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!

Flying Carp (WATCH OUT!!) on the Missouri River

Last week, I attended the Nebraska Natural Legacy Project’s annual conference, which was terrific.  At the end of the conference, I had the chance to go on a boat tour of the Missouri River south of Nebraska City.  The tour was led by Gerald Mestl (Nebraska Game and Parks Commission) who did a great job of explaining both the history and current status of the river.  We also got to see examples of side channel and bank restoration efforts and hear about ongoing research and monitoring efforts on fish and other Missouri River species.


I’d love to give you a full recap of the information Gerald gave us on the tour, but I honestly don’t remember much of it.  Unfortunately for Gerald, but to the great entertainment of those of us on the tour, our attention much of the boat trip was diverted by numerous flying carp.  Yes, you read that correctly.

Asian carp, particularly silver carp, have invaded the stretch of the Missouri we were touring.  Silver carp are an invasive fish species that quickly become the dominant fish species in a lake or river (by biomass), though it’s not always clear what or how much negative impact they have on the ecosystem they invade.  Because silver carp are plankton feeders, they probably compete most with other species utilizing that same food source, including many larval fishes, paddlefish, and freshwater mussels.  Gerald said that so far they’ve not seen any obvious impacts from the Asian carp invasion of the Missouri, with the possible exception that paddlefish weights seem to be less than they used to be.  Of course, that doesn’t mean other impacts won’t arise as time goes by and more research is conducted.  Unfortunately, once Asian carp become established, there doesn’t seem to be a way to remove them from an ecosystem.

While there are concerns about what the impact of silver carp (and other Asian carp, including bighead and grass carp) will be on the Missouri River ecosystem, those of us on the boat tour last week were mostly concerned about ducking them as they came flying past or into our boats.  It turns out that silver carp have a propensity to jump (up to 10 feet!) out of the water in response to the vibrations caused by boat motors.  This wasn’t much of an issue in the main channel of the river, but as soon as we entered any side channel or backwater area where the water wasn’t strongly flowing, silver carp started flying out of the water like big slimy popcorn (or something).  Thus, my recollection of the tour and the information poor Gerald was trying to impart to us goes something like this:

“Historically, the banks of the Missouri River often consisted of steep banks that were being actively eroded by the flowing channel.  Some estimates are that those banks could shift an average of 180 feet a year!  That’s really important to understand because LOOK OUT!!


“Ok, anyway…  Another important thing about the historic river is that it was full of snags (dead trees).  Navigation was really tricky because of all the big cottonwood skeletons along the bank and shallow islands.  Today, we don’t see much of that kind of habitat, which was probably really importWATCH YOUR HEAD!!



“Yeah, so there are a number of fish species that aren’t doing well in the Missouri or lots of other similar rivers that have been severely altered.  Benthic (bottom-feeding) fish, in particular, are having a hard time, including species such asWHOA!! DID YOU SEE THAT??



“We’ve been doing quite a bit of research on some of these rare fish and learning a lot.  For example, it appears possible that pallid sturgeon (a federally-listed species) might actually be periodically moving from the Missouri into the lower Platte River for certain reasons.  If that turns out to be true we might have to re-evaluaWOW! LOOK AT THE SIZE OF THAT ONE!!

Nelson Winkel (The Nature Conservancy) holds a siver carp that jumped into the boat while RaeAnn Powers looks on.

Nelson Winkel (The Nature Conservancy) holds one of the smaller silver carp that jumped into our boat while RaeAnn Powers looks on.


“Here at Hamburg Bend, there was a big restoration project that included the creation of a side channel that now forms a shortcut through a big bend of the river channel.  That side channel provides important habitat for a number of species, includOH MAN!  DID THAT ONE HIT YOU IN THE HEAD??



“Ok, where were we… oh yeah, so the current main channel of the Missouri River has been constrained to a 600 foot width.  That’s much less wide than the historic channel, and of course because it no longer is allowed to move around the floodplain, we don’t see the kind of bank erosion and associated habitat that used to be so important for thiWOOHOO!! DID YOU SEE HOW HIGH THAT ONE JUMPED??



“On another subject, there has been some interesting recent research on turtles and their use of the Missouri River.  Among other things, they’re seeing some surprisingly long-range movement of turtles – including one that traveled 50 miles upstream!  That’s really interesting, and makes you think abouHEY! HEADS UP!!



“You know, one of the main challenges of the Missouri River is the balance between recreational, flood control, and navigational needs (among others).  Trying to figure out how to restore and manage flows and habitats is complicated by LOOK – TWO MORE!!



“Oh never mind… let’s just watch carp for a while.  At least that way we canI THINK THAT ONE JUMPED OVER THE WHOLE BOAT!!



“So, that’s our tour for the day.  Thanks for coming out, and I hope you enjoyed it.  Sorry about all the fish slime on your shirt.  I hope that bruise on the side of your face heals ok…”