Thatch management

Tom CookOregon State University

2008

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. Sometimesleaves will form a layer of pseudo thatch at the juncture of thethatch and the green grass. It disappears quickly.

T. Cook Photo

Importance of thatch components

Roots(major)

Stems(major)

Leaf tissue/ pseudo thatch(minor)

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.

>Thatch prone speciesKy.Bluegrass, fine fescues, bentgrasses

>Acidic soilspH below 6

>Anaerobic soils

>Dry soils

>High or low N fertility

>High mowing heights

There are many factors that affect thatch 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.

Factors that increase thatch buildup

Thatch in Ky. Bluegrass + Fine Fescue sod

Thatch build up can occur very quickly on new turf areas as demonstrated by this sodthat is less than a year old. Both Ky. bluegrass and fine fescues are heavy thatch formers.

T. Cook Photo

Thatch depth increases with mowing height

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.

Mechanical Thatch Control

Timing: Target April to mid-May

Strategy:Set flail at or above ground level.Make one to three passes. Remove debrisScalp 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 rapid recovery.

Adjust blade depth on a hard surface before you destroy the lawn!

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.

T. Cook Photo

Spring is the normal time for dethatching

T. Cook Photo

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.

First pass

Second pass

T. Cook Photos

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 lighterbut make several passes. That way I can slowly remove as much thatch as the lawn can tolerate.

Third pass

Turf appearance after 3 passes

Fertilize after cleaning up debris.

Three passes on this bentgrass lawn have thinnedit 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 willlikely have serious weed invasion.

T. Cook Photos

Appearance two weeks after dethatching

Weak area Strong area

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 the flatter parts of the lawn. Ultimately, it recovered just fine. T. Cook Photos

Debris from a standard size lawn

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.

Brian McDonald Photo

Result of severely dethatching a heavily thatched lawn

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 aftereach 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 the soil.

T. Cook Photo

How often do you need to dethatch?

It all depends:1.Every 2-3 yrs in spring2.When thatch exceeds ¾”3.When moss takes over 4.Never

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 avoiddeveloping 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.

What about Fall Dethatching?

T. Cook Photo

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 roughbluegrass 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. bluegrassand/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 areno 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 lessprone to thatch build up.

T. Cook Photo

Core depth is approx. 2”

Coring is not an effective way to remove thatch, but it does make it easierto water thatchy lawns, thus eliminating the biggest single problem associated with excess thatch (i.e. dry spots due to poor wetting).

T. Cook Photos

Cultural Thatch Control

1. Keep pH in 6-7 range2. Keep root zone aerated3. Return clippings & use moderate N 4. Encourage earthworms5. 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 mixed with thatchspeeds decomposition.

Highland Bentgrass

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.

T. Cook Photo

Earthworms help mix thatch with soil, which speeds microbial decomposition.

Thatch

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 53.6Bandane 13.6 0Benefin 0 64.7Bensulide 0 49.2Calcium Arsenate 18.6 0Dacthal 0 47.2Siduron 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 0Daconil 2787 8.8 6.2 81 0Terraclor 9.3 6.1 29 0Dithane M-45 16.7 5.9 93 426Tersan 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 ThatchCultural thatch control is in reality biological thatch control, meaning that by creating a healthy environmentfor 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 naturalsoil 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 decompositionThatch 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 17Bio-Dethatch 9.8 16 16 18Thatch-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 Changemm mm mm

BGK-pre 4.3 oz 20.8 20.0 -.8BGK-plus 0.4 lb N 20.9 20.4 -.520-2-3 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 thatchcontrol 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 ReportGaussoin 1992

TRT N/1000/yr. Thatch Change mm

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 thatch control.

OSU Research ReportCook 1994

Product Gal./acre Apps. Compressed Thatch (mm) 7/15/93 4/8/94 Change

Biodigest 0.5 6 17.8 17.2 -0.6Bac.Gro.Enhan. 0.5 6 16.2 16.7 +0.5

Biodigest + 0.5 6 15.8 18.9 +3.1Bac.Gro.Enhan. 0.5 6Untreated ---- -- 16.6 18.4 +1.8

LSD (.05) NS NSIn 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.