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!


The emotional response you have to this photo will say a lot about your background, experience, and cultural influences.

A profusion of sunflowers in sandhill prairie at The Nature Conservancy's Niobrara Valley Preserve in north central Nebraska.

A profusion of annual sunflowers (Helianthus petiolaris) in sandhill prairie at The Nature Conservancy’s Niobrara Valley Preserve in north central Nebraska.

The sandhills of Nebraska consist of nearly 20,000 square miles of prairie.  The scale can be hard to comprehend until you have driven through it for hour after hour, gaping at the beauty spreading out all around you.  When I drove through a good portion of the eastern sandhills this week, a lot of it looked like this photo – covered with blooming yellow sunflowers.

Many readers of this post will be thinking, “Wow!  What a beautiful year in the sandhills!”  But I know others of you are thinking, “Ugh, what do we have to do to get rid of these invasive weeds?”

I’m going to get to that discrepancy, but let’s first back up and look at why the sunflowers are so abundant this year.  First, the sunflower species we’re talking about here is an annual called plains sunflower (Helianthus petiolaris).  It germinates from seed in the spring, flowers in the summer, and dies at the end of the same year.

During the drought of 2012, annual sunflowers were among the few plant species able to continue growing and flowering during the hot dry summer.  Because of that, sunflowers were able to produce copious amounts of seed, many of which ended up on the ground at the end of the year.  Few other grassland plants produced anything comparable to the seed crop of those sunflowers.

The spring of 2013 brought abundant rain to the dry sandhills.  In addition, the plant litter from last year’s dry growing season was thin and sparse, allowing a lot of light to hit the soil. That combination of abundant light and moisture was exactly what all those plains sunflower seeds needed, and they germinated.

Of course, germination doesn’t ensure survival, and many annual plants germinate each year, only to be quickly overshadowed and outcompeted by strong perennial plants.  Perennials have the advantage of a pre-existing root system that can monopolize moisture and nutrients from the soil while annuals are still struggling to get started.  In years when perennial grasses and wildflowers are strong, there is very little space for annuals to grow, except in places where the soil and plant community were disturbed by digging animals or intensive grazing/trampling.

However, in the spring of 2013, not only were conditions perfect for plains sunflower germination, competing perennial plants were also weak from drought and grazing in 2012, leaving lots of open space belowground for sunflower roots to take advantage of.  In short, you couldn’t have designed a better situation for the sunflower.  It was one of the few plant species to produce seed in 2012, and then it got light, moisture, and weak competition in 2013.  It’s no wonder the hills are yellow!

Some people will look at this photo and see an amazing abundance of pretty wildflowers.  Others will see weeds running amuck.

Some people will look at this photo and see an amazing abundance of pretty wildflowers. Others will see weeds running amuck.  At the Niobrara Valley Preserve, last year’s wildfire increased the favorable conditions for plains sunflower by creating massive amounts of bare ground for germination.  While it looks like a monoculture from a distance, hidden among the sunflowers are lots of grasses and other plants that are slowly regaining their vigor.  By next season, this will be a very different looking prairie.

Ok, back to the perception issue.  Sunflowers are one of the most popular and well-known flowers in the world.  They are big, attractive, and easy to recognize.  On the other hand, many farmers and ranchers have grown up learning that sunflowers (of any kind) are weeds.  The presence of sunflowers in a field or pasture – especially an abundance of them – can be seen as a badge of shame for the landowner who is clearly not managing his/her weed problems adequately.

The important thing to remember if you’re a rancher, however, is that the sunflowers are not outcompeting perennial grasses.  Instead, the sunflowers are opportunists, taking advantage of the fact that grasses are weak.  As perennial grasses recover from last year’s drought and/or grazing, they will reclaim the root space they lost in 2012 and sunflowers will have much less room to grow next year.  Plains sunflower is a native prairie plant, and it’s role is to fill the space left when other plants are weakened (similar to ragweeds and other opportunistic species).  If sunflower wasn’t filling that space, another “weedy” species would, and the alternative could be much worse.

Some ranchers will be tempted to spray their pastures to kill off the “invading” sunflowers, but that’s actually a counterproductive strategy.  First, the annual sunflowers are going to die at the end of the season anyway, so if you want fewer sunflowers next year, the best strategy is to focus on limiting the germination and growth of next year’s crop by allowing perennial grasses and wildflowers to regain their dominance.  Second, herbicide spraying will kill a number of other plant species that are both valuable as forage and competitors with sunflowers and other annuals.  Why spend money to weaken the long-term viability of your grassland?

It’s also important to remember that cattle do eat sunflowers – they particularly like them early in the season when the leaves and stems are tender, but will also seek out the nutritious buds and flowers later in the season.  The evidence of that can be seen right now; pastures grazed at certain times this year have many fewer blooming sunflowers than those that haven’t yet been grazed this season.  In addition, of course, sunflowers are among the most valuable grassland plants in a prairie for wildlife and pollinators.  They produce large nutritious seeds for birds and other wildlife, and have abundant and accessible supplies of nectar and pollen that attract numerous pollinator species.  In short, sunflowers may not be everyone’s favorite plant, but they’re far from a useless weed or invasive threat.

For those of you who started out reading this post as fans of sunflowers, good for you!  If you get the chance, you should take a drive through Nebraska’s sandhills this summer and enjoy the scenery – it’s not likely that we’ll see another year like this for a while.  For those who are appalled by the abundance of sunflowers this year, maybe you can take some comfort from the fact that it’s a temporary phenomenon, and one tied to a particular combination of weather factors more than anything you or others did as land managers.  Things will be different next year.

Regardless of whether or not you like sunflowers, I guess there’s one thing we can all agree on.  The year 2013 will be one to remember!

Note:  Nebraska has nine species of native sunflowers, seven of which are perennials.  All of them are valuable for wildlife and pollinators, and important components of a healthy grassland community.