Lawn Thatch: The Good, the Bad and the Ugly

Lawn thatch is the layer of dead and alive stems between the green grass leaves up top and the soil down below.  Thatch is not made up of grass clippings, contrary to popular belief.  Too much thatch restricts air and water movement within the soil.  Thatch of ½ inch or less is not detrimental to lawns, and can help keep moisture in and weeds out.  From time to time, it is necessary to de-thatch to prevent the layer of thatch from taking over the lawn.

What Causes Lawn Thatch?

Thatch naturally occurs in all lawns, but becomes a problem over ½ inch in depth.  Overwhelming thatch can build up when these conditions are met:
  • Lack of earthworms in the soil
  • Excess compaction
  • Overuse of nitrogen fertilizer over a period of years
  • Insect or disease problems
  • Dry spots, or variations in soil texture
To prevent thatch, follow these lawn care tips:
  • Water deeply and infrequently
  • Always mow to the proper height for the grass species:  ½ inch for bermudagrass, 2 inches for bluegrass
  • Treat pest and disease problems immediately to prevent grass from becoming stressed
  • Do not over-fertilize

Lawn Thatch:  The Bad

Too much lawn thatch can cause a variety of problems.  It can restrict air movement through to the soil, which makes it more difficult for plants to convert the stored sugars to food.  Plants need oxygen to use the sugars they make during photosynthesis, and they take in that oxygen through their roots.  Too much thatch can also keep the soil too moist, and trap fungus and bacterial diseases in this critical layer between the grass leaf blades and the soil.  Insects also find a thick thatch layer to be a cozy habitat, with plenty of space to live and lots of food to eat, namely, the grass.

Lawn Thatch:  The Good

A layer of thatch of ½ inch or less helps moderate the soil temperature and moisture.  Thatch can also help grass out-compete weeds by shading the ground where the weeds would otherwise sprout.  A healthy lawn will not develop problems with thatch.

How to De-Thatch a Lawn

You always want to de-thatch your lawn at a time when the species of grass in your lawn is actively growing.  That is when the lawn can best recover from the treatment.  You need to use a core aerator for best results.  A core aerator will remove small cores from the soil, and will not simply poke holes in the lawn.  Water the lawn with at least one inch of water up to two days before aerating.  (If you are not sure how long it takes to give the lawn 1 inch of water, put straight-sided containers around your lawn and turn on the water.  Measure the length of time it takes to accumulate one inch of water in the container.   Then you know how long you will need to water.)  After aerating, be sure to water the lawn adequately so that the lawn can recover.  With several waterings, the cores will break up.  If your lawn needs more organic matter, topdress (spread a thin layer of) with compost. You can also use a thatch rake or a vertical mower which cuts vertical spaces in the lawn, a similar principle to core aerators.  You will find that once your lawn recovers from the de-thatching process, it is healthier and more lush.
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