AWARENESS. The importance of controlling thatch is clear – for the health of the turf. Defined, thatch consists of a tightly intermingled layer of dead or decaying turfgrass tissues derived from stems, roots and leaves just above the soil.

According to Roch Gaussoin, extension turfgrass specialist at the University of Nebraska, Lincoln, Neb., as thatch accumulates, there is a tendency for root and rhizome growth to occur primarily in the thatch layer rather than in the soil. This results in a weakened, poorly rooted turf that is prone to stress injury and requires increased management.

Physical Properties Profile. . . . . . . . .

THATCH SAND SOIL AERATION Good Good Fair COMPACTION Resistant Moderately Resistant Susceptible MOISTURE RETENTION Poor Poor Good NUTRIENT RETENTION Poor Poor Good

Source: University of Nebraska Cooperative Extension

“The real importance of controlling thatch is ultimately a healthier turf,” commented John Chiera, sales director of professional lawn care products at Textron Turf Care and Specialty Products, Racine, Wis. “Controlling thatch makes turf less susceptible to stress caused by wear and tear. A means of control also makes turf less susceptible to disease.”

“Controlling thatch offers the ability to allow nutrients, such as air and water, into the soil,” noted Jim Doring, product development, JRCO, Minneapolis, Minn.

“Thatch limits the water penetration into the soil and causes problems with the effectiveness of some pesticides,” remarked Bob Shearman, integrated turfgrass management specialist, University of Nebraska, Department of Horticulture. Shearman added that thatch can limit some insecticides’ ability to control insects such as grubs.

Once the thatch accumulates to ½ to ¾ inches thick, contractors should dethatch, according to Shearman.

“Turf should have a certain amount of thatch (¼ inch) because it becomes a protective layer from too much traffic and ultraviolet rays,” Brophy noted. “For example, thatch protects the nodes on the bluegrass. When thatch gets to be near ½ inch, the lawn can develop problems. If there is 1 inch of thatch, the turf has severe problems.”

A small thatch layer is also beneficial because it increases the turf’s resiliency, improves its wear tolerance and insulates against soil temperature changes, according to Gaussoin.

One way to tell if thatch is becoming a problem is by periodically taking a soil sample. “Contractors can tell by the sight of the grass or they can probe the turf,” Chiera mentioned. “Cut a plug of turf out – a 2-inch square down to the root base. Thatch problems can be created with regular watering and fertilization.”

Agreeing with Chiera, Ray Christopherson, president of TerraCare, Pardeeville, Wis., also recommended taking a soil sample. “This way a contractor can tell if the thatch is more than ¼ inch thick,” he added.

When taking these samples, it is important to measure the accumulation from several different areas in the turf, according to Gaussoin, because thatch is normally not uniformly distributed.

IN CONTROL. The headaches of thatch can be avoided with a few pro-active steps.

According to Bob Shearman, there are four major principles to follow in order to escape thatch problems:

Fertilizing – Practice judicial fertilizer applications and do not overstimulate.

Aeration – This practice takes the root system out of the thatch layer. For cool-season grasses, aerate once each year. For warm-season grasses, aerate once in late spring when the grass is actively growing. For clay or compacted soil, aerate twice per year.

Mowing – Contractors do not need to worry about leaving clippings on the grass because they are not a factor in thatch accumulation.

Watering – Deep and infrequent waterings stimulate root growth into the soil.

Preventative Practices

Thatch accumulation can be minimized by using proper cultural practices and selecting appro-priate turfgrass, according to Roch Gaussoin, extension turfgrass specialist at the University of Nebraska-Lincoln. This is not necessarily easy because controlling thatch requires proper management to encourage organic matter decomposition rather than accumulation. Thatch prevention and control cannot just be an occasional practice. MOWING – Proper mowing frequency and height are the principle cultural practices that can be used to reduce thatching tendency, according to Gaussoin. Mowing frequency should be dictated by the turfgrass growth rate. No more than 1/3 of the leaf blade should be removed with any mowing. If proper mowing frequency is maintained, clippings do not need to be removed because turfgrass leaf clippings contribute very little to thatch buildup. They break down rapidly and recycle nutrients slowly when returned to the turf. As mowing height increases, thatching tendency increases as well. It is best to cycle mowing height from low in the spring to high in the summer and back to low in the fall. This reduces thatching tendency and minimizes environmental stress and weed encroachment, according to Gaussoin. FERTILIZER – Another key in controlling thatch is the application of fertilizers. They should be applied at rates and in programs that meet, but do not exceed, the nutritional needs of the turf. Excessive nitrogen applications may result in organic matter production rates that exceed breakdown, encouraging thatch accumulation. IRRIGATION – Avoid light, frequent irrigations. Instead, irrigating turf deeply and infrequently is best, and water when the turf shows moderate signs of moisture stress. PESTICIDE – Lastly, use pesticides only as needed. Thatch can be minimized by avoiding unnecessary use of pesticides. Pesticides may affect desirable microorganism and earthworm populations. Earthworms digest thatch, improve soil aeration and drainage. They also introduce soil and microorganisms into thatch. Pesticides play an important role in thatch control and, where feasible, should be encouraged through proper management. – Angela Dyer

“Aerate, don’t overfeed and don’t over-water,” echoed Chiera. “A certain amount of stress is actually healthy for generating a stronger plant. If contractors don’t over-water, then the roots will go into the ground looking for water. This will create a stronger root system. I personally recommend aerating once a year and then spot aerate, for example, when an area isn’t draining well.”

Brophy suggested that contractors have a good growth regulating program. “A high nitrogen treatment gives instant growth, but this rapid growth creates cellulose in stems and causes thatch,” he said.

Another preventive measure can be blending cultivars, according to Gaussoin. Mixing cultivars with varying thatching tendencies is recommended to reduce the thatch build-up rate. The blends should be based on cultivars that have superior site adaptation and reduced thatching tendency (see chart).

Thatching Tendency of Turfgrass

HIGH MEDIUM LOW Zoysiagrass Kentucky Bluegrass Perennial ryegrass Bermudagrass Creeping bentgrass Tall fescue Creeping red fescue Hard fescue Buffalograss Chewings fescue

Source: University of Nebraska Cooperative Extension

If these pro-active precautions have not been taken, thatch may have accumulated and contractors should know how to control thatch buildup once it is already present.

“If the thatch is 1 inch or less, contractors need to do proper aeration, which is twice each year,” Brophy stated. “This will reduce the buildup eventually. If there is 1½ inches of thatch, the problem will require mechanical removal with a vertical cutting machine.”

Vertical cutting machines and power rakes use rigid wire tines or steel blades to lift thatch debris and a small amount of soil to the lawn surface, according to Gaussoin, who recommended that the soil should have some moderate moisture for best results. Power raking where these is excessive soil moisture tears and pulls the turf from the soil instead of slicing and lifting the thatch debris as desired.

Shearman recommended aeration, which is also known as core cultivation. “This takes the cores out of the lawn,” Shearman said. “Aeration allows the cores to lay on the surface and as the contractors mow, the cores break up.”

Core cultivation or aeration can be used to minimize and reduce thatch accumulation, to modify its physical characteristics and remove certain amounts of thatch, according to Gaussoin. However, core cultivation is not as effective as power raking in removing thatch debris, but it is less injurious and disruptive. When the broken-up soil cores are redistributed throughout the thatch, the soil modifies the physical structure of the thatch, thereby making it a better growing medium.

The best times to remove thatch are during periods of active turfgrass growth. Gaussoin recommended this because the removal encourages recovery from injury that may occur during dethatching. Cool-season turfgrasses, such as Kentucky bluegrass, grow best in the spring and fall. Thatch removal in this type of turf is best when done in spring prior to green-up or in the fall after Labor Day.

POINT OF PURCHASE. Last, but not least, is the equipment. Once contractors know the how’s, when’s and why’s of controlling thatch, they just need the equipment to get the job done.

“Contractors need dethatching equipment that is going to offer them a return on their investment and increase their productivity,” Doring said.

“It’s important for the equipment to bring the soil up,” explained Larry Classen, vice president of Classen Manufacturing, Norfolk, Neb. “Equipment should be heavy enough to get into the dirt and bring up a good portion of soil to examine. A long enough plug to pull a good soil sample is also necessary. For example, a 2¾-inch plug allows contractors to see the soil, thatch and grass.”

Shearman explained that an aerator should pull as many cores per square feet as possible. “The more cores the better.”

“An aerator should offer a minimum of nine holes per square foot,” Brophy added. “You should go across the lawn at least twice to get that amount of holes. You can’t have too many holes, but you can definitely have too few.”

The author is Assistant Editor of Lawn & Landscape magazine.

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