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Understanding Lawn Thatch
By 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.”

What is "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 rye grasses.  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.

Thatch is Padding

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 re-wet easily once dry, and once wet stays wet,
providing excellent conditions for disease.

Excessive Thatch is Not Good for a Healthy Lawn

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 over-fertilize, 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 Clipping Don't Cause Thatch

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 de-
thatching.  Thatch will appear as a horizontal layer that is brown and spongy,
perhaps like felt.  De-thatching 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 de-thatchers,
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 de-thatch
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.

Test pH of Soil

In addition to de-thatching, 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.