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Do You Have a Thatch Problem?

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lawn with green and dead portions
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What is thatch?

Thatch is a layer of roots, shoots and stems, both living and dead. This brown mat develops between the green vegetation and the topsoil.

 

How does thatch develop?

Your lawn is a living system. It produces organic matter (grass blades, stems and crowns) at the same time it decomposes organic matter. When your lawn produces more organic matter than what it can decompose, thatch starts to build up.

How much thatch should I have?

A thin layer is good. It insulates the lawn from temperature and moisture extremes. A little thatch makes the lawn more resilient.

A thick layer is bad. When thatch gets to be 1 inch thick, it makes roots more susceptible to drought, diseases and rot. 

How do I measure thatch?

Cut a wedge out of your turf. Pretend you are cutting a piece of pie and go down, 2 inches into the soil. Pull out the wedge. Measure the mat’s thickness.

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What causes excessive thatch?

Too much fertilizer, pesticides and irrigation. Aggressive fertilization creates too much plant growth for the microbes to decompose. The excessive use of pesticides kills microbes and earthworms that decompose thatch. Excessive irrigation will drown and kill soil microbes. 

Do grass clippings create thatch?

Not if the lawn is mowed regularly. Young grass blades are almost all water, and they decompose readily.

The problem is when you mow irregularly. Old, tall grass blades will get fibrous and decompose slowly. 

How can I prevent thatch?

Limit your fertilization to only what is needed. This is subjective, but the more you fertilize, the greater your risk of thatch build-up.

Core aeration is very helpful. Core aeration stimulates microbial activity in the soil. Core aeration will physically remove thatch, too.

Limit your use of pesticides. A lawn rarely if ever needs a fungicide or insecticide application.

What if I have too much thatch?

Dethatching is recommended if your thatch is 1 inch or thicker.

Dethatching is a destructive process. It should be done when the turf grows actively and can recover quickly. Fall is a great time.

You can remove thatch using a vertical mower. These are available at rental dealers, garden centers and hardware stores. Dethatching is also done by lawn care services.

Set the tines at a depth where they rake the thatch but cause minimal damage to the soil.

Rake up the debris. This debris may be composted. Irrigate the lawn to activate its healing.

You may aerate, overseed and fertilize the lawn after dethatching. Use a light, half-strength fertilization. Your lawn will recover in a few weeks.

Sources:

Bauer, S. 2018. How to control thatch in your lawn. https://extension.umn.edu/lawncare/how-control-thatch-your-lawn. University of Minnesota.

Landschoot, P. 2020. Managing thatch in lawns. https://extension.psu/edu/managing-thatch-in-lawns. Penn State University.

Kansas State University, https://blogs.k-state.edu/turf/declining-turf-look-below-the-surface/

Tom Kalb, Extension Horticulturist, North Dakota State University.
Adapted from NDSU Yard & Garden Report, September 5, 2021.