A Tough Plant, Not A Weed

I blame whomever named the plant.  Giving a plant the name “ironweed”, apparently – according to Google – because of its tough stem, creates an unnecessarily negative connotation right from the start.  It’s an unfair connotation for a plant that is both beautiful and important.  It’s also a big favorite of butterflies; something I can attest to after spending a couple hours last weekend chasing monarchs and others around ironweed patches at our family prairie.

Ironweed at our family prairie, growing abundantly in a smooth brome-filled draw.  The abundance of the plant goes up and down each year, but it never spreads beyond the draw or shades out the grass around and beneath it.

There are three species of ironweed (genus Vernonia) in Nebraska, and two that are common in the prairies I am most familiar with.  Both of those – V. fasciculata and V. baldwinii – seem to act in similar ways, but the first likes a little wetter sites than the second.  Both species can occur as scattered plants across a prairie, but are also often found in fairly dense patches where conditions favor them.  That patchy local abundance is the first mark against them by people who don’t appreciate their value.  The second mark is that cattle absolutely refuse to eat them.  This both helps them stand out (especially when blooming) in heavily grazed pastures and helps them spread across those same sites since they gain a strong competitive edge when surrounding plants are all being grazed hard.

Like many other plant species I tend to admire and write about, however, ironweed is not an invasive plant – it’s an opportunist.  It takes advantage of soil and management conditions that favor it, but doesn’t just spread aggressively across pastures.  If you look online, it’s not hard to find websites that encourage its control in pastures.  I dispute that.  At least in my experience, ironweed has its favorite locations (often in draws or other low spots where moisture and nitrogen are high) and pulses in abundance within those locations as grazing treatments and weather vary from year to year.  At our family prairie, ironweed is fairly abundant in some of the low draws where high nitrogen also strongly favors smooth brome, but while there are years when those patches are thicker than others, the overall patch sizes and stem densities of ironweed aren’t any higher today than they were 15 years ago.  That matches what I see elsewhere in central and eastern Nebraska.

(I found a university website online that blamed ironweed for making cattle have to look harder to find grass, thus reducing grazing efficiency.  Give me a break.  That’s the same attitude that leads to people spraying pastures to remove everything that isn’t grass, and then wondering why they need to fertilize their grass and supplement their cattle’s diet.  The same people blame others for the lack of wildlife and pollinators on their land.  …Ok, I’m done ranting – let’s talk about butterflies.)

When I arrived at our family prairie last weekend, I immediately noticed monarch butterflies flying all over the place.  I’d seen a surprising number of larvae back in July, so figured we might have a good August, but I was still impressed with how many adults I saw.  I’m guessing there were 40-50 or more across our 100 acres of prairie.  They kept moving, so it was hard to count them…

Almost every monarch I spotted was either flying or feeding on ironweed.  A few other flowers got attention too, including wild bergamot (Monarda fistulosa), Flodman’s thistle (Cirsium flodmanii), and some early tall thistle blossoms (Cirsium altissimum), but ironweed clearly monopolized most of their attention.  I started stalking monarchs with my camera and eventually found a couple that let me get close enough for to capture reasonable photographs.  While I was doing that, I also spotted myriad bees, along with quite a few other butterfly and moth species.

Here are some photos of the butterflies and moths that were kind enough to let me get close.  I didn’t ever get a good shot of a bee, though there were at least a dozen species feeding on the ironweed flowers, and I also never caught up to one of the many silver-spotted skipper butterflies that were all over the place.

This is one of many monarchs that were floating from plant to plant across ironweed patches last weekend.
I haven’t looked up this moth yet. Maybe one of you can save me the trouble? Thanks in advance.
I’ve been seeing a lot of adult swallowtails around lately, including this tiger swallowtail , which was pretty easy to spot, even from across a large draw.
Ok, this black swallowtail wasn’t on ironweed when I photographed it, but it went to ironweed after feeding on this native thistle.  I was taking bets (in my head) about whether or not the crab spider on that thistle would be able to take down the big butterfly. The butterfly eventually moved within striking distance, but the spider didn’t attack, so I’m guessing it decided to wait for something a little smaller.
Thanks to Neil Dankert, I can tell you that this gorgeous little brown skipper butterfly is a tawny-edged skipper.

Ironweed is too beautiful and important for its name.  Maybe we need a campaign to rename it, and maybe that campaign would help convince people, including those at a certain unnamed university, to leave this plant alone to do its job.  Either way, it might be fun to think about potential names.  Any ideas?

Aggressive Weed or Opportunistic Plant? It’s Good to Know the Difference

“Those weeds are really taking over my pasture!”

I cringe when I hear that sentence because it’s often a precursor to broadcast spraying of herbicide and the subsequent loss of most plant diversity in a prairie.  That’s really bad.  What’s most frustrating, however, is that the sentence is rarely true.

East Dahms pasture. Ragweed in degraded pasture.
The ragweed in this pasture is not acting aggressively.  It is filling spaces left open by grasses weakened by intensive grazing.  Within a year or two of this photo, this site was dominated by big bluestem.

It’s easy to understand how a landowner would look at a pasture that is visually dominated by ragweed, buffalo bur, snow-on-the-mountain, hoary vervain, or a number of other weedy plants and think those plants are aggressively pushing grasses out of the way.  In almost every case, however, the opposite is true.  Grasses are usually the bullies of the plant community, and only when they are suppressed by fire, grazing, or some other pressure do the “weeds” thrive.

There are a few weeds that can out bully grasses, of course.  Leafy spurge, crown vetch, and sericea lespedeza are good examples in the central United States.  They seem to be able to invade and spread regardless of the vigor of grasses and other competing plants.  Landowners should absolutely work to control those aggressive perennial species before they get a foothold across large areas.

Leafy spurge at The Nature Conservancy's Broken Kettle Grasslands in the northern Loess Hills of Iowa.
Leafy spurge can be a serious threat to prairies and should be dealt with quickly to prevent it from spreading throughout a site.

However, while there are some important exceptions, most pasture weeds are more opportunistic than aggressive.  Opportunistic plants don’t compete well with grasses or other perennial plants when those plants are at full strength, but can move quickly to fill spaces left between plants that are weakened by intensive grazing or drought.  Many opportunistic species are short-lived, and produce huge numbers of seeds, and those seeds sit in the soil waiting for a chance to germinate and grow.  When the tops of grasses are grazed off, the roots below shrink up as well, creating the perfect opportunity for seeds to germinate and new plants to establish.

The majority of those new plants will survive only as long as the vigor of the surrounding grasses remains low.  As those grasses recover, they regain their advantages, both above and below ground.  Annual plants may bloom and drop more seed, but those seeds have to wait until the grasses are weakened again before they can germinate and grow.  Perennial opportunistic plants might stick around a little longer, but most of those will also lose out to recovering grasses because of their poor competitive ability.

These "weedy" species are filling in while grasses recover from a grazing bout. In the meantime, the hoary vervain (purple) and upright coneflower (yellow) are providing important pollinator resources and great habitat for other species, including insects, reptiles, and birds like northern bobwhite.
These “weedy” species are filling in while grasses recover from a grazing bout. In the meantime, the hoary vervain (purple) and upright coneflower (yellow in foreground) are providing important pollinator resources and great habitat for other species, including insects, reptiles, and brood-rearing habitat for birds like northern bobwhite.

There’s an easy way to find out whether or not the “weeds” in a pasture are aggressive or opportunistic – build an exclosure or two to keep grazing out for a year or more.  If the grasses within those exclosures regain their vigor and dominance, you’ll know it was the grazing pressure that was creating opportunities for weeds.  If the weeds continue to dominate the area inside the exclosure for a couple years (assuming you’re not in the middle of a drought that is keeping those grasses down), you’ll know that either the grasses have been debilitated to the point of no return or the weeds are truly aggressive and in need of control.

Grazing exclosure at the Dahms Tract.
Even a small and simple exclosure can help determine whether weeds are suppressing grasses or just taking advantage of grasses already weakened by grazing.

As a final note, it’s important to understand that grazing hard enough to suppress grasses and allow weedy plants to flourish temporarily is not at a bad thing.  Ecologically, the habitat conditions created by those tall weedy plants are critically important for many wildlife species, including upland game birds.  Many important wildflowers also benefit from the opportunity to reproduce during short periods (a year or two) of weakened grasses.  As long as the grasses are allowed to recover before they are intensively grazed again, they’ll be fine, and the wildlife, pollinators, and plant diversity of your prairie will all benefit from the temporary reprieve from grass dominance.  Shifting intensive grazing and subsequent rest periods around a large prairie, especially when those grazing/rest periods are a couple months long or longer, seems to be a great strategy for maintaining prairie health.

Opportunistic plants suffer from a public relations crisis.  While they are scorned by most people, these valuable plants are doing exactly what they’re supposed to do.  They are the temp workers of the plant community – the substitute teachers, backup quarterbacks, and house sitters that keep prairies humming along when dominant grasses are on sick leave.  By filling spaces between temporarily shrunken grass plants, opportunistic plants can help prevent the truly aggressive weeds from easily gaining a foothold.  They can also provide much needed habitat for wildlife and pollinators.  Ragweed, hoary vervain, and buffalo bur aren’t the villains of the story at all – they’re the heroes!  We just have to get used to seeing them that way.