The Root of it All: Is lawn dethatching necessary?

The Root of it All: Is lawn dethatching necessary?

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My question lies in the area of dethatching our lawns. I see many neighbors renting dethatchers in attempts to try and take of their thatch problems. Could you help clarify the most effective method in best dealing with thatch? 

I have been told that the dethatching method (power raking) adds unnecessary stress to our grass. And that the better way to deal with this thatch is to aerate our lawns (renting a machine that pulls out small plugs of soil and grass.) I have been advised that this in turn will help allow air and nutrients to pass through as to better allow the underlying dead grass to naturally decompose.

So, bottom line, which is the safest and most effective method in caring for thatch in our lawns, dethatching or aerating? And secondly, what is the best time of the year to do either one? Also, as a general rule of thumb, how often should we expect to do this? Curious and just wondering! Thank you. - Lisa, Racine.

That is a great question, Lisa. Dethatching used to be recommended as a way to "open up" our lawns and allow nutrients, water and air to penetrate the root systems. Through research, it has been found that dethatching can be hard on the grasses and cause more damage than benefit.

The dethatching machine, sometimes called a vertical mower, rips the grass blades while tearing up the thatch, and can damage the roots as well.

Let's start by identifying the thatch layer so we are clear on what is being discussed. The thatch layer is a normal accumulation of organic matter that benefits the lawn. It is nutrient rich and full of beneficial microorganisms. In a healthy lawn the thatch layer could be half an inch to 1 inch thick, and that is OK. If the thatch has accumulated to more than 1 inch, which is not that common, then dethatching could be helpful.

But it should not be done annually using a dethatching machine or vertical mower. Core aeration is a healthier alternative and it can be done annually. Core aeration will help reduce the thatch layer and in addition, will help smooth out bumpy lawns over time. Core aeration should be done when the soil is moist and the grass is actively growing. Do not core aerate if it is exceptionally hot or dry.

To prevent thatch from getting too thick, do not over fertilize. No more than 4 pounds of nitrogen per 1,000 square feet per year is needed for a healthy lawn.

Thatch is not due to leaving grass clippings on the lawn or from using a mulching lawn mower to chip up leaves. The mulching practice is a healthy one, and results in a richer, organic soil and can provide significant nutrients to your lawn. If you are using a mulching lawn mower, you can decrease the amount of fertilizer you are using on your lawn by one pound per thousand square feet per year.

For more information on establishing and maintaining a healthy lawn, see the numerous publications on the UW-Extension Publications website at:

Click on "Lawns & Turf" and read through "Lawn Aeration and Topdressing" by Dr. John Stier, which explains in great detail, different ways to aerate your lawn.

More questions?

UW-Extension Master Gardener volunteers serving as plant health advisers can help answer your questions at or (262) 886-8451 at the Racine Horticulture Helpline.

Dr. Patti Nagai is the horticulture educator for Racine County UW-Extension. Submit your questions for the Journal Times Q&A column to Dr. Nagai at and put "Question for RJT" in the subject line.



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