University of Vermont
Department of Plant and Soil Science

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UNDERSTANDING LAWN THATCH

Dr. Leonard Perry, Horticulture Professor Emeritus
University of Vermont

If you want a healthy, attractive, and soft lawn to walk on, then you need to understand the basics of the lawn component called “thatch.”  Some is good, more is not.

Thatch is a layer under the growing grass you see, and of the roots, composed of tightly interwoven or compacted stems, leaves and roots.  It is composed of both living and dead plant parts. A common misconception is that it comes about from leaving grass clippings on lawns.  If you don’t follow good culture, including proper mowing, clippings can contribute to thatch but don’t cause it initially.

You generally see thatch problems more with cool climate Kentucky bluegrass, creeping red fescue, and creeping bentgrass. rather than tall fescue and perennial ryegrasses.  In warm climates, zoysia and Bermuda grasses are prone to forming thatch.  Some Kentucky bluegrass varieties, bred for more vigorous growth, can have thatch while more common varieties don’t.  Fine fescues, although not as vigorous, can develop thatch as their blades are tough and decompose more slowly.

Most lawns have thatch, and in small amounts it is good in that it provides a resilient and springy surface to walk on.  Think of the padding under a carpet.  If too thick though, over an inch or so, thatch begin to cause problems.  It is thick thatch that gives this otherwise normal part of lawns a bad name.

Too thick thatch keeps water, fertilizer, and air from penetrating to the roots, and can harbor insects and diseases. Thick thatch can bind up fungicides and insecticides, keeping them from penetrating and being effective.  Roots begin growing in the thatch layer to get what they need, so are more susceptible to even slight droughts and stresses.  Thatch does not rewet easily once dry, and once wet stays wet, providing excellent conditions for disease.
   
With thick thatch you can get “scalping” when mowing.  The wheels of the mower sink down into the soft thatch, mowing it too low.  Also in thick thatch the grass crowns grow higher, above the soil surface, which also contributes to scalping when mowing.

Excessive thatch comes about from cultural practices that make the grass grow too rapidly, faster that soil organisms can break it down, or that reduce these beneficial soil organisms such as earthworms, insects, and microscopic species.  To avoid excessive thatch, aim for a soil pH of about 6.5.  Don’t overfertilize, as this will lead to too much growth that won’t break down and instead will accumulate as thatch.  Keep lawns watered, at least minimally if possible, during drought.  It is better to water deeply, less often, to encourage deep roots.  And don’t routinely use pesticides if not really needed, as some can kill the organisms you need to keep thatch under control.
   
Grass clippings don’t cause thatch, and won’t contribute to it if you mow regularly so the clippings are small and easily broken down.  But mow regularly only if grass is growing; if grass is dormant during heat of summer or drought stress, mow only as needed.  When you wait too long to mow, the clippings are too long and may accumulate rather than break down.  If grass gets too long, it’s better to mow high, then again in a few days slightly lower, so no more than 1/3 of the leaf blades are removed each time.  An ideal mowing height to maintain is 2-1/2 to 3 inches.  If a “country lawn” such as mine, with a mix of grass and plant species (like clover), you might even mow 3 to 4-inches high. 

You should remove clippings if you already have a thatch problem, or grass gets too long and you can’t mow it high enough.  Mulching mowers help cut up clippings so they break down faster, if you mow regularly.  They keep clippings from piling up in windrows-- thick piles that may accumulate rather than totally break down.

If you cut a small square or triangle of turf and soil and remove it, then notice a thatch layer an inch or so thick above the soil, you should consider “dethatching”.  Thatch will appear as a horizontal layer that is brown and spongy, perhaps like felt.  Dethatching is done in late summer or early fall when weather has cooled and grass is growing so will recover quickly, and weeds are not germinating so they won’t compete.  Don’t try to dethatch all at once if it is thick, maybe do some this year and some next.  And don’t detach when soil is wet, to avoid damaging the soil structure.  If the thatch is two inches or more thick, most the roots will be growing in it, so after dethatching you’ll need to overseed the lawn.

There are machines you can rent from rental supply firms called “dethatchers”, “vertical mowers”, or “power rakes”.  They basically have vertical blades that cut through the thatch layer, and bring some to the surface.  Get some experience first with vertical mowers, or you may thin the thatch and lawn too much. 
   
In addition to dethatching, test your soil to see if you need to alter the pH or soil acidity, such as by liming to raise it.  If the soil is compacted, and water doesn’t enter quickly, you may want to rent an “aerator” as well.  This makes small cores into the soil to allow water, nutrients and air to penetrate.  In turn, this helps microorganisms and roots both grow better.  Since aerating and dethatching can stress otherwise healthy lawns, only use these if needed, and when grass is growing and not under drought or heat stress.

If thatch isn’t too bad, merely work on changing your cultural practices.  Or use a thatch hand rake with vertical teeth.  A thin layer of soil (1/4-inch or so) can be applied over the lawn to help decompose the thatch layer. This “topdressing” also may be combined with aerating. The soil introduced through the aeration cores provides microorganisms, which help decay the thatch.  If topdressing, use a sandy soil or soil similar to that existing, or it may not mix well and end up causing more problems.
 

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