Thatch can be described as a tightly
intermingled layer of partially decomposed
stems and roots of grasses which develops beneath the actively-growing green
vegetation at the soil surface.
Excess thatch accumulation is undesirable
because it decreases the vigor of turf grasses
by restricting the downward movement of air, water, plant nutrients and other materials
applied to the soil. During wet periods, thatch may act as a sponge and hold excessive
amounts of water, which can reduce the oxygen supply to the roots. During hot, dry
weather, thatch may become dry and very resistant to wetting, even shedding water until
it is re-moistened.
Turf grass disease organisms and insects
can be found in thatch accumulations.
Pesticide effectiveness is reduced since some of the materials can be adsorbed to the
organic matter and not reach and/or control the organism it is intended to control.
Mowing height is also affected by thatch
accumulations. As undecomposed material
builds up, the mower tends to ride on the thatch and does not cut at the desired height.
If the cutting height is lowered in an attempt to overcome this problem, scalping and
brown areas develop.
Thatch decomposition during periods of
hot, moist weather can generate sufficient heat
decomposition by-products to injure or kill turf grasses.
Factors Causing Thatch Development
Thatch accumulation in lawns tends to
be governed by one or more of the following
1. The vigorous growth habit of grasses
that produce creeping stems. The creeping
bent grasses and Kentucky bluegrasses, for example, have a tendency to produce more thatch.
2. The more decay resistant stems and
roots contribute more to thatch development than
the leaves in all grass varieties.
3. Fertilizer deficiencies produce roots
and stems that are more resistant to decay.
Sufficient nitrogen is required for growth and to stimulate bacterial decomposition of
thatch. The other essential plant nutrients must also be present in the proper balance.
4. Acid soil conditions reduce bacterial
activity which may result in slow stem and
5. Over watering causes a reduction in
soil oxygen, reducing microbial activity necessary
for thatch decomposition. Extended periods of saturated soil will induce surface rooting
thereby reducing/delaying the breakdown of the material responsible for the thatch buildup.
Practice thatch removal on an annual
basis. A good raking in the spring and/or fall will
remove some of the undecomposed material. If thatch becomes too thick, more than half an
inch, renovation may be the only answer. Normal thatch removal will not injure the lawn
severely enough to necessitate reseeding.
Timing. Mid August-September
is the best period for thatch removal, particularly
if large amounts need to be removed. At this time, all dead stems and roots which accumulate
throughout the fall and winter can be removed. Turf grasses can recover quickly at this time
because the soil is warm and the rain is more likely to be regular. If properly de thatched,
grasses will recover quickly and resume normal appearance in a short period of time. If light
thatch removal is all that is required, de thatching can be done any time of the year.
Power rakes or other mechanically driven
de thatching machines are usually superior to hand
rakes. Considerable force is necessary to slice or scratch into the grass mat and remove the
dead material. Operate the de thatching machines across the turf in a north-south direction
and repeat in an east-west pattern if the thatch is very thick. Remove loosened material before
After thatch has been removed from the
lawn, mow at the recommended mowing height. Do not
increase or decrease the mowing height of lawn turf at any time during the season. There is
no valid reason for changing the mowing height.
Suggestions for Thatch Prevention
1. Maintain soil moisture by watering
thoroughly and infrequently, applying one inch of water
per week in one application. Never use light, frequent irrigations.
2. Aerate the soil with a core-type aerator
if the soil is compacted or if water is not soaking
into the ground.
3. Maintain proper pH levels. Have the
soil tested every three or four years. If the soil
becomes too acid (pH of 5.8 or lower) apply limestone to correct the condition. A slightly acid
pH provides the best environment for nutrient uptake by the grasses and for microbial activity.
4. Maintain adequate nutrition for normal
growth. Nitrogen is especially important to stimulate
heavy populations of bacterial organisms.
Good turf grass management programs will
help prevent thatch accumulations and will aid in
a healthy and good-looking lawn.
Edmond Marrotte, Consumer Horticulturist & Mohamed Dhinbil, Assistant Extension Educator, Horticulture
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