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University
of Rhode Island GreenShare Factsheets
What
is Thatch?
Thatch is a tightly intermingled layer of living and dead
stems, leaves and roots which accumulates between the layer
of actively-growing grass and the soil underneath. Thatch is
a normal component of an actively growing turfgrass. As long
as the thatch is not too thick, it can increase the resilience
of the turf to heavy traffic. Thatch develops more readily
on high-maintenance lawns than on low-maintenance lawns.
An excessive thatch layer (more than 1 inch) can restrict
the movement of air, water, fertilizer and other materials
to the roots. This air- and water-impervious layer causes the
grass to restrict rooting to the thatch layer to get air and
water and, hence, reduces drought resistance in the turf. Furthermore,
if the thatch dries out, it cannot be re-wetted easily. Even
if the thatch stays moist, it can harbor fungi which can cause
turf diseases. When temperature and moisture conditions are
right, a disease infestation can kill the already weakened
turf. The overall effect of a thick thatch layer is an unthrifty
lawn which does not respond well to management practices and
is easily injured by adverse weather conditions and pests.
The major causes of thick thatch accumulation are management
practices that reduce the population of organisms which decompose
thatch and/or that cause plant material to build up more quickly
than the microorganisms present can break it down. Research
has shown that earthworm and microorganism activity play a
vital role in preventing excess thatch accumulation. Good aeration,
soil pH around 6.5 and adequate moisture favor the build-up
and activity of beneficial microorganisms. Thatch problems
have been noticed more often in acidic and compacted soils,
as a healthy microorganism population does not flourish under
these conditions. Unbalanced fertilization and indiscriminate
use of fungicides and insecticides are the most detrimental
to beneficial organisms in turf. Too much nitrogen can promote
disease development. High-nitrogen fertilizing speeds up lush,
succulent growth that is more prone to insect pests and disease.
Moreover, many disease-causing fungi can live on cut clippings
as well as on living grass plants. These clippings are then
added to the thatch layer that harbors disease organisms until
the environmental conditions are right for disease development.
Leaving grass clippings on the lawn, however, does not necessarily
cause an increase in thatch buildup. Leaving short clippings
after mowing does not add to thatch development--soil microorganisms
break the clippings down quickly and return a valuable source
of balanced nutrients back to the turf. Proper management practices,
therefore, promote healthier turf that will need fewer pesticides
and can maintain a healthy population of beneficial organisms.
Grasses which produce a large amount of side shoots, such as
Kentucky bluegrass (produces rhizomes) and creeping bentgrass
(produces stolons), tend to produce thatch readily.
To determine if your lawn has a thatch problem, cut out
a small, triangular-shaped plug of turf several inches deep
and examine. Note the spongy layer of material above the mineral
soil. If this layer is more than 3/4 to 1 inch thick when you
compress it, you should consider having your lawn dethatched
or beginning a management program which will encourage thatch
decomposition.
When thatch has accumulated to an excessive thickness,
it is best reduced by mechanical means. Dethatcher machines
known as vertical mowers, verticutters, dethatchers or power
rakes have vertically spinning blades which pull some of the
material to the surface as they slice the thatch layer. Most
equipment rental outlets have dethatching machines available
for renting. Mechanical dethatching should be done in either
late summer or fall when cool weather prevails. Caution! DO
NOT attempt to remove the entire thatch layer in one treatment;
DO NOT dethatch when soil is wet; only dethatch your lawn when
it is needed rather than on a routine basis. In addition to
dethatching, the lawn should be aerated and limed if necessary.
Lime and aeration stimulate bacterial decomposition by improving
air, water and nutrient relationships. Some garden centers
and equipment rental companies rent aerators.
You can also topdress your lawn after dethatching with
about 1/8 inch of topsoil similar to the soil underlying the
turf. Topdressing helps even out bumpiness and fill in holes
left from dethatching and/or aerating. Topdressing is usually
only necessary in special cases, however, and using the wrong
topdressing material can be more detrimental than the benefit
of evening a bumpy turf area. Do not topdress with a different
type of soil. A dissimilar soil will not mix well enough with
the underlying soil.
Other maintenance practices which discourage thatch build-up
are frequent mowings to maintain the grass at a height of 2
1/2 inches, keeping clippings on the lawn, reducing nitrogen
fertilization and amending the soil with phosphorus, potassium,
and lime according to a soil nutrient analysis.
Adapted from the University
of Massachusetts Extension, 1999
Pesticides
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
For more
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
University
of Rhode Island Cooperative Extension provides equal program
opportunities.
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