University of Vermont
Department of Plant and Soil Science
Summer News Article
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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