Categorizing Invasive Plants

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!

Dealing With a Pervasive Invasive – Kentucky Bluegrass in Prairies

Many of the prairies we manage have pretty degraded plant communities, characterized by low plant diversity and dominance by a few grass species – including the invasive Kentucky bluegrass (Poa pratensis).  Our primary objective for these prairies is to increase plant diversity, which, in turn, bolsters ecological resilience and improves habitat quality for a wide range of prairie species.  Because bluegrass is so pervasive in our prairies, we’ve had to modify our objectives and strategies from those we use to address most other invasive species.

Kentucky bluegrass can stifle plant diversity by crowding out other plants. In many prairies degraded by years of overgrazing and/or broadcast herbicide use, bluegrass is now the dominant plant species and few other plants can compete with it.

When attacking invasive plant species, a common strategy is to contain, and (hopefully) shrink, patches of invasive plants in order to protect plant diversity in non-invaded areas.  In the case of Kentucky bluegrass, however, we have to take a different approach because the species already spans the entire prairie.  Kentucky bluegrass acts like a thick blanket of interwoven stems, roots, and rhizomes – smothering most other plant species beneath it.  Because our goal is to increase plant diversity, we want to make that blanket thin and porous enough that a wide variety of other plant species can grow up through it.   

An illustration of a floristically diverse prairie (left) with several large patches of an invasive plant.  When dealing with many invasive species, we can focus on reducing the size of infestations in order to restore a more diverse plant community.


Kentucky bluegrass acts as a thick blanket that smothers most other plant species.  In a case like this, we need to manage the prairie in a way that increases the mesh size of that blanket and allows a more diverse plant community to poke through.

Our primary strategy for suppressing Kentucky bluegrass is the periodic application of prescribed fire and grazing.  We can weaken bluegrass by burning prairies when bluegrass is just starting to flower and/or by grazing prairies harder in the spring than in the summer.  We mix those treatments with rest periods within a patch-burn grazing regime.  The result has been a steady increase in plant diversity in most of our degraded prairies.

If we were fighting a different invasive species, we might expect that if plant diversity was increasing, the amount of territory occupied by the invasive species would be decreasing.  With Kentucky bluegrass, however, our bluegrass blanket is getting thinner, but still covers the whole prairie – something that shows up clearly in data I’ve been collecting over the last decade.  Through the use of nested sampling plots of 1m2, 1/10m2, and 1/100m2, I’ve been tracking plant diversity and floristic quality through time, along with changes in the frequency of various plant species (the percentage of plots in which they occur).  Over the last 8-10 years, as plant diversity within 1m2 plots has increased, the frequency of Kentucky bluegrass has stayed about the same.   Even at smaller plot sizes, which are more sensitive to changes in the frequency of very abundant species, bluegrass is still in nearly every plot.

We also have a number of restored (reconstructed) prairies in and around our remnant prairies.  Within restored prairies, plant diversity is in pretty good shape, but Kentucky bluegrass is rapidly invading.  In one particular prairie, Kentucky bluegrass is now in almost 90% of 1m2 plots and more than 50% of 1/100m2 plots.  That sounds bad, but as bluegrass becomes more abundant, plant diversity – and the frequency of other prairie plant species – is actually holding steady.  There are a few small areas in which bluegrass appears to be forming near monocultures, but for the most part, it looks like bluegrass is just filling in around the other plants instead of actually displacing them.  My guess is that some soil conditions provide such ideal growing conditions for bluegrass, it’s going to be king of those areas no matter what we do.  Elsewhere, however, I think our management is preventing it from becoming dominant.

Here’s the take home lesson for me:  When trying to manage for plant diversity in the face of a pervasive species such as Kentucky bluegrass, it’s more important to track plant diversity than to worry about how much territory bluegrass occupies. 

Just consider the data from our prairies… if I concentrated only on how much Kentucky bluegrass is in our prairies, it would look like we’re failing miserably in our management attempts.  We’ve got just as much bluegrass as we ever did in our degraded remnant (unplowed) prairies, and we’re quickly losing ground in some of our restored prairies.  However, my data also shows that plant diversity is increasing in remnant prairies and holding steady in restored prairies.  Since the ultimate goal is to have a diverse plant community, that’s success! 

Click here to see a PDF showing some of the data I mentioned in this post.