Remembering Why We’re Fighting Invasive Plants

Why do we worry about invasive plants?  Most of us aren’t trying to stave off invasives simply because they aren’t part of the native plant community.  Instead, those invasive plants are causing some kind of problem that disrupts or degrades the diversity and function of an ecosystem.  Plants like crown vetch and Caucasian bluestem, for example, can form near or total monocultures, displacing other plant species.  The result is a simplified plant community that also provides much lower quality habitat for most prairie animals (and livestock, where applicable) and is less resilient to drought or other challenges.  Invasion by eastern red cedar and other trees converts prairie habitat into woodland habitat, making it inhospitable to many prairie animals.  At the same time, tree invasion also decreases available forage for livestock, displaces native prairie plants, and facilitates the encroachment of other invasive plants around and beneath trees.

Eastern red cedar trees can shade out large areas of vegetation and transform grassland habitat into woodland habitat.

As land managers, we need to be able to specifically define the desired condition of our prairies and the precise ways in which invasive plants prevent us from achieving those goals.  After all, our ultimate objective isn’t to eradicate any particular invasive species, it is to create or maintain those defined site conditions.  In the heat of battle, however, it can be easy to lose sight of what we’re fighting for.  We can become so focused on eliminating an invasive plant that we ignore the collateral damage and cause negative impacts to our site that can rival the impacts of the invasive plant.

Eye on the Prize

There are a number of ways invasive species control can go awry when our focus is misplaced.  One example occurs when a land manager goes after the biggest patches of an invasive instead of first concentrating on new infestations.  It can be tempting to start with the largest and most dense areas of invasion, but in many cases, those areas should actually be the lowest priority.  If the goal is to prevent the invasive species from damaging the site, the focus should be on the edges of those big patches, and more importantly, any small satellite invasions that are just starting.  By first attacking new patches, we can contain the invasive threat.  It’s easy to think that killing the most enemies is the best strategy, and that it’s easiest to do that in the most invaded areas.  Usually, however, the best strategy for the condition of the overall prairie is to prioritize the portions that are still in good shape before wading into the already damaged patches.

Another big example of misplaced focus happens when herbicides are applied to large swaths of a prairie, instead of through a targeted spot-treatment approach.  In the worst case scenario, this occurs in a battle against a species that like ragweed or others that aren’t really an invasive threat in the first place.  Let’s assume, however, that the targeted plant is a real problem.  There are certainly times when a broadcast application of herbicides across large dense patches is the best tactic (while remembering the lesson from the previous paragraph), but that comes at a tremendous cost.  Even the more selective herbicide options available today still affect non-target species (the label often provides a long list of species that are resistant to the chemical, but that list is a lot smaller than the species list of most prairies).  Reducing the plant diversity of an area through herbicide application runs counter to most prairie managers’ goals and can also make the same area more vulnerable to future invasion.

When Kentucky bluegrass or other invasive plants become ubiquitous, attacking them with broadcast herbicide application can affect the diversity and resilience of the remaining community. It is usually better to attempt to suppress the vigor of the invasives through the use of fire, grazing, mowing, or other targeted strategies that allows the remaining community to better compete with the invasive plants.

Applying herbicide to individual plants or small dense patches is usually much better than spraying a larger area that contains populations of invasive and non-invasive plants.  In some cases, it’s even more effective to dig out or chop individual plants and avoid herbicide altogether.  Even applying herbicide to a single plant at a time can affect surrounding vegetation.  Sometimes this happens as some spray droplets simply miss the target and hit other plants.  Other times, however, it can happen via volatilization – a process in which some herbicides can transform into a gas and travel away from the plant they’ve been sprayed upon.  Unfortunately, many invasive plants require herbicides to kill them because their growth strategies make digging or chopping attacks unsuccessful.  Just remember that even if only a few non-target plants are killed each time an invasive plant is sprayed, that can add up to a huge impact over many years.

Applying herbicide to individual plants is time consuming (and still affects some nearby plants) but can help preserve the diversity of the surrounding prairie much better than broadcast herbicide application.

Of course, approaches that target individual plants instead of larger areas come with tricky trade-offs regarding time and resources.  Dedicating a lot of time and people to a slow and meticulous approach to invasive plant control can drain resources away from other important land management tasks – including work on other invasive threats.  The flip side, though, is that if our quicker, more aggressive approach is degrading the plant diversity of a site over time, what are we really fighting for anyway?  Creative approaches to finding resources (volunteer labor, better equipment, etc.) may really pay off in the long run.  Regardless, the key is to recognize our overall goals and how each attack option will impact those goals.

Finally, it’s important to focus on broader goals when deciding whether to attempt eradication versus suppression of a particular invasive plant species.  In many cases, invasive plants can become so embedded within a plant community that eradication is impossible without serious impacts on the rest of the community.  Often, designing a management strategy aimed at reducing the dominance of those invasives can maintain a diverse plant community despite the presence of a ubiquitous invasive species.  In prairies invaded by Kentucky bluegrass, smooth brome, or other invasive grasses, for example, strategic burning and/or grazing can suppress the vigor of those plants often enough to allow the rest of the community to thrive.

This prescribed fire helped set back Kentucky bluegrass and smooth brome, allowing a more diverse plant community to flourish. It is a short-term effect and needs to be repeated, but can help maintain a diverse prairie despite a heavy dose of invasive plants.

Measuring Success

Whether we are evaluating the success of our invasive species work via observation or hardcore data collection, we need to be sure we’re looking at the right metrics.  It can be tempting to measure success by counting how many plants have been killed, or how much area has been cleaned up.  However, if our broader goal is to sustain plant diversity, forage productivity, or habitat quality, we should really be measuring the status of those attributes. 

Measuring how many acres have been cleared of eastern red cedar or crown vetch doesn’t tell us whether or not plant diversity is high or nesting bird success has increased.  Focusing on the potential impacts of invasive species to our goals will indicate what to measure.  It might be more difficult to measure nesting success or plant diversity than to count invasive plants or acres.  However, focusing the impacts rather than the abundance of invasive plants is both a better indication of success and a way to be sure our approach to attacking invasive species is actually achieving our real goals.

If prairie plant diversity is the attribute we want to protect from invasive plants, we should measure success by quantifying plant diversity rather than invasive plant abundance or the number of individual invasive plants removed.

The Upshot

If you’re fighting invasive species at your prairie, and of course you are, it can be useful to pause now and then to make sure you’re still focusing on your broader goals.  There will always be difficult decisions to make when combating invasive plants and there are many right and wrong approaches to any situation.  The key is to be sure you’re designing strategies and evaluating them based on the long-term success of the prairie, rather than on how many invasive plants you can kill.  Good luck out there!

This entry was posted in Uncategorized by Chris Helzer. Bookmark the permalink.

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.

15 thoughts on “Remembering Why We’re Fighting Invasive Plants

  1. May I have permission to copy this article and distribute it to our park volunteers?
    Thank you,
    Patty Trnka
    MN Master Naturalist Volunteer

  2. The most important piece of instruction that invasive species training classes have neglected is to measure, evaluate, and work to improve. This is important both from the stand point that you have accomplished your objective of controlling the invasive species and also not harming desired vegetation.

    Usually, in remnant ecosystems the improvement from selectively removing invasive species is so apparent that continually measuring the changes in plant diversity or the nesting success of birds is not going to help you improve your control efforts. This being said, plant diversity and statistics on birds are being measured locally. These numbers and the variation over time are interesting. However, I don’t see how this data directly helps those doing invasive species control work get improved results. To me, it just confirms what is already apparent.

    • I tend to agree with you James, depending on the species. For example, if historical photos show a site was grassland, but it has been overtaken by cedars, the time it takes to measure ecosystem recovery after cedar removal may take valuable time away from actually removing cedars.

      That said, there may be better ways to remove cedars than cutting them all down at once. Here I would agree with Chris that focusing on minimally invades sites first, and working outward toward heavier stands (limbing them up first, to increase light and allow for some plant recovery before cutting the tree) might be better than mulching a whole stand, or creating rows of downed timber, even if the entire process takes longer.

      The first goal then would be to remove cedars, and then worry about other grassland invaders as it returns to an open grassy state. This may require interspersing cedar removal and spot treatment of other invasive species.

      • To clarify my larger point, I was trying to make sure people are remembering that invasive species control is about more than just removing a plant species we don’t like. Identifying the threat from those species is really important, and measuring whether or not our work is affecting that threat – and the value(s) being threatened is the way to evaluate whether or not our work is worthwhile. I’m not saying we should necessarily be collecting intensive data on birds or plant diversity, or trying to use existing data on those as measurements. Observation is often fine as a way to evaluate success. The key is to recognize what to look for. If there isn’t a clear threat from an invasive species, we should probably be focusing on other challenges that present clear threats. If there IS a clear threat, let’s make sure our control efforts are addressing it. Seeing that removal of an invasive species is improving plant composition in an area is an excellent way of measuring success if the diversity/composition of that community is the main concern – and we can measure that with intensive data collection or with observation, depending upon time constraints and other factors. Measuring success by saying we dug out 150 knotted slapweed plants is not sufficient. (Knotted slapweed is an excellent name for my plant game that I’ve just squandered here in this comment.)

        • Trying to comment on your post, I could not help but thinking that the topic of invasive species control is so broad that it would be difficult to make a comment that was general enough to apply universally. I expect writing the post was many times more difficult because of the aforementioned reason.

        • I’ve been evaluating my work from last year recently. I tried using triclopyr ester to control buckthorn and Asian honeysuckle in late fall/winter. In the thickets were I did a careful basal bark application with a paint roller (20, 25, and 30 % a.i. to about one foot of stem length) no forbs are growing, not even weeds. On the edge of these thickets, existing forbs are showing some damage. In contrast, when I applied glyphosate to frills during late fall/winter in a different location during previous years the vegetative response was excellent where the buckthorn had not cast too much shade. Where the buckthorn shade was deep the weedy species grew very well after the buckthorn were killed.

          Last year, I also tried carefully hand wiping glyphosate (8 % a.i.) foam on crown vetch during late summer in a remnant. The crown vetch died and weeks later I thought the application had worked great. When I returned to the area this year some spots seem spares. Now I am wishing I had collected data before I did the application, so I had measurements of my work’s impact.

          My point is, I think I was wrong before when I said continually measuring plant diversity would not help improve control efforts. Considering results are often site specific and highly dependent on who does the application, continual measurement is needed to identify when the expected outcome is not being achieved. This will let people know when further investigation is needed to determine the cause of a problem and what changes need to be made to prevent it from reoccurring.

          • When I said, (20, 25, and 30 % a.i. …) above I was incorrect. I applied 20, 25, and 30 percent product (61.6 percent triclopyr) mixed in oil on a volume to volume basis. This comes out to percent active ingredient of about 12, 15, and 18 respectively. I only noticed my mistake after I had reviewed the notes I had made after performing the application.

    • When I said “selectively removing invasive species” in my previous comment I really meant “targeted removal of invasive species.” I’ve seen a lot of “selective” herbicides really mess up plants that the label said would not be harmed. I don’t know if these plants will recover. I do know that some dicots are killed by grass selective herbicide. The impact was not visible until almost a year later (after winter). Other times even targeted control harms adjacent plants. The herbicide must move through the soil. This is the reason it is important to test on a small scale and then continually measure, evaluate, and work to improve.

      I also wanted to thank Chris for mentioning digging out plants. This is what I prefer to do when plants are small. However, ecologists in my county won’t allow this in remnant ecosystems because careful targeted application of herbicide can kill the invasive species without disturbing the soil. In my experience the soil disturbance caused by pulling up/digging out small things is not a problem. In contrast, broad cast spraying to kill all the seedling of invasive species that pop up after initial clearing creates a more ruderal response if my one small test is repeatable.

      Regarding Patrick’s comments on cedars, I haven’t been cutting them. I have been frilling them, apply herbicide to the frill, and leaving them standing. They will burn up in a prescribed fire. Even without fire they will eventually decompose. If I don’t cut the cedars then I don’t have to worry about them killing plants where they drop. I also don’t have to worry about the soil being killed under a pile, especially if that pile gets burned. I have been using the method discussed in the link below.
      https://grasslandrestorationnetwork.files.wordpress.com/2018/02/results-of-frilling-in-winter-xiii-james-mcgee.pdf

      I think collecting data on plant diversity and bird nesting is important. However, this is more of a benefit for those doing reconstructions than the people who stewards the small remnants left in my area.

      • Yes, that is also a reasonable approach James. I didn’t want a lot of standing deadwood because that would make moving through the property difficult, and getting a burn done with that much dead standing fuel is not always something contractors are willing to do. I should have mentioned that I remove the branches off the prairie remnants (but leave the log), and burn them in winter to avoid scarring. It’s a bit of labor doing it that way, but I enjoy the exercise.

        • Yes, heavy fuels, like dead wood, can cause a problem when conducting prescribed burns. I have seen dead limbs on an oak tree catch fire and drop embers. Since it was very windy during this burn, the embers were blown outside of the unit and started a spot fire. I will leave it to people who are more experienced to determine the conditions when a fire can be safely conducted. Although, I do think fires can be safely conducted with standing wood present if conditions are appropriate and the standing dead wood is far enough away from fire breaks.

          • My success using herbicide in a targeted manner has all been when doing control of woody species in winter. It seems no matter how carefully I paint or wipe herbicide onto the leaves of invasive species some adjacent desirable plants are always impacted.

            For example, I wiped 3 % a.i. glyphosate from a foaming dispenser onto quackgrass (Elymus repens) leaves with my double gloved hand. I was careful to not get any herbicide on adjacent plants. If I did get herbicide on them then I immediately removed that part of the plant. I am convinced glyphosate volatilized even though references say it does not volatilize readily. Even if volatilization was not the problem, then the wind moving leaves or rain (probably both) must have gotten herbicide on adjacent plants I was trying not to harm. I only use herbicide on plants that are difficult to control by digging out the roots like quackgrass and field thistle. Still, the reduction in labor that can be achieved with herbicide is often not worth the damage sustained to other non-target species.

            I can remove quackgrass by digging. Although this highly disturbs the soil because this species has long rather deep (~5 inches deep) rhizomes. I can also control field thistle by removing the rosettes repeatedly throughout the season. This greatly reduces the amount of field thistle until the prairie plants out compete it. I often think digging or repeated defoliation will get me to the condition I want faster than constantly having good plants set back or killed by unintended herbicide exposure.

          • I was wrong in my previous comment. Quack grass roots are only about two inches deep. I’ve decided digging out the roots of this species is better than using herbicide and damaging adjacent plants.

  3. I do management consulting and the same focus applies to all training and development work: Know your objectives and evaluate all actions in terms of whether or not the actions help accomplish the objectives.

  4. This is valuable information to concentrate on the invasive outliers moving into the native populations rather than the overwhelming mass of non-natives in any location. I learned something here. Thanks.

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