Using Defoliation of Dominant Grasses to Increase Prairie Plant Diversity

 In many prairies, the primary suppressors of plant diversity are dominant grasses – both native and non-native.  These grasses, left unchecked, can monopolize light, moisture, and nutrients to the point that few other plant species can coexist with them.  I’m not sure why some prairies suffer from this more than others.  There is some evidence that hemi-parasitic and allelopathic plants such as pussy toes, false toadflax, and wood betony can play a role in suppressing grasses and facilitating forb diversity, but I don’t think that’s the whole answer because I’ve seen very diverse prairie plant communities without those species – or with only a few scattered populations of them.  Regardless of the reasons, we are left with many prairies that have lost – or are losing – plant diversity through domination by grasses, and we have to decide what to do with them.  Some of those prairies are restored (reconstructed) prairies that started out with high plant diversity but have since lost much of that diversity.  Others are remnant prairies that have been degraded by overgrazing and/or broadcast herbicide application.  Still others are relatively diverse remnant prairies that are slowly losing diversity as individual forbs die without reproducing.

Big bluestem. A good native grass, but sometimes so dominant that overall plant diversity suffers.

I think one of the best tools we have for combating grass domination in prairies is defoliation – the removal of above-ground portions of plants.  Defoliation has always been a major component of prairie ecosystems through fire and herbivory, and mowing and (non-lethal “burn back”) herbicide applications are additional options at our disposal today.  Plants respond to defoliation in various ways, depending upon the severity of defoliation, the frequency and/or duration of the defoliating event, the stage of the plant’s growth at the time of defoliation, and each species’ genetic programming.  The way each plant, and its neighbors, respond to a defoliation event determines which plants will gain or lose territory.  In other words, defoliation influences the competition between plants – and manipulating competition between plants is really what most prairie management is all about. 

Some of the earliest research I’m aware of on the effects of prairie plant defoliation can be found in range management research from the 1950’s and 1960’s.  The earliest paper that is often cited from that era is by F.J. Crider, who documented the effects of defoliation on the root growth of grasses.  He (and others since) found that a severe defoliation of a grass plant resulted in an immediate cessation of root growth as plants reallocated resources from root growth to regrow leaves and stems.  More importantly, those grass plants actually abandoned sections of living roots as well – shrinking the total root mass of the plant fairly dramatically.  This makes sense, since the plant has to support those roots through photosynthesis, and a severe defoliation takes away most of the plant’s ability to photosynthesize. 

A simplified look at how dominant grasses can affect plant diversity. In the top example (A) grasses have monopolized both aboveground (light) and belowground resources (moisture and nutrients). After defoliation (B), both the above and belowground parts of the grasses have shrunk, freeing up resources and allowing other plants to establish within that lost territory. As the grasses recover from the stress of defoliation, their vigor and size increase, but the new plants have a fighting chance - at least for a while - to hold the new ground they've taken.

Those early range science research data provide some useful context for today’s prairie management, but those researchers were primarily trying to figure out the intensity of grazing they could employ while still maintaining a dominant stand of grass.  As prairie managers, by contrast, we want to reduce the dominance of grass to increase the diversity of other plants.  We can still learn from what those range scientists discovered; we just want to employ it in a different way.  Since plants primarily compete for light, moisture, and nutrients, we want to find ways to make those three kinds of resources more available to plants other than dominant grasses.  Defoliation can reduce shading aboveground (removal of leaves and stems), while simultaneously freeing up the availability of moisture and nutrients belowground (reduction of root masses).

In prairies where dominant grass species are suppressing plant diversity, we want to defoliate those grasses in a way that forces them to cede territory to other species.  In order for that to work, the first important factor is that the defoliation has to happen during the growing season.  Defoliating a dormant plant (e.g. with an early spring burn) doesn’t have any impact on its root system, which is a critically-important part of its competitive ability.  In order to force a plant to reallocate resources away from its roots, defoliation needs to take place after the plant has already invested significant resources in above-ground growth.  This is why a late-spring burn can have a significant (if temporary) impact on cool-season exotic grasses such as smooth brome.  Burning, grazing, or mowing grasses when they are just starting to flower has the biggest impact on most species because they have invested the maximum amount in their above-ground growth by that point.  Alternatively, repetitive mowing or grazing of grasses can also have a strong – and perhaps longer lasting – impact on their root systems because every time the grasses start to regrow, they get nipped off again, forcing them to regroup and reallocate resources time after time.

The immediate result of that kind of severe and/or repeated defoliation of dominant grasses is a release of opportunistic plants that thrive under low levels of competition.  This includes many annual and biennial plants, but also perennial plants that are built to move quickly into open space.  The quick flush of these plants often turns people off of defoliation because of a widely-held misperception that those “weedy” plants are outcompeting “good” plants.  In truth, the weedy plants are only able to grow because the competition that normally holds them in check has been suppressed.  When the defoliation event is over, the dominant grasses and other perennial plants will slowly recover their vigor – at which point the weedy plants will retreat and wait for another opportunity.  Rather than indicating a problem, I use the presence of weedy plants to tell me that my defoliation treatment has succeeded in weakening dominant grasses and has opened up space for other plants to take advantage of. 

As common as the overly-dominant grass problem is in prairie conservation, there is a frustrating scarcity of research that addresses it.  However, a recently-published research project by Kat McCain and others at Kansas State University provides some very nice insight into what can happen when dominant grass species are suppressed in a restored (reconstructed) prairie.  Kat and her colleagues studied plots of seeded prairie that had become heavily dominated by big bluestem and switchgrass over time.  They found that removing half or all of the big bluestem tillers (stems) – by clipping and herbicide application – from a plot significantly increased plant diversity.  Interestingly, removing switchgrass tillers in the same way had much less impact.  Following the removal of big bluestem tillers, the researchers saw increases in the vegetative cover of some forb species, including roundheaded bushclover (Lespedeza capitata),  pitcher sage (Salvia azurea), and blue wild indigo (Baptisia australis) within those plots, as well as new establishment of forb species including whorled milkweed (Asclepias verticillata), green antelopehorn milkweed (Asclepias viridis), leadplant (Amorpha canescens), roundheaded bushclover, and heath aster (Symphyotrichum ericoides).  In other words, suppression of big bluestem competition led to increased vigor among existing forbs and also allowed new plants to establish in the territory previously held by the dominant grass.  The study bolsters the theory that grass competition is suppressing forb diversity in many prairies, but also provides information on how plant communities might respond if that grass competition is reduced.

Of course there is a difference between simple defoliation and the kind of clipping/herbicide combination used by McCain and her colleagues.  In addition, most defoliation treatments (especially prescribed fire and haying) in prairie management are non-selective, meaning that all plants are simultaneously defoliated – not just the ones we want to suppress.  Uniform defoliation likely decreases some of the benefits of suppressing grass vigor because the vigor of the plants we hope will respond is suppressed as well.  However, there will be still be plants that can take advantage of the newly available light and soil resources following a uniform defoliation treatment, and by altering the timing of defoliations from year to year, we can ensure that a variety of species get the opportunity to respond.

Haying is a good example of uniform defoliation. Every plant gets cut at the same height and at the same time.


Ideally, though, we would like the ability to defoliate only those species that are suppressing plant diversity.  One way to do that is by using a selective herbicide such as Poast, which affects only grasses (not forbs, sedges, or other plants).  Poast is labeled for control of annual grasses, but at light rates can also provide short-term burn-back (defoliation) of perennial grasses as well, and some prairie managers have seen plant diversity increase following treatments.  Because it can kill annual grasses, and calibrating the appropriate application rate with the desired result can be tricky, it’s probably best to use this treatment on restored prairie rather than on remnant prairies for now – and to test it on small patches first. 

Another way to get selective defoliation is by the use of grazing.  In an earlier post, I described our use of patch-burn grazing in our Platte River Prairies as a way to increase and maintain plant diversity.  Patch-burn grazing is essentially a technique that uses patches of burned prairie within a larger prairie to attract grazing animals, concentrating grazing activity in those burned areas while allowing other areas to recover.  Under a light stocking rate, we find that cattle – even in the burned patches – are very selective about the plants they choose to eat.  Their top choice of grasses in the spring is smooth brome, and their summer favorite is big bluestem.  These happen to be two of the top three grasses that appear to stifle plant diversity in our prairies (the third is Kentucky bluegrass, which cattle like less well).  In our application of patch-burn grazing, a burned patch of prairie is normally grazed intensively for an entire season before the next patch is burned and cattle shift their attention to that.  That length of intense defoliation has significant impacts on the plants that are grazed – and again, under light stocking rates, the primary plants that are defoliated are smooth brome and big bluestem.  Interestingly, switchgrass is much less attractive to cattle and is often left ungrazed – or lightly grazed – in our prairies.  I found it intriguing (and encouraging!) that McCain and her colleagues found that switchgrass appeared to have much less impact on plant diversity than big bluestem did.

The effects of selective grazing in a restored prairie. This photo shows the burned patch with a patch-burn grazing system where a light stocking rate allows cattle to be selective about their eating preferences. Big bluestem is cropped very short, while other grasses and forbs are ungrazed or lightly grazed. Species such as hoary vervain aren't typically grazed even under high stocking rates, but many species such as purple prairie clover (front left), illinois bundleflower (middle left) and stiff sunflower (blooming) are commonly considered to be favorites of cattle - but only at higher stocking rates.

It would stand to reason that selective grazing of big bluestem and smooth brome would favor the expansion of the ungrazed plant species growing with those grasses.  While I’ve not had the time or resources to conduct much full-scale research (help wanted!), I do have data that supports that idea.  Through annual data collection of plant species frequency, I’ve found that the density of species (the number of plant species per 1m2 plot) increases by 20 to 30 percent in the year following the burn/graze treatment in a patch of prairie – in both restored and remnant prairies. 

Data from two prairies under patch-burn grazing. In both cases, the graphs show the number of plant species per square meter over time from the year of fire and intense grazing through the two subsequent years. The East Dahms site is a degraded remnant prairie and the Dahms 95 site is a restored prairie that was seeded in 1995 with over 150 plant species. The error bars represent 95% confidence intervals.

Of course, the increase I see in species density following grazing includes many plants such as ragweed and other opportunistic species, but I also see species like purple prairie clover (Dalea purpurea), Illinois bundleflower (Desmanthus illinoiensis), and stiff sunflower (Helianthus laetiflorus) respond as well.   In addition to seeing this in my plot data, I can walk out into the prairies and see seedlings of these species around the adult ungrazed plants.  Not all of those young plants survive their first year or two, but some permanent plot data I’ve looked at shows that at least some of them do.  By the way, I see similar post-grazing increases in plant species density and establishment of species like prairie clover under patch-burn grazing with higher stocking rates (less selective grazing) as well. 

As I said in my previous post on grazing, I’m not advocating that all prairies need to be grazed.  I’m not even advocating grazing as the solution to all prairies that suffer from overly-dominant grasses.  However, as we search for answers to address grass suppression of plant diversity, grazing certainly appears to be one viable alternative that is worth more investigation.  I’m continuing to experiment with variations in the way we employ fire and grazing treatments, and will keep learning as I go.  I’m also combining seed additions with those grazing treatments to see if I can take advantage of the open space created by defoliation to help establish new plants from seed – something that appears to happen rarely without some kind of suppression of surrounding vegetation.  I’m seeing some positive results, but it’s too early to know how well it will work long-term, and I’m still tweaking seeding rates and other factors.

Whether it’s grazing, prescribed fire, haying, or herbicide application, defoliation may be the most powerful tool available to help us suppress dominant grasses and increase plant diversity in prairies – where that is an issue.  The biggest obstacle to its application is probably the fear of causing damage to prairie by burning, cutting, or grazing plants during the growing season, but I think that fear ignores the resilience of prairie communities.  We still have a lot to learn about the most effective ways to apply defoliation to achieve our objectives, but the only way we’ll learn is by experimentation.  I hope you’ll join me in testing these methods and tracking the results.  If you do, please share what you learn with the rest of us so we can all work to figure this out.

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About Chris Helzer

Chris Helzer is an ecologist and Eastern Nebraska Program Director for The Nature Conservancy. He supervises the management and restoration of approximately 4,000 acres of land in central and eastern Nebraska - primarily along the central Platte River. Chris is also the author of "The Ecology and Management of Prairies in the Central United States", published by the University of Iowa Press.
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23 Responses to Using Defoliation of Dominant Grasses to Increase Prairie Plant Diversity

  1. Rich says:


    Last fall I conducted strip disking in a stand of overly-dominant WSG (big bluestem and indiangrass, primarily) and overseeded the disked area with a mix of forbs, in an effort to set back the grasses and encourage forb diversity. Can you speak of any experience, good or bad, with disking to boost diversity?

    • Chris Helzer says:

      Rich – I have limited personal experience with disking (we’ve only tried it 3-4 times). However, from what I know of it and from what I’ve learned from others, it can be very effective. I would VERY cautious about disking unplowed prairie, of course, because there are some important soil flora and fauna communities as well as a number of plant species that might not recover well. However, in seeded prairies dominated by grasses it can sometimes be a good tool. I’ve been surprised by how strongly warm-season grasses can recover within just a year or two of a fairly aggressive disking treatment. In fact, we once tried to get rid of eastern gamagrass by triple disking a site that was planted to a monoculture of it and the stuff wouldn’t die!

      One caution with disking is that it can release some pretty nasty weeds if those are present in the seed bank, so you’d want to be aware of that possibility. On the other hand, I’ve also done some light disking/seeding in some very degraded prairie (just small test areas) and a year later we couldn’t even tell where we’d disked – and found almost no new seedlings from our seed. So if you’re going to commit to disking, it probably makes sense to be fairly aggressive (in seeded prairie…)

      I know there are others who read this blog regularly who have more experience with disking/seeding than I do, so I’m hoping they’ll add comments here as well.

      • Stephen Winter says:

        I’d like to echo Chris’ caution about disking remnant prairie. I’m inclined to think that any tap-rooted forb that has it’s tap-root severed will die under typical conditions of soil moisture.

        In contrast, rhizomatous species should respond well to disking because having a rhizome severed may stimulate associated buds to initiate new growth/tillers.

  2. James C. Trager says:

    Interesting, Rich:
    A friend of mine has three acres of planted prairie on sandy loam soil where he has disked, then sowed forbs, to reduce grass dominance in several sections over the years. The results are pretty and suggestive, and he swears by it, recommending it to others frequently. More, controlled studies would be desirable, though…

  3. Stephen Winter says:

    Chris – one thing I’ve been wondering about lately is how some of the conservative forbs may in fact be quite competitive themselves, and not subject to the same competitive dynamics with grasses that we typically envision. Looking at some of the eastern Nebraska hay meadows the last couple of years I’ve been seeing what appear to me to be a high diversity and abundance of conservative forbs, in spite of the fact that they’re growing in what I assume are vigorous stands of grass.

    This might go back to the eastern/western prairie discussion some of us are familiar with (especially recently) and the sub-topic within that discussion about the prevalence of herbivory (i.e., defoliation in the context of your post) in the east vs the west. IF eastern prairies had historically low levels of herbivory, PERHAPS selection has favored associated forbs in those prairies that are capable of competing with the grasses instead of having to rely on herbivory to provide them with competitive release.

    My Konza world-view leads me to believe that frequent spring fires benefit the dominant grasses to the detriment of sub-dominants. However, here’s a citation from the east that seems to suggest the entire prairie flora (in their study region) does just fine with frequent spring fires:

    Rooney and Leach. 2010. Replacing Hay-mowing with Prescribed Fire Restores Species Diversity and Conservation Value in a Tallgrass Prairie Sampled
    Thrice: A 59-Year Study. American Midland Naturalist 164:311-321.

    • Chris Helzer says:

      Steve – there’s a lot to respond to in your comment…!

      First, I think you’re right that there are a number of forbs that are very competitive with grasses, and can hold their own – at least in many prairies. Whether that’s because of something intrinsic to those species or something in the prairie environment that balances competition I don’t know.
      Second, I think it’s likely that hay meadows have a somewhat weaker grass stand than a prairie that is not annually hayed. Obviously, the timing of the haying (especially relative to the phenology of the grasses) makes a big difference. But I think annual haying, in general, probably helps those species that bloom prior to cutting and hurts those that bloom later. So even though those sites have a good diversity of early and mid-season plants, they don’t have much going on in the late summer and fall because the prairies are only a few inches tall!
      Which brings me to:
      Third, some would say (and you’re well aware of this, I know) that high plant diversity is the major measure of success in prairies, and others would say that habitat structure and the inclusion of historic (or at least important) processes such as fire and grazing are also necessary. My November 9, 2010 post dealt with that topic. A prairie that is maintaining high plant diversity through annual haying or annual prescribed fire (I’ve seen good examples of both) may be in great shape, depending upon what the objectives are for that prairie. On the other hand, it might be missing plant species that require different management, and is surely missing animal and insect species that require habitat structure not provided by that management.
      I’m not trying to sidestep the question about whether or not annual fire or haying can maintain high diversity plant communities in prairies. I’m confident that they can and have. I also have questions, though, about whether that’s sustainable – or if it’s largely because many prairie forbs live so dang long that it takes a very long time to see species loss through lack of sexual reproduction. I don’t honestly know the answer to those questions. I also worry about whether or not those prairies could have even HIGHER plant species richness and/or diversity if they had a more varied management regime applied to them. The only way to know is to try. And finally, – again – there are a number of insect and animals species that need a more varied habitat structure than can be provided by a repetitive uniform management regime like annual fire and haying. The importance of those animal species has to be determined by local objectives.
      Anyway, yes, it’s an interesting discussion to have. I certainly don’t know the answers, but enjoy learning more all the time!

  4. Mel says:

    Sounds pretty complicated but what I’d expect from a thriving multi-culture. Were the original prairies diversified via fire and grazing only? Or were there other factors involved – say harsh winters?

    • Chris Helzer says:

      Good question, Mel. We don’t really know. There’s lots of discussion, and much opinion that is divided regionally. That could be because prairies acted very differently in each region, or it could be the varied perspective on what “good” prairies look and act like.

      I doubt that harsh winters had too much to do with prairie plant diversity – other than to help keep woody species out. I think most prairie plants are pretty equal in their ability to withstand winter conditions because they’re dormant at that time (I could be wrong about that). I think fire and grazing were large components – at least in the prairies I know best. And climate was also extremely important.

    • Stephen Winter says:

      I’m inclined to agree with Chris that harsh winters have little to no meaningful effect. Droughts are another matter and we have good descriptions from the early to mid 20th century of these substantial effects from the work of John Weaver.

      A biotic effect that we’ve lost (although I’m not sure how prevalent it would have been in the tallgrass region) is episodic, severe herbivory by Rocky Mountain locust, the now-extinct migratory grasshopper that was the basis of pioneer many tales.

  5. James C. Trager says:

    Fascinating post! I think it’s worth mentioning that the prairie natural areas in Missouri, about which there has been some controversy lately (about whether to do patch-burn grazing on them), were hayed annually before they became protected and designated as natural areas for their diverse florulas. Any thoughts on that…?

  6. Good thing you live in Nebraska. Mowing, haying, herbicides, patch grazing? Them’s fightin’ words here in Missouri!

  7. Dennis Fitzwilliam says:

    A 2 acre remnant loess hill prairie in southern Illinois, that has been rescued from significant woody invasion over the past 5 years, is now overly dominated by Bluestems and Indian grass.
    Forbs have noticeably diminished in the past 2 years. June grass and Canadian rye are the cool season components. The prairie has had 3 spring burns, 2007, 2008 and 2011(only 50% ). The plan is to start burning in Fall to stop favoring the grasses. A 1973 inventory showed 60 species and presently it’s probably about 50. In dealing with the grass dominance how does one decide between a growing season burn or a weed eater and for either method what should the timing be. Either method would be performed as an experiment on a relatively small area.

    • Chris Helzer says:

      Dennis – it’s a good question, and I’m not going to give you a solid answer. I think it’s a great idea to try a small experiment, and I’m not sure there’s a good way to decide between a fire and a weed eater. (Try both!) In terms of timing, there are a couple choices. If you use the weed eater, you could hit bluestem/indiangrass repeatedly through the season (and mow around forbs as feasible) to keep the vigor of those grasses down. I’d mow as short as possible – if you leave too much height on those grasses they can actually increase their horizontal spread. Another option would be to mow or burn as the grasses are just starting to flower and try to knock their legs out from under them just as they’re at the peak of their energy expenditure for the season.

      I think either way could work well – but you’ll have to see what happens. I’m glad to hear that you’re willing to try some experimental ideas. Please let me know what you do and how it works! It’ll be important to watch for any invasive species that might try to fill the void left by the big grasses, but hopefully you’ll see that open space colonized by prairie forbs instead.

      Good luck!

  8. Suzanne says:

    Chris – do you have any thoughts about how using bison for grazing might differ from using cattle?

    • Chris Helzer says:

      Suzanne – the biggest differences are behavioral rather than related to diet selection. We’re finding that the diet selection of bison and cattle is very similar when they’re managed in the same way. They’d both rather eat grasses than forbs. Behaviorally, bison are less likely to trail in single file, sit and poop in water, or congregate in shade.

      However, you don’t see many of the behavioral “benefits” of bison unless they’re managed on a pretty big scale (several thousand acres or more) where they actually can act like bison. Also, there are some big logistical challenges in running bison, including the need to keep them year-round (as opposed to bringing cattle in for short periods – up to 6 months – through a lease arrangement). In addition, there are strict innoculation requirements for bison that requires them to be rounded up and medicated each year, and fences need to be a bit stouter than for cattle.

      In most cases, cattle make a good substitute for bison, although you have to be thoughtful about how they are applied to minimize some of their drawbacks (trailing, pooping in water, etc.).

  9. Rex Peterson says:

    A few weeks ago, the range ecologist for our local NRD and a landowner who is delighted with the range ecology improvement on their pasture after leasing it to us, asked us to come quick, quick and fence a stand of 5 and 6 year old CRP and graze it. We developed a four year program to rotate the grazing. Yesterday, we received an email announcing that the spring grazing treatment was off limits. I found your site while trying to research why such a decision was made. I appreciate your insights, they help me understand what might have been the thinking. Unfortunately, in the case of this parcel, that early spring grasses like brome and the annual weed grasses are the limiters, rather than the seeded switchgrass.

    • Chris Helzer says:

      Hi Rex,

      Yeah, the CRP program can be tough to deal with sometimes because the primary objectives – and most of the strategies – have been set by the federal governmnet, leaving fewer options for the landowner. On the other hand, the landowner gets paid to go along with those objectives…

      Because one of the objective is wildlife habitat, and grassland birds are what most people consider to be wildlife, nesting season gets a little more priority than it probably should. I’m glad you’ll be able to do any grazing at all, actually, because that’s a relatively new phenomena in CRP. It might be worthwhile to run some questions up the chain in NRCS – if you can show them that the brome is causing problems and that there are long-term benefits to suppressing it, you might have some luck.

      Good luck!


  10. Mike Suiter says:

    Hi Chris,

    Just found out about your site from NE games and parks. Looks like I have a lot of reading to do! I live in Springfield and in 2009 had an acre of brome killed off. In spring 2010 I had it drill seeded with prairie grasses and wildflowers. The grasses are little bluestem, side oats grama, blue grama, and buffalo grass. The flowers are 9 annuals and 21 perennials.

    The wet season gave me good establishment of the prairie but also was good for yellow nutsedge and foxtail. I used Sedgehammer and got the nutsedge under control. In November I over seeded areas that didn’t establish as well. Here are some pictures of last year.

    This spring a lot of flowers are coming up and some areas of brome coming back. I zapped the brome with roundup over the weekend. I know that some flowers will die but think it’s better to control the brome now than worry about flowers. The grasses are starting to wake up and I’m hoping the 2nd year will be more impressive. Here is what things looked like a week ago.

    Looking forward to reading more on the site.


    • Chris Helzer says:

      Sounds like you’re doing the right kinds of things to get your plants established. I think you’re right to hit the brome now while you’ve got a chance. It’s also important to do the best you can to kill it out along the edges of the site so it doesn’t just creep in over time.


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