Oregon State University
T. Cook Photo
Thatch is a layer of organic material between the green grass and
the soil. Thatch is composed of living and dead stems and roots.
Leaves make up only a very small percentage of thatch. Sometimes
leaves will form a layer of pseudo thatch at the juncture of the
thatch and the green grass. It disappears quickly.
Importance of thatch components
Leaf tissue/ pseudo thatch
Roots contain lots of sclerified vascular tissue and are relatively high in lignin. Stems composed of nodes and
internodes also have a lot of sclerified vascular tissue. Leaves are relatively poorly developed compared to stems and
roots and have very low lignin levels. This means that leaves are quick to decompose and disappear quickly from the
profile. Stems and roots are more persistent and make up the bulk of the thatch profile.
Factors that increase thatch buildup
>Thatch prone species
Ky.Bluegrass, fine fescues, bentgrasses
>Acidic soils There are many factors that affect thatch
pH below 6 accumulation. Grass type is a major factor. Beyond
grass type, conditions that prevent decomposition
from occurring (acid soils, anaerobic soils, dry
soils, low nitrogen) or stimulate accumulation
faster than decomposition can occur (high nitrogen,
high mowing heights.) will result in thatch buildup.
>High or low N fertility
>High mowing heights
Thatch build up can occur very quickly on new turf areas as demonstrated by this sod
that is less than a year old. Both Ky. bluegrass and fine fescues are heavy thatch
Thatch in Ky. Bluegrass + Fine Fescue sod
T. Cook Photo
Grass mowed tall produces bigger plants, more rhizomes, and a bigger root system than grass mowed shorter. As
a result, there is more stem and root tissue to contribute to thatch.
Thatch depth increases with mowing height
T. Cook Photo
Mechanical Thatch Control
Target April to mid-May
Set flail at or above ground level.
Make one to three passes.
Scalp down remaining grass.
Fertilize with Nitrogen @ 1-2 lb N/1000
Spring is a good time to dethatch because conditions are optimum for rapid recovery and you are preparing the lawn
for the coming growing season. The object is to thin out the lawn and remove as much thatch as possible without
destroying the stand of grass. I try to evaluate after each pass to make sure I quit before it is too late. Scalping
after all debris is removed forces new growth to come from basal crown buds. Fertilizer stimulates new growth and
Adjust blade depth on a hard surface before you destroy the lawn!
T. Cook Photo
A flail type dethatcher can do a lot of damage if it is set too low. Always adjust it on the high side until you figure
out how severe of a dethatching the grass can handle. You can always go over the lawn again but you can’t put back the
grass you ripped out.
Spring is the normal time for dethatching
Spring is good because it is a time for major growth of both shoots and roots, so the stand will recover quickly and
enter the summer period in good condition. Summer is generally a bad time to dethatch. Because temperatures are
high, root growth is often weak and recovery may be slow.
T. Cook Photo
This is about right for a first time over the lawn. As you can see the second pass took almost
as much off as the first pass, but still has not damaged the lawn. I prefer to go a bit lighter
First pass but make several passes. That way I can slowly remove as much thatch as the lawn can tolerate.
T. Cook Photos
Turf appearance after 3 passes
Fertilize after cleaning up debris.
Three passes on this bentgrass lawn have thinned
it out a lot. I would never go over a lawn like this
more than three times.
Without fertilizer to stimulate fresh growth, the
lawn will be slow to recover and will
likely have serious weed invasion.
T. Cook Photos
Recovery can be quick and as you can see here, most of the lawn has filled in nicely.
The weak looking area by the driveway is a ridge that got thinned out more severely than
T. Cook Photos
the flatter parts of the lawn. Ultimately, it recovered just fine.
Appearance two weeks after dethatching
Weak area Strong area
Dethatching can remove a lot of debris. In general, people tend to wait too long before dethatching new
lawns so they remove a lot more debris than they probably are prepared for. I prefer to dethatch more
often rather than less often. A rule of thumb is to dethatch the third spring after planting and then
every two to three years after that if needed. The magic thatch depth is about ¾”. You can dethatch
pretty vigorously without any serious damage at that depth of thatch. Once you get more than 2” of
thatch, it is hard to effectively remove the thatch without destroying the lawn.
Debris from a standard size lawn
Brian McDonald Photo
Result of severely
dethatching a heavily
This lawn was sodded 7 years ago. The person
who dethatched, set the flail at ground level
and kept going over the lawn until he reached
the original soil.
The grass you see in this picture is dryland
bentgrass and annual bluegrass. The fine fescue
that dominated the lawn before dethatching,
is completely gone.
A better strategy would have been to set the
machine higher initially, check the lawn after
each pass, and quit before big chunks started
coming up. This lawn might have been saved if
it had been lightly dethatched and then aerified
with a coring machine to break through the
thatch and facilitate development of roots into
T. Cook Photo
How often do you need to dethatch?
It all depends:
1.Every 2-3 yrs in spring
2.When thatch exceeds ¾”
3.When moss takes over
The key to successful dethatching is constant checking to see how the lawn is performing and how deep the thatch is.
Lawns planted to Ky. bluegrass and/or fine fescues may need programmed dethatching about every other year to avoid
developing excessive thatch. Perennial ryegrass or tall fescue may never need dethatching at all. Bentgrass under high
fertility and high mowing may need dethatching annually while the same grass mowed low and fertilized sparingly may
need dethatching once every five years or so.
Winter active grasses often dominate lawns that are dethatched in fall
Fall dethatching works okay, but there are some problems to look out for. In this case a bentgrass climax lawn was
dethatched in mid September resulting in lots of thatch being removed and severe thinning of the bentgrass. When
the weather turned cool and wet, the bentgrass was slow to recover and over winter the lawn was dominated by rough
bluegrass and had a significant increase in annual bluegrass. Dethatching in spring would have favored the bentgrass.
T. Cook Photo
Central Oregon may well be the thatch capital of the
PNW. I routinely find thatch 4-6” deep on Ky. bluegrass
and/or fine fescue lawns. Once thatch gets that deep, the
only way to manage the lawn is by consistent over watering
in summer, since nearly all roots are in the thatch layer.
Coring won’t even make much difference because there are
no machines that will get through the thatch.
To manage thatch in areas east of the Cascade Mountains,
plan on dethatching about every other year right from the
start. You need to remove thatch before it gets too deep.
Another approach is to plant more tall fescue, which is less
prone to thatch build up.
T. Cook Photo
Coring is not an effective way to remove thatch, but it does make it easier
to water thatchy lawns, thus eliminating the biggest single problem
associated with excess thatch (i.e. dry spots due to poor wetting).
Core depth is approx. 2” T. Cook Photos
Cultural Thatch Control
1. Keep pH in 6-7 range
2. Keep root zone aerated
3. Return clippings & use moderate N
4. Encourage earthworms
5. Break up and drag in aerifier cores
Thatch decomposition is an aerobic process involving fungi and bacteria that attack stems, nodes, vascular tissue,
roots, and other cell components and break them down eventually into carbon, hydrogen, and oxygen. Hard to
decompose materials such as lignin are left behind as residual thatch.
Soil in a pH range of 6-7, adequate aeration, and healthy earthworm populations facilitate thatch breakdown. Worms
don’t actually breakdown thatch, but they do ingest it and break it up into smaller pieces and mix soil and thatch
together so bacteria and fungi can do their job. By maintaining a healthy soil and using only moderate amounts of
nitrogen fertilizer, you can minimize the amount of thatch that accumulates.
Grasses that are naturally prone to thatch will still tend to accumulate thatch, but at a slower rate.
Soil incorporation into thatch during coring operations or via topdressing speeds decomposition. When soil and thatch
are mixed together, the result is mat. Mat tends to be firmer and less spongy than straight thatch. If you dethatch a
lawn and then use a coring machine to remove plugs, you can go over the plugs once they have dried a bit to break them
up and mix the soil with the remaining thatch.
Soil mixed with thatch
T. Cook Photo
Earthworms help mix thatch with soil, which speeds microbial decomposition.
There are a host of organisms associated with the turf ecosystem. Earthworms are important macro-organisms in the
chain of events leading to thatch decomposition. We don’t know how other soil fauna influence thatch.
Adapted from Streu, H.T., Bull. Entomol.Soc.Am., 19, 89, 1973
Earthworms excluded (left) vs. earthworms allowed (right)
When earthworms are excluded from thatch, the thatch tends to stay tight and dense and is slow to break down. When
earthworms are allowed to work in the thatch layer, it becomes mixed with soil and is broken down into smaller pieces.
This process creates an environment more conducive to decomposition by soil microbes.
Photo adapted from Potter, D.A., Powell, A.J., and Smith, M.S., J. Econ. Entomol., 83, 205, 1990
Herbicide effects on thatch accumulation*
Herbicide Thatch Earthworms
(g/plug) ( #/sq. m.)
Untreated 0 53.6
Bandane 13.6 0
Benefin 0 64.7
Bensulide 0 49.2
Calcium Arsenate 18.6 0
Dacthal 0 47.2
Siduron 0 51.9
This trial evaluated the effect of common pre-emergence herbicides on thatch accumulation and earthworm
populations. The two treatments that resulted in thatch build up killed all of the earthworms in the treated plots.
This dramatic correlation implies that earthworms appear to be an important link in the sequence of events leading
to thatch decomposition. In general, earthworms are thought to facilitate litter decomposition by breaking litter
into smaller pieces and mixing it with soil containing bacteria and fungi that are capable of decomposing cellulose,
hemicellulose, and lignin. Note: Bandane and Calcium arsenate are no longer registered for use in the USA.
*Hort Science 7:64-65 Randell, Butler, and Hughes 1972
Effects of fungicides on thatch accumulation*
Fungicide Thatch Soil N S
(mm) pH (g/1000) (g/1000)
Untreated 6.3 6.3 0 0
Daconil 2787 8.8 6.2 81 0
Terraclor 9.3 6.1 29 0
Dithane M-45 16.7 5.9 93 426
Tersan 1991* 17.0 5.8 53 0
Products applied 9 times / year for 3 years
* Toxic to earthworms
This trial looked at long term effects of repeated fungicide applications on the biology of micro organisms and thatch
accumulation in a Ky. bluegrass turf growing in soil.. A steady diet of Daconil or Terraclor had only modest impact on
thatch build up, but Dithane M-45 and Tersan 1991 produced 2-3 times as much thatch. It is interesting to note that
even though Tersan 1991 is known to be toxic to earthworms, the authors found no indication that thatch accumulation
was related to reduced earthworm populations in this trial. They concluded that while some fungicides clearly caused
thatch buildup, there were many different ways in which they could have caused thatch to accumulate.
*Agronomy Journal 70:1013-1019 Smiley and Craven 1978
Biological Control of Thatch
Cultural thatch control is in reality biological thatch control, meaning that by creating a healthy environment
for indigenous micro and macro organisms we can maximize thatch decomposition. The popular connotation,
however, is that biological thatch control involves adding some kind of preparation via a spreader or sprayer much
like we add fertilizer to stimulate grass growth.
The idea of biological additives has been around for several decades. Along the way, many different products
have been marketed as the ultimate solution to thatch buildup. The idea is simple. By augmenting the natural
soil flora with a commercial preparation, the microbes will go to work on the thatch and decompose it much faster
than the beleaguered natural microbes already working in the thatch and root zone.
You can see the obvious appeal of a product that will stimulate decomposition of thatch with none of the messy
realities associated with mechanical dethatching. Anybody who has ever tried to control thatch mechanically on a
commercial scale property or even as a service for residential customers knows this is almost an impossible task
and leaves you with a mountain of debris to dispose of. Few people have been able to do commercial thatch removal
profitably. Most people who have tried have quickly abandoned the idea.
The next step is to come up with a product that will do the job. The question now is, what is needed to accomplish
the task? What are the organisms that are needed, what are the steps in decomposition, what factors affect the
process, how much product do we need, how often do we need to apply it, how fast will it work, and what will we see
to demonstrate its effectiveness?
As you might imagine, the answers to these questions are hard to come by. The following slides depict processes
involved in thatch decomposition and provide examples of products that have been tested and illustrates how
effective they have been.
Processes in thatch decomposition
Thatch deposition occurs due to vigorous growth of grass, presence of thatch prone
grasses such as Ky. bluegrass, fine fescue, and bentgrasses, or other factors.
Earthworms, insect larvae, and other macrofauna ingest and process thatch tissue. This
process results in increased surface area, increased leakage from lysed cells, and
facilitates colonization by primary saprotrophs.
Primary saprotrophs (bacteria, fungi, yeasts, amoebae, etc.) excrete extracellular enzymes that
initiate hydrolysis of complex organic molecules and polymers to smaller molecules that can be
absorbed through microbe cell membranes and be further metabolized. Breakdown of cellulose,
hemicellulose, protein, pectin, and lignin is initiated.
Secondary saprotrophs consume primary saprotrophs and feed on partially digested
plant litter. Key organisms include bacteria, fungi, protozoa, nematodes, rotifers,
earthworms, microarthropods, etc. Further decomposition occurs.
Thatch changes from recognizable stem, node, and root structures to unrecognizable organic matter.
Cellulose, hemicellulose, pectin, protein, etc. decrease and lignin increases as a proportion of remaining
organic matter in the lower thatch profile. Soil incorporation via worms or turf cultural practices produces
mat generally at the lower interface of thatch and soil.
Effects of Bio-Dethatch and Thatch-Away on
thatch depth in bermudagrass
Trt. Rate Thatch Depth(mm)
g/sq.m 6/25 9/3 11/20
No Trt. 0 17 16 17
Bio-Dethatch 9.8 16 16 18
Thatch-Away 9.8 16 17 17
NS NS NS
To my knowledge, this is the first published trial that evaluated products that claimed to control thatch biologically.
In Hawaii on bermudagrass, neither Bio-Dethatch nor Thatch Away had any impact on thatch depth.
*Hort Science 1976 11(5): 488-489
Wisconsin Turf Research, Kusow 1992
TRT Rate/mo. Start Finish Change
mm mm mm
BGK-pre 4.3 oz 20.8 20.0 -.8
BGK-plus 0.4 lb N 20.9 20.4 -.5
20-2-3 0.4 lb N 22.4 25.2 2.8
LSD (.05) 1.8 2.0 2.5
In the 1990’s Bio-Greenskeeper was tested at several universities to determine if it was an effective thatch
control product. In this trial, Kusow reported statistically significant reductions in thatch depth where two
different BGK formulations were applied. The reductions in depth reported were approximately equal to the
thickness of a quarter. While the statistics may be significant, the functional impact on thatch is minor.
Nebraska Research Report
TRT N/1000/yr. Thatch Change
BGK-pre 0 14.0 -22%
BGK-plus 4.2 lb 12.7 -28%
20-2-3 4.2 lb 12.1 -33%
LSD (.05) NS NS
In a similar trial at Nebraska, Gaussoin found no significant differences in thatch depth when BGK products were
compared to fertilizer alone. It is hard to conclude from these two trials that BGK products provide effective
OSU Research Report
Product Gal./acre Apps. Compressed Thatch (mm)
7/15/93 4/8/94 Change
Biodigest 0.5 6 17.8 17.2 -0.6
Bac.Gro.Enhan. 0.5 6 16.2 16.7 +0.5
Biodigest + 0.5 6 15.8 18.9 +3.1
Bac.Gro.Enhan. 0.5 6
Untreated ---- -- 16.6 18.4 +1.8
LSD (.05) NS NS
In our trials at Oregon State with a local product, we found no significant differences in thatch depth due to
treatments even with 6 applications.
Will we ever find an effective biological additive that controls thatch?
This is a hard question to answer. It is hard to imagine finding a mix of microbes that can magically attack and
decompose thatch and then survive in the thatch/rootzone to prevent thatch buildup from occurring in the
future. The various products that have been offered for sale have often been by products of pharmaceutical
manufacturing or have targeted one specific component of thatch such as cellulose. All have been proprietary
products so there is no way of knowing exactly what they are composed of. Some are bacterial, some are fungal,
some include humates, some have been fortified with feed grade molasses, etc. All of the products we have
tested smell horrible and none of the products we have tested worked.
It seems that if we could develop a product that worked, it would have to be better researched than products
that have been offered up so far. The product microbes would need to integrate into the natural thatch
decomposition system and they would need to be compatible with other microbes. In my opinion, any product
offered for this purpose would have to measurably reduce thatch depth by at least 50% of the before treated
depth to be worth the effort to apply.
I remain skeptical about current products that promise thatch control. Right now the best plan for managing
thatch is to create a healthy thatch/rootzone environment and monitor thatch depth annually. When thatch
approaches ¾” use a mechanical dethatcher in spring.