Tag Archives: control

Categorizing Invasive Plants

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Managing invasive plant species is often the greatest challenge faced by land managers.  Because there are so many invasives and so little time, it’s critically important to be thoughtful about how to approach them.  There is much good advice available about how to prioritize which species to focus on and how to approach those priority species.  My own approach to invasives continues to change over time.  For what it’s worth, here’s how I think (today) about categorizing invasive plants on our sites here in Nebraska.

I can put most invasive plant species into one of four categories that describe how we approach control.

  1. Thin the matrix
  2. Beat them back
  3. Nip them in the bud
  4. Live with them

 

Category 1 – Thin the matrix.

Invasive grasses, such as Kentucky bluegrass, smooth brome, and tall fescue are excellent examples of species in this group.  They are plant species that are common enough that they occur throughout most of our sites, and – if left unmanaged – can form thick monocultures that exclude most other plant species.  Eradication of these species is not possible without losing many of the other plants we are hoping to conserve.  Instead, our general approach is to reduce their dominance and limit their impact on the diversity of the plant community.  (I wrote about this approach in an earlier post, using Kentucky bluegrass as an example.)

Essentially, we try to use fire, grazing, and/or mowing to weaken invasive grass plants and open up space between them for other species to flourish.  Because these suppression strategies also have negative impacts on some native species, we are careful not to use them too many years in a row.  Instead, we apply them periodically, whenever it looks like the invasives are starting to exert their dominance a little too much.

Prescribed fire and grazing are the two best ways we attack invasive grasses such as smooth brome and Kentucky bluegrass.

Prescribed fire and grazing are the two best ways we attack invasive grasses such as smooth brome and Kentucky bluegrass.

Category 2. Beat them back.

This group includes perennial plants that radiate outward other otherwise spread from an established source population.  Species we commonly deal with that fit into this category include purple loosestrife, sericea lespedeza, and Canada thistle.  It can be tempting to jump in and start attacking (with herbicides or other approaches) the biggest thickest patches of these, but that’s rarely the smartest strategy.  Instead it usually makes more sense to work from the outside edges of an infestation toward the middle – or source – so that the problem doesn’t continue to get worse as you attack.  I wrote an earlier post on that topic as well…

An exception to the “work from the outside edge first” rule applies to species such as Siberian elms that may be spreading from a single discrete patch of parent trees.  If it’s possible to eliminate that source population by cutting down a handful of trees, it absolutely makes sense to do that first.  Next, it’s smart to target other elms that are big enough to produce seed before working on the smaller ones – rather than blindly following the rule about working from the edges of an infestation.  Rules are meant to be broken, after all.

 

Category 3. Nip them in the bud.

This category contains invasive species that are just starting to show up in our area or at a particular site.  Here, the tactic is to seek and destroy new plants as soon as they arrive, to prevent the species from becoming established.  After all, it’s always better to attack an invasive species before it gains a foothold.

Many species can fit within this group.  For us, Common reed (Phragmites australis), crown vetch, garlic mustard, and Queen Anne’s Lace (aka wild carrot) are good examples.  Sometimes, we’re not sure if a species will really cause serious problems if it becomes established at our sites (e.g., Queen Anne’s Lace) but if minimal effort can prevent that establishment, it seems  like time well spent.

Our approach to musk thistle sort of fits into this “nip it in the bud” category, but for other reasons.  Musk thistle is an officially-designated noxious weed in Nebraska, and all landowners (including us) are required to eradicate it from their property each year.  If it weren’t for the state law, musk thistle would not be among our highest priority species because it really doesn’t cause big problems in most cases (on our sites).  It is most abundant where the dominant vegetation has been recently weakened by fire, grazing, or drought, but quickly diminishes in abundance when grasses recover their dominance.  However, to abide by the law and to prevent thistles on our land from going to seed and affecting our neighbors, we do our best each year to eradicate musk thistle.

 

Tier 4. Live with them.

This last group includes species we don’t actually consider to be invasives, at least by the criteria that a truly invasive plant acts to reduce biological diversity or otherwise simplify (and thus weaken) natural communities.  Many native species are considered weeds by some of our neighbors, but we like having them around.  Prime examples include annual sunflower and ragweed species.  However, many non-native plants fall within this category as well, including common mullein, dandelions, goatsbeard, marestail, and sweet clover.  Sometimes, we spend some time collecting and/or analyzing data to help ensure that we’re categorizing these species correctly.  A good example of that was discussed in an earlier post on sweet clover.

Dandelions are a species we just live with. They're great for early season pollinators, and aren't aggressive - they just fill space when the perennial plant community is weakened (as they are doing here in the lot around our shop buildings).

Dandelions are a species we just live with. They’re great for early season pollinators, and aren’t aggressive – they just fill space when the perennial plant community is weakened (as they are doing here in the lot around our shop buildings).

Switching Categories.

Ideally, of course, we’d be able to move some invasive plant species from one category to another, so that populations of “matrix” invasives shrink to the point they are in discrete patches and we can “beat them back.”  Likewise, it’d be great if “beat them back” invasives became rare enough that we could eventually “nip them in the bud”.  Unfortunately, reality usually goes the other direction.  For example, we’re dangerously close to having to shift garlic mustard from “nip it in the bud” into the “beat them back” category (if not the “thin the matrix” category!) at a nice woodland site we manage.  We’ll see what the next year or two brings.

We hope that using more frequent fire (along with other methods) will help us prevent garlic mustard from spreading across our whole property at the Rulo Bluffs Preserve, but it's a big challenge.

We hope that using more frequent fire (along with other methods) will help us prevent garlic mustard from spreading across our whole property at the Rulo Bluffs Preserve, but it’s a big challenge.

Regardless, I find it helpful to think about our invasive plants within categories like these because we can more easily define both our control strategies and objectives.  Putting a species into the “nip it in the bud” category helps make it a top priority and we can prioritize resources toward keeping it rare.  Just as importantly, if we know that we’re only trying to suppress the dominance of smooth brome, not eradicate it, we don’t have to beat our heads against the wall in frustration because smooth brome is still present.

We spend more time on invasive plant control than on any other land management activity.  Unfortunately, that’s true of most land managers I know.  More unfortunately, invasive species numbers are going up, not down.  It’s not time to throw in the towel just yet, but it is absolutely critical to be organized and thoughtful about how we approach these invaders.

Hopefully, being organized will allow us to spend less time on invasives and more time enjoying the sites we manage!

Blowing Against the Wind?

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As I mentioned last week, I recently spent a couple days helping our land manager, Nelson Winkel, pull garlic mustard at our Rulo Bluffs Preserve in southeast Nebraska.  The invasive species has just started to invade our property within the last several years.  We’ve heard stories from colleagues in other places about beautiful woodland plant communities turning into monocultures of garlic mustard within a relatively short time period.  We’d sure like to keep that from happening at our Preserve.

So, we pulled garlic mustard plants.  A lot of them.  On the first day, I figured we pulled at least 25,000 plants.  That’s a very conservative estimate.  The second day was longer, but we did more searching and less pulling.  This wasn’t the first trip to pull either, so we were just trying to get what was leftover from the previous efforts.

Nelson Winkel, showing off one patch's worth of pulled garlic mustard plants. The Nature Conservancy's Rulo Bluffs Preserve - Nebraska.

Nelson Winkel, showing off one patch’s worth of pulled garlic mustard plants. The Nature Conservancy’s Rulo Bluffs Preserve – Nebraska.

The Rulo Bluffs Preserve is 444 acres.  Hand-pulling weeds doesn’t seem like a very sustainable strategy for invasive species control at that scale.  In fact, it’s downright depressing because we pull more plants from more locations every year.  We’re clearly not winning.  So why bother?

It’s a good question, with several answers.  The first answer is that we’ve got some ideas for increasing our effectiveness.  Nelson and I talked as we worked about how we might put together a small army of volunteers to come help us pull each spring.  The big challenges are that the site is far from population centers (more than two hours from Lincoln and Omaha), has difficult terrain to hike in, and garlic mustard doesn’t bloom at exactly the same time each year, so we’d have to schedule work days on fairly short notice.  On the other hand, I think there are people who’d be glad to help, and it is a beautiful place to work in the spring time – lots of warblers and other birds above, and plenty of woodland wildflowers below.

In addition to finding more people to help hand pull, we hope to decrease the number of plants we need to pull in the bigger, more established, patches by doing some herbicide work in the late winter.  Garlic mustard is a winter annual or biennial which germinates in one season, overwinters as a rosette (a few leaves, low to the ground), and then flowers in the late spring of the next year.  Our colleagues in more eastern states have been dealing with garlic mustard longer than we have, and have had luck spraying the rosettes with Glyphosate herbicide on warm February days.  Spraying in the winter works well because there are very few other woodland plants that are green (and thus susceptible to Glyphosate) in February.  They don’t usually spray in the early winter because many rosettes die on their own over the winter, and by waiting until February, they can focus only on those most likely to bloom in the coming year.  Nelson was marking the bigger patches we found with a GPS unit so he can find them next winter and try the spraying technique.

Small patches of garlic mustard such as this one might eventually be eliminated by hand-pulling.

Small patches of garlic mustard such as this one might eventually be eliminated by hand-pulling – especially if we find and treat them every year.  Larger patches are much more problematic.

The second reason we’re still trying to suppress garlic mustard is that I hope we can buy some time until better control options become available.  There has been some work to develop a biocontrol technique (using insects from the native range of garlic mustard), for example, and if something like that turns out to be effective, I want to be sure we still have some woodland left to save.  Unfortunately, I’m hearing that biocontrol development has stalled at the moment.  Apparently, in at least some places, people are seeing garlic mustard populations decline steeply on their own – as if the plants are outcompeting themselves and self-thinning.  That could be great news, but only if the native plant community rebounds as the garlic mustard declines, and I haven’t been able to find anyone who can tell me whether or not that’s the case.  I sure hope it is, but I’d feel better if the biocontrol folks kept forging ahead on the development of that control option anyway.  Regardless, I’m holding out hope that either garlic mustard will turn out to be a temporary nuisance (seems unlikely?) or that biocontrol or better control options will be developed in the next several years.  I could be naive, but at least it gives us something positive to think about while we’re pulling up thousands of garlic mustard plants…

While we look for better control options, we’re also trying to change the playing field for plant competition at Rulo Bluffs and give garlic mustard less of an advantage.  With considerable help from Kent Pfeiffer of Northern Prairies Land Trust, and funding from the Nebraska Game and Parks Commission and others, we’ve been trying to ramp up our stewardship work during the last several years.  We’ve not done as much burning as we’d like to, but are earnestly trying to change that.  Last fall, a contractor did some “hack-and-squirt” herbicide treatment to kill many of the smaller understory trees that are shading out the herbaceous plants on the ground.  We’ve also been doing mechanical shredding of brush on ridgetops to help the grassland, savanna, and open woodland plants there.  All of this work is aimed at getting more light to the ground, which should stimulate increased oak regneration and a stronger, more diverse, herbaceous community on the woodland floor.  In addition, we hope that increased light will put shade-loving garlic mustard at a disadvantage, at least in some parts of the woodland.  From talking with others around the country, they’ve seen mixed results from similar work.  I guess since we want that light on the ground anyway, we’re going to forge ahead – and hope we don’t make things worse.

Jack-in-the-pulpit is one of many woodland flower species that help make the Rulo Bluffs Preserve unique and valuable.

Jack-in-the-pulpit is one of many woodland flower species that help make the Rulo Bluffs Preserve unique and valuable.

Finally, we’re pulling garlic mustard because the Rulo Bluffs Preserve is worth the effort.  It’s one of the few remaining high-quality oak woodlands in Nebraska, and hosts a wide diversity of plant and animal species – many living at the edge of their geographic range.  In addition to lots of mayapples, jack-in-the-pulpit plants, and woodland phlox, we also found two orchid species blooming last week – the showy orchid and the yellow lady’s slipper orchid.  We walked around beneath eastern deciduous tree species such as chinkapin oak, black oak, and Ohio buckeye.  Several animal species at the preserve, including zebra swallowtails, timber rattlesnakes, and southern flying squirrels, are only found on the very eastern edge of Nebraska.  While some of those species are common to the east of us, it is probably important to protect their genetic diversity by maintaining populations across their entire range.  That should allow the species to better adapt and survive in changing conditions over time.

Genetic and biological diversity aside, the Rulo Bluffs Preserve is also important because it’s a beautiful place.  We need to keep some aesthetically-pleasing natural areas around for people to enjoy.  Despite our aching backs, Nelson and I had a great time exploring the preserve last week, marveling at warblers, flowers, velvet mites, and other wonders.  It’s possible that we’ll invest a tremendous amount of time and money into stewardship and restoration at the Rulo Bluffs Preserve over the next several years and still lose out to garlic mustard.  There are plenty of examples of that happening elsewhere.  I guess we’re not ready to concede the battle, however – there’s too much at stake.