University of Vermont Extension
Department of Plant and Soil Science
Summer News
Article
UNDERSTANDING
LAWN THATCH
Dr. Leonard Perry, Extension
Professor
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.”
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 mowing, clippings can contribute to thatch but
don’t
cause it initially.
You
generally see thatch problems more with aggressive spreaders such as
Kentucky
bluegrass, and the warm climate zoysia and Bermuda grasses, rather than
tall fescue
and perennial ryegrasses. 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.
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.
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 wont 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 these can kill the
organisms
you need to keep thatch under control.
Grass clippings don’t cause thatch,
and wont contribute to it if you mow regularly, so the clippings are
small and
easily broken down. 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. 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 won’t compete. 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.
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.
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.
Microorganisms and roots will both grow better. Since aerating
and dethatching can stress
otherwise healthy lawns, only use these if needed.
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 coring. The soil introduces
microorganisms that help decay the thatch. Use a soil similar to
the existing, or it may
not mix well and end up causing more problems.
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