Aggressive Weed or Opportunistic Plant? It’s Good to Know the Difference

“Those weeds are really taking over my pasture!”

I cringe when I hear that sentence because it’s often a precursor to broadcast spraying of herbicide and the subsequent loss of most plant diversity in a prairie.  That’s really bad.  What’s most frustrating, however, is that the sentence is rarely true.

East Dahms pasture. Ragweed in degraded pasture.

The ragweed in this pasture is not acting aggressively.  It is filling spaces left open by grasses weakened by intensive grazing.  Within a year or two of this photo, this site was dominated by big bluestem.

It’s easy to understand how a landowner would look at a pasture that is visually dominated by ragweed, buffalo bur, snow-on-the-mountain, hoary vervain, or a number of other weedy plants and think those plants are aggressively pushing grasses out of the way.  In almost every case, however, the opposite is true.  Grasses are usually the bullies of the plant community, and only when they are suppressed by fire, grazing, or some other pressure do the “weeds” thrive.

There are a few weeds that can out bully grasses, of course.  Leafy spurge, crown vetch, and sericea lespedeza are good examples in the central United States.  They seem to be able to invade and spread regardless of the vigor of grasses and other competing plants.  Landowners should absolutely work to control those aggressive perennial species before they get a foothold across large areas.

Leafy spurge at The Nature Conservancy's Broken Kettle Grasslands in the northern Loess Hills of Iowa.

Leafy spurge can be a serious threat to prairies and should be dealt with quickly to prevent it from spreading throughout a site.

However, while there are some important exceptions, most pasture weeds are more opportunistic than aggressive.  Opportunistic plants don’t compete well with grasses or other perennial plants when those plants are at full strength, but can move quickly to fill spaces left between plants that are weakened by intensive grazing or drought.  Many opportunistic species are short-lived, and produce huge numbers of seeds, and those seeds sit in the soil waiting for a chance to germinate and grow.  When the tops of grasses are grazed off, the roots below shrink up as well, creating the perfect opportunity for seeds to germinate and new plants to establish.

The majority of those new plants will survive only as long as the vigor of the surrounding grasses remains low.  As those grasses recover, they regain their advantages, both above and below ground.  Annual plants may bloom and drop more seed, but those seeds have to wait until the grasses are weakened again before they can germinate and grow.  Perennial opportunistic plants might stick around a little longer, but most of those will also lose out to recovering grasses because of their poor competitive ability.

These "weedy" species are filling in while grasses recover from a grazing bout. In the meantime, the hoary vervain (purple) and upright coneflower (yellow) are providing important pollinator resources and great habitat for other species, including insects, reptiles, and birds like northern bobwhite.

These “weedy” species are filling in while grasses recover from a grazing bout. In the meantime, the hoary vervain (purple) and upright coneflower (yellow in foreground) are providing important pollinator resources and great habitat for other species, including insects, reptiles, and brood-rearing habitat for birds like northern bobwhite.

There’s an easy way to find out whether or not the “weeds” in a pasture are aggressive or opportunistic – build an exclosure or two to keep grazing out for a year or more.  If the grasses within those exclosures regain their vigor and dominance, you’ll know it was the grazing pressure that was creating opportunities for weeds.  If the weeds continue to dominate the area inside the exclosure for a couple years (assuming you’re not in the middle of a drought that is keeping those grasses down), you’ll know that either the grasses have been debilitated to the point of no return or the weeds are truly aggressive and in need of control.

Grazing exclosure at the Dahms Tract.

Even a small and simple exclosure can help determine whether weeds are suppressing grasses or just taking advantage of grasses already weakened by grazing.

As a final note, it’s important to understand that grazing hard enough to suppress grasses and allow weedy plants to flourish temporarily is not at a bad thing.  Ecologically, the habitat conditions created by those tall weedy plants are critically important for many wildlife species, including upland game birds.  Many important wildflowers also benefit from the opportunity to reproduce during short periods (a year or two) of weakened grasses.  As long as the grasses are allowed to recover before they are intensively grazed again, they’ll be fine, and the wildlife, pollinators, and plant diversity of your prairie will all benefit from the temporary reprieve from grass dominance.  Shifting intensive grazing and subsequent rest periods around a large prairie, especially when those grazing/rest periods are a couple months long or longer, seems to be a great strategy for maintaining prairie health.

Opportunistic plants suffer from a public relations crisis.  While they are scorned by most people, these valuable plants are doing exactly what they’re supposed to do.  They are the temp workers of the plant community – the substitute teachers, backup quarterbacks, and house sitters that keep prairies humming along when dominant grasses are on sick leave.  By filling spaces between temporarily shrunken grass plants, opportunistic plants can help prevent the truly aggressive weeds from easily gaining a foothold.  They can also provide much needed habitat for wildlife and pollinators.  Ragweed, hoary vervain, and buffalo bur aren’t the villains of the story at all – they’re the heroes!  We just have to get used to seeing them that way.

 

 

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About Chris Helzer

Chris Helzer is the Director of Science for The Nature Conservancy in Nebraska. His main role is to evaluate and capture lessons from the Conservancy’s land management and restoration work and then share those lessons with other landowners – both private and public. In addition, Chris works to raise awareness about the importance of prairies and their conservation through his writing, photography, and presentations to various groups. Chris is also the author of "The Ecology and Management of Prairies in the Central United States", published by the University of Iowa Press. He lives in Aurora, Nebraska with his wife Kim and their children.
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17 Responses to Aggressive Weed or Opportunistic Plant? It’s Good to Know the Difference

  1. Patrick says:

    These weeds are sort of like aphids. Abundant and a good food source for other critters, but eventually decline. I especially like the vervains…great butterfly attractors.

  2. Pat says:

    Thank you for explaining that with such clarity!

  3. Chris Muldoon says:

    Ah! I’ve been waiting for another one of your insightful posts that pulls all the ends together!

  4. Peter Dunwiddie says:

    Another insightful piece, Chris. Thanks! I’d also note that our aggravation with such plants (and I include myself in this) often is particularly intense if they are non-native. Yet, it is interesting to note how often such “pests” actually seem to just co-exist with, rather than outcompete and replace, many if not most native species. What I find particularly interesting are those that now play an important role in sustaining rare species, including butterflies and other pollinators, some birds, etc. It is a perspective on non-native “invaders” that is not widely embraced yet in the conservation and restoration communities, and certainly does not apply to all such species. Nonetheless, it is one that many of us need to be more open-minded about.

    • Chris Helzer says:

      Thanks Peter. I agree. Just discriminating against plant species because they are non-native seems to be a waste of energy that should be directed at species that are actually causing harm (which can be either non-native or native). If a plant species is not displacing others, lowering overall plant diversity, or causing other significant problems, I’ve gotten to a place where I’m at peace with it.

      • James McGee says:

        In my experience, native plants that are reducing diversity are always the result of changes to ecological processes. Whereas, non-native plants are a problem regardless of management and often become more of a problem when ecological process necessary to maintain a plant community are continued. I think this is the reason non-native plants often receive more attention (or aggravation) than opportunistic native species.

  5. Justin Thomas says:

    The picture you paint is certainly beneficial to weeds and to generalist grasses, but what about the conservative species (the doctors, lawyers, professors and movie stars). How do they fit in to the frequent intensive grazing regime? I’m concerned, as are many professionals in the ecology community here in Missouri, that this type of management philosophy is detrimental to those species (mostly conservative forbs, but also some conservative grasses) that require long-term stable communities; those species that colonize, thrive and persist long after the generalist grasses reestablish dominance. This concern comes from data which show very scary trends. In Missouri, several of our prairies in the Natural Areas program were intensively grazed (others still are) more than 10 years ago. They have not recovered. Some have suggested that they no longer qualify as Natural Areas and should be dropped from the program. They appear to be stuck in a degraded stable state. My research with the Missouri Prairie Foundation has demonstrated that once the conservative flora is lost, it might bounce back a little, but not to its former state. Continuing degrading processes only further degrades. If intense grazing degrades prairies and they bounce back to the grass dominant state and then they are intensely grazed again, and again, when do they recover the conservative flora that makes up 50-75% of the species richness of a given site? When does the late successional flora that may very well be remnant from now gone climatically driven dynamics recolonize? The specialist flora that provides for the specialist animals. The high-end merchandise. A community of generalist pioneers is fine, but a rich, functional community also must have room for specialists. Specialists take time and stability. That is, after all, how they became specialists.

    • Chris Helzer says:

      Justin, thanks for the question (and for continuing my corny metaphors!) An appropriate response takes more space than a comment reply gives me, but let me give it a shot. First, the more conservative species in our prairies (rosinweed, compass plant, leadplant, prairie clovers, Canada milkvetch, etc.) seem to respond very well to this kind of intensive grazing/long recovery periods. I find lots of little prairie clover seedlings, for example, around the base of adult plants during and after grazing, and at least some of those make it to maturity. I have 15 years or more of data from some sites tracking populations of these species through patch-burn grazing in both wet and dry years. In some sites, our grazing exclosures that are burned but not grazed are significantly more grass dominated and less forby than than those that are both burned and grazed. That’s not universally true, however – some sites seem to hold on to good plant communities with a fire only management, and that’s a research topic that is extremely intriguing to me. What I’ve not seen is prairies that have lost species (including conservative ones) from the kind of fire/grazing we’re doing. I’m actually pretty familiar with some of the prairies in Missouri that have been patch-burn grazed, and have seen them before and after that grazing. To my non-Missouri eyes, they look very good, and I’ve seen many healthy populations of conservative species in places that were grazed down to the nubbin. I’ve spoken to Missouri botanists who were skeptical until they actually visited the sites and were pleasantly surprised at how well they looked, and who are now willing to watch and learn to see what happens in the long term. I’ve also seen and read some of the responses from people who are not happy with the grazing, and I respect their viewpoints. I know MDC is trying hard to collect better data to see what’s happening with those plant communities, and I will be watching – along with many others, I know – to see what comes of that effort. It’s a tricky thing to study. I’ve never seen a long-lived perennial plant die just because it was cropped off. If that were the case, I’d think haying would be a terrible idea, but hayed prairies tend to have very strong conservative plant populations. Maybe grazing creates different responses, but I’ve not seen conservative plants die after one or two years of being grazed off either. They seem pretty tough. I also need to follow up with people who know more about western Missouri Prairies because some of the ones that are most respected as having high quality plant communities also seem to have what look like old stock ponds on them, making me wonder about their grazing history (in other words, did they go through some tough grazing years after European settlement but then recover?) I’m not saying they did, I’m just curious to hear more about that and need to talk to people who might know. Anyway, thanks again for the comment. We’re all trying to figure this stuff out.

      • Justin Thomas says:

        Thanks for the reply, Chris. And I’m sorry to be such a thorn on this topic. All the field ecologists and field botanist of any clout that I know have long since shook their heads in dismay and walked away from this issue; which is really depressing. I can’t do that. So…

        I think the only thing we really know, in the grazing prairie debate, is that SOME level/degree of grazing is harmful. There is no shortage of examples. It saddens me deeply and really makes me question the spirit and science behind prairie conservation knowing that no one has spent the requisite money to investigate, with sound scientific methodology, where the tipping point is; if some degree is beneficial. Too many assumptions are being ignored and too much opinion is taken as fact. What is clear, at least in Missouri, is that high quality prairies (in terms of floristic integrity) that have not been grazed, continue to be ecological hot spots of diversity, richness, structure and function. They are as ecologically stable as one could possibly imagine, while supporting highly conservative species at amazing levels of richness and diversity. Since grazing is such a giant throbbing question mark for the best ecologists and botanists in the region, I see no point in gambling with the irreplaceable, or proceeding as though the mystery has been solved. Yesterday’s good idea is too often today’s regret.

        • James McGee says:

          Hi Justin, I tend to agree with you. Would you please provide examples of which plants tend to not recover after intense grazing? If you step on virgin prairie you can immediately tell the soil is special. It is like walking on a sponge. This is true even on gravel hill prairies. Once repeated trampling or plowing has destroyed the soil structure the recovery times are so long that I have been unable to see much progress within my lifetime. The effect of trampling is not just a problem from intense grazing, it can easily be observed in natural areas that have received too much love from the visiting public.

          The people who make their living from ranching should not be concerned about this discussion. So little prairie remains in a virgin state that protecting the little that is left will not have any effect on them when compared to obstacles like fluctuations in price or weather.

          • Justin Thomas says:

            Great points, James. Soil variables are yet another thing we know next to nothing about in this debate. Trampling by large beasts is certain to change soil dynamics that may take, as you say, many, many years to recover; assuming recovery is even possible. Heck, for all we know, it is good for prairie soil. That’s why this is all so frustrating. Wet areas get especially hammered and high quality swales are quickly turned into weedy mud holes. Some managers hotwire around riparian zones. I’ve always found this to be a contradiction.

            You asked for a list of species that don’t recover from intense grazing. One would think that there would be some sort of published research out there to answer this very important question. Unfortunately there is none (none published, at least). Over the past few years Doug Ladd of TNC and I have collected quadrat data on a prairie that was patch-burn grazed. We found that there was an impressive initial spike in richness the growing season after the treatment (largely due to increased weedy species), but that was followed by two years of dramatic decline that ultimately resulted in a net loss of richness and quality (quality being measured by FQA protocols). My own research comparing those species that occur on high quality prairies but not on low quality prairies (having been degraded by variable influences including grazing) have demonstrated an interesting phenomenon called the Floristic Integrity Curve. This curve show that high richness and diversity can occur at low average C-values and high average C-values and that there is a grass dominant state between the two. Much like Chris’ blog topic, weediness and richness increase with disturbance, then grass out competes the weeds (thus dropping overall richness but increasing floristic quality) until a grass dominant state is achieved. At this point, conservative species, which are slow to colonize, move in when grasses have bad years and create a rich mosaic of co-dominance. High quality prairies thus have several forb species co-dominating with grasses rather than just grasses dominating. The more equal the ratio, the higher the richness, quality, diversity, evenness and, hypothetically, function and ecological stability. These are the species that degraded prairies have lost and that we fear losing from existing prairies. Here is a short list of vascular plants:

            Eurybia hemispherica, Echinacea pallida, Scleria triglomerata, Solidago gattingeri, Parthenium hispidum, Dichanthelium praecocius, Camassia angusta, Fimbristylis puberula, Maianthemum virginicum, Solidago missouriensis, Nemastylis nuttallii, Calopogon oklahomensis, Dichanthelium perlongum, Dichanthelium wilcoxianum.

            There are more, but these are good examples. The list is also SW Missouri-centric.

            If you are interested in reading more about the Floristic Integrity Curve, here is a link to a report I conducted for the Missouri Prairie Foundation that outlines it well.

            http://www.moprairie.org/wp-content/uploads/2015/03/PennSylvania-Prairie-Report-Final.pdf

          • James McGee says:

            I have often said that the prairie is like a forest of grass. The difference you are seeing is probably like the difference in old growth forest and second growth forest. Second growth forest can have a lot of diversity. However, there is very little remaining old growth forest with its unique life. This makes the old growth forest very special.

  6. Teresa Lombard says:

    This was a great timely read for me because I have a prairie that I suspect was very heavily grazed for years so that the conservative species mentioned have not appeared in the last three to four years with no grazing, one haying year and 1/3 of the 9 acres burned once but the pasture is also compromised by brome infestation. I highly value the opportunistic ‘weeds’ for the pollinator and wildlife value (ragweed, vervain, ironweed, etc.) but I am a little uncertain whether the conservative species can still be brought back – either from seedbank or broadcast seeding. Longtime residents have said the pasture was never cropped, but I tend to doubt that since the conservative species are mostly missing (one leadplant plant in the nine acres, no dalea, no lespedeza, no compass plant, cup plant etc). I’m interested in the ‘tipping point’ – at what stage has the landscape been so degraded, that the conservative species won’t come back. I intend to burn the whole nine acres this spring and so something different could pop up after that, but I’m also looking at options such as August burning followed by mid to late October spraying (when brome will hopefully be the only thing still active) followed by December or January seeding of the ‘missing’ forb species. But I have lots of concern about the spraying, and whether or not the conservative species, even heavily seeded, will be able to find and fill a ‘niche’ in the new less diverse current ecosystem. So I guess I am echoing Justin’s questions about recovery of prairies that were heavily grazed for many years – can conservative species come back once they go missing and diversity has dropped below a certain point. I think I do still have perhaps 40 native species on this pasture including grasses – but 200 – 400 would likely have been there originally.

    • James McGee says:

      Yes, conservative species can come back once eliminated and diversity has dropped below a certain point. It just tends not to happen on its own and takes a lot of work. Here is a picture of a local restoration that was an agricultural field. Active restoration of this site started 26 years ago.

    • James C. Trager says:

      Teresa – I am concerned about this statement, “I intend to burn the whole nine acres this spring”. If the intent is to restore diversity, I’ll hazard a guess that the spring-flowering component, including sedges and native cool-season grasses, is a significant part of what is lost. A fence line to fence line fire in spring will not help those species. Subdividing into units that will be burned during the full-dormant season (i.e. – the limits of the hard frost season) on staggered 2-3 year intervals would be a lot safer for those “high end merchandise” species (specialist invertebrates) Justin mentioned.

      • Teresa Lombard says:

        Thanks for that comment! I do have sedges, scribners panicum, and one year (I think the year after I burned, but in an unburned area) I had carolina anemone in just a couple of spots). A neighbor divided by us by a road only hays his, so there will still be some refugia nearby, and there are other pastures (not ours) in the same section and next section over, but I admit I don’t really like burning the entire thing at once. If my primary intent to remove thatch to stress the brome and gain better seed contact with soil in the fall/winter, I would probably try to just burn a third every year.

  7. Warner, Tyler says:

    Hey Chris, I’ve been following your blog for a couple years and read most of your posts. Keep it up and keep them coming.

    Thanks,

    *Tyler Warner* *District Wildlife Biologist* *Kansas Department of Wildlife, Parks & Tourism* *7760 174th Street* *Valley Falls, KS 66088* *785-945-6615*

    On Tue, Mar 22, 2016 at 4:08 PM, The Prairie Ecologist wrote:

    > Chris Helzer posted: ““Those weeds are really taking over my pasture!” I > cringe when I hear that sentence because it’s often a precursor to > broadcast spraying of herbicide and the subsequent loss of most plant > diversity in a prairie. That’s really bad. What’s most frustra” >

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