Admirable, Abundant, and Adaptable, But Not Aggressive

During the last month or so, I’ve had several people tell me how aggressive marestail (horseweed, aka Conyza canadensis) is, and how this is a particularly bad year for it.  One person suggested marestail should be added to Nebraska’s noxious weed list.  This week, Olivia and I drove from our Platte River Prairies to the Niobrara Valley Preserve – right through the center of our state – and I tried to document what is certainly a summer of abundance for marestail.

Marestail is an annual/biennial plant with tiny flowers and fluffy wind-blown seeds. Marestail is abundant across much of Nebraska this year, including in this flat area at The Nature Conservancy’s Niobrara Valley Preserve.

Here are a few things you should know about marestail right off the bat.  First, it is native to Nebraska and most of North America.  It acts as an annual plant in states to the east of us, but acts as a biennial here, usually germinating in the fall and blooming the following summer.  In its native habitats (including grasslands), marestail is a colonizer of bare ground, filling spaces between plants left open by disturbances like grazing, trampling, animal burrowing, drought, or fire.  Because marestail loves open soil conditions, it isn’t surprising that it has become a weed in crop fields.  It has garnered special attention lately because it has a strong ability to become resistant to herbicides, including glyphosate, which it started showing resistance to way back in 2006.

In other words, marestail is a tough native plant that has always scraped out a living when and where it can.  However, it’s not a plant that can push other plants around.  Instead, it sits in the soil (as a seed) and waits for a time when surrounding plants are weakened and abundant light is hitting the soil.  Then it pops out of the ground and tries to grow, bloom, and produce as many seeds as it can during its short window of opportunity.  In any particular year, marestail can be found here and there in most Nebraska prairies, especially those in the western 2/3 of the state.  However, it also seems periodically to respond to certain weather patterns and exhibit a flush of abundance across a larger region – as it is doing this year.  Many short-lived plants do the same thing, each with its own individual preferences for weather patterns.  Many Nebraskans might remember the huge sunflower party across the Sandhills back in 2013, for example, following the big drought of 2012.

During late summer, marestail flowers and produces seed heads, helping it stand out from the surrounding vegetation.

Whether it’s sunflowers or marestail, huge regional flushes in abundance don’t last long.  By 2014, annual sunflower numbers in the Sandhills had returned to normal – patches of yellow flowers here and there, around livestock tanks and fence corners, and wherever else there was open soil to grow in.  Marestail will do the same thing in 2019.  That pattern of boom and bust is not evidence of an invasive plant.  Instead it characterizes a plant that is too weak to compete most of the time and has to take ultimate advantage of the few windows of opportunity it gets.  When it is abundant, marestail isn’t stealing resources from other plants, it is taking resources that weren’t being used.  I don’t know for sure what weather patterns led to rampant marestail germination last fall, but I’m sure this year’s abundant rains have played a big role in the survival of a large percentage of those seedlings.

Call me crazy, but I think marestail adds an attractive texture to the grassland this year.

When short-lived plants like marestail and sunflower (along with ragweed, gumweed, and many more) are in the middle of a short-term explosion in your prairie, you could choose to fight them.  You could, for example, mow them off, trying to prevent them from making seed.  However, that’s a lot of work, and the plants will do everything they can to regrow and still produce seed – it’s what they do, and they only get one year to do it.  Even if you do keep them from going to seed, there are many thousands of seed already in the soil, ready to spawn the next generation of plants whenever they get the chance.  You could also spray short-lived opportunistic plants with herbicide, but I wouldn’t recommend it.  First, you’ll likely kill the surrounding plants (the ones that normally out-compete marestail and sunflower) and just trigger another explosion of opportunistic plants the f0llowing year.  Second, with most short-lived plants, by the time they’re big enough that you notice them (especially by the time they’re flowering) herbicide treatments just make them produce seed more quickly, so are counterproductive.

Marestail might not have big showy flowers, but it can still be an attractive part of a landscape, especially if you know it isn’t dangerous.  If you’re still not a fan after reading this, just squint your eyes and pretend it’s a funny-looking grass.

The smartest choice is to just sit back and marvel at these periodic phenemona, knowing you’re watching a short-term and harmless event.  Marestail, of course, doesn’t have the wide aesthetic appeal of sunflowers (though not everyone likes sunflowers either), but it has its own distinctive charm.  I think it adds an attractive texture to the landscape, but I’ll admit I’m a little odd.  Regardless of whether you find it attractive or not, it’s here, and it’ll be here whether you like it or not.

Fighting back against these periodic flushes of marestail and other opportunists is expensive and futile, and usually results in weakening the plant community that normally keeps them in check.  Most importantly, remember that, at least in grasslands, marestail doesn’t steal resources from the plants you like, it just takes what they can’t use.  What’s to dislike about that?

Now You See Them, Now You Don’t (But They Might Still Be There!)

Grazing, especially by goats and/or sheep, is often promoted as a control method for weeds or shrubs.  Depending upon the life strategy of the weeds being targeted, grazing can be effective, but it’s important to set realistic objectives.  As you might expect, many perennial grasses, forbs, and shrubs have evolved strategies for surviving repeated defoliation.  In those cases, grazing may appear to effectively control plants while grazers are present, but the plants bounce back right after grazers are removed.

One of my all-time favorite research projects showcases this exact phenomenon at a site in South Dakota owned by The Nature Conservancy.  Back in the early 1990’s, an estimated 75% of the Conservancy’s Altamont Prairie Preserve was covered by leafy spurge (Euphorbia esula).  In 1994, goats and sheep were installed in separate pastures and spurge was treated by using periodic high-intensity grazing sessions during both early summer and early fall.  Both the goats and sheep were very effective at eating the spurge plants, and after five years, managers conducting walk-through inspections the site felt like excellent long-term control of spurge had been achieved.  Inside small exclosures, spurge was still abundant and vigorous, but outside the exclosures, almost no plants could be seen.  As a result, the goats and sheep were removed and everyone was happy.

One of the goats used at Altamont Prairie and an exclosure showing a dramatic difference between abundant and blooming leafy spurge in ungrazed areas and no apparent spurge in grazed areas. Photos courtesy of TNC’s Minnesota, North Dakota and South Dakota Chapter.

…Until the next season when spurge plants popped right back out of the ground and the pasture looked essentially as it had before the grazing treatment had started.  In dismay, the managers looked for another option and decided upon flea beetles (Apthona spp.), which ended up being a much more successful choice, greatly reducing the footprint of leafy spurge over the next several years.

You’d be excused for thinking the use of sheep and goats was a total waste of effort, but additional data collected at Altamont Prairie adds some interesting nuance.  As it happens, mean Floristic Quality (a kind of qualified plant diversity metric) stayed relatively stable within the grazed area during the five years sheep and goats were present.  During the same time period, mean Floristic Quality decreased significantly in exclosures.  In other words, while grazing didn’t eliminate the spurge problem, it may have stabilized some of its negative impacts for a while.

This, to me, is one of the best attributes of many grazing-for-weed-control efforts.  Even if grazing can’t eradicate many weeds/shrubs from a prairie, it might be a strategy that prevents further spread (eliminating flowers and reducing vigor for belowground reproduction) and/or reduces the weed’s ability to compete with desirable plants.  In a large site where more effective long-term strategies (such as selective herbicide application or biocontrol releases) aren’t feasible across the whole area, using grazing as a suppression tactic in some areas of the site while you kill it in others can make a lot of sense.  In other words, grazing might buy you time to work on a problem that would otherwise seem overwhelming in scope.  (However, it’s also important to remember that grazers will also be eating and suppressing the vigor and reproduction of desirable species with similar growth strategies to the invader you’re targeting.  If you do succeed in reducing populations of invaders, you might also reduce populations of those desirable plants.)

Grazing can sometimes provide effective control of short-lived plants if it prevents flowering and seed production and forces plants to die without reproducing.  Just remember that more seeds are likely waiting in the soil, so it will likely take repeated grazing treatments to reach your goal.  Here in Nebraska, we often use short-term intensive grazing as a tool to knock back the competitive ability of perennial cool-season grasses such as smooth brome (Bromus inermis) or Kentucky bluegrass (Poa pratensis).  We don’t expect the grazing to kill those grass plants (and it doesn’t) but we can allow other plants a chance to flourish for a few years until the invasive grasses regain their vigor.  By repeating the treatment periodically, we can maintain a more diverse plant community.

Periodic early-season cattle intensive grazing helps us temporarily suppress cool-season invasive grasses like smooth brome and reduce its ability to outcompete many native grasses and wildflowers.

Personally, I’ve never used goats or sheep to help with a management challenge.  In contrast with cattle, goats and sheep, feed preferentially on forbs, and I’m usually trying to suppress grasses and encourage forb growth.  However, I do think goats, sheep, and cattle can all play important roles in controlling invasives as long as you don’t expect them to do more than they can.  I worry that landowners and land managers can sometimes end up paying an exorbitant price to someone that brings animals in with the promise of weed control.  It’s important to remember that if you do that, you’re providing food for that contractor’s animals, and that should be factored into whatever price one of you pays the other.  When we use cattle for prairie management, the cattle owner always pays us.  That seems not to be the case with many goat grazing operations.  I’m not saying it’s wrong to pay someone to graze their goats on your land, I’m just saying it’s important to fully process what each party is getting from the transaction.  That includes the forage provided to the animals from your land, the time and expenses incurred by the owner of the animals, and  – importantly – the actual effectiveness of the treatment.

As long as you have clear objectives and a good understanding of the plant(s) you’re targeting, grazing may be a great tool for invasive species control.  Just remember one of the biggest lessons from the South Dakota spurge experiment: just because you can’t see the invasive plant anymore doesn’t mean it’s gone!

Photo of the Week – February 9, 2017

Snow-on-the-mountain (Euphorbia marginata) is a showy plant, but not because of its flowers.  In fact, the flowers are tiny and very simple.  It’s the leaves (and some bracts beneath the flowers) that make the plant outstanding in its field.

Sno-
Snow-on-the-mountain (Euphorbia marginata)

Like other relatives in the spurge family, snow-on-the-mountain’s flowers have no petals or sepals.  The small round white things that look like petals beneath the anthers are actually bracts, and all the other white parts are leaves.  Most of the leaves of snow-on-the-mountain are green, but they become variegated toward the top of the plant.  The leaves closest to the flowers are often nearly or completely white.

More snow-on-the-mountain.
More snow-on-the-mountain, showing the rounded white bracts beneath the flowers, and the variegated green and white leaves below those.

The weird flower structures and variegated leaves are not the only unique features of snow-on-the-mountain.  Some of you with biology backgrounds know that plants are often divided by their photosynthesis strategy.  There are C4 plants (often casually referred to as “warm-season” plants) which are most efficient at photosynthesis during hot temperatures and drought conditions, and there are C3 plants (“cool-season” plants) which are more efficient during other kinds of conditions.  A few of you might even know that there is a third photosynthetic pathway called CAM, which is particularly effective in very arid conditions.  As it happens, snow-on-the-mountain and other Euphorbs actually use all three photosynthetic pathways – the only genus of plants for which that is true (as far as I know).  There, now you all have something to talk about at your next family gathering.

Snow-on-the-mountain is often maligned as a weed that needs to be controlled via herbicide or mowing.  In truth, while it is closely related to leafy spurge (Euphorbia esula), snow-on-the-mountain is an annual plant that is opportunistic but not invasive.  Cattle don’t like the taste of it, so it is often left ungrazed when everything else around it is nipped off near the ground.  Snow-on-the mountain can thrive in overgrazed pastures and other places where the vigor of dominant grasses is suppressed, but it competes poorly against those grasses when they’re allowed to grow again.

I appreciate periodically seeing snow-on-the-mountain in prairies because I know that if it was able to germinate and grow, other plants (including less “weedy” perennials) probably got the same opportunity.  Except around water tanks or other places where repeated cattle impacts or other factors keep grasses suppressed, snow-on-the-mountain doesn’t hang around very long.

A small beetle feeds on pollen, seemingly unaware of the camouflaged danger lurking nearby (crab spider).
A small beetle feeds on snow-on-the-mountain pollen, seemingly unaware of the camouflaged danger lurking nearby (crab spider).

As if the weird flowers, variegated leaves, and triple threat photosynthesis strategy weren’t enough to make you love this unique and beautiful plant, here’s one more cool fact.  Snow-on-the-mountain and other spurges produce white milky latex in their leaves and stems just like milkweed plants and rubber trees.  The latex isn’t sap, it’s made by a completely separate production system and doesn’t travel through the plant.  It’s a defense mechanism that is bad tasting and irritating to the skin of many animals (including humans with latex allergies).

Snow-on-the-mountain is a gorgeous native wildflower.  It used to be more commonly planted as an ornamental because of its beauty, but also because it is relatively free of pests and diseases (due in part to its toxic latex).  Unfortunately, because cattle don’t like to eat it and it can be abundant (and very conspicuous) in overgrazed areas of pastures, it has gotten an undeserved reputation as a nasty weed.  Snow-on-the-mountain doesn’t want to take over the world.  It just likes to grow in places where nothing else is growing anyway, and it gives way politely when the neighbors start getting pushy.

Does that sound like an invasive plant to you?  It sounds to me like a plant that needs a friend.  Let’s be friends with snow-on-the-mountain.  What do you say?

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.

 

 

Life on a Weedy Plant

Daisy fleabane (Erigeron strigosus) is considered by many people to be a weed.  It’s a biennial with very pretty, albeit small, daisy-like flowers that flourishes when the dominant plants around it have been weakened.  As a prairie manager, I’ve always appreciated daisy fleabane as an indicator that we’ve created conditions for new wildflowers (short- and long-lived) to insert themselves between the grasses in our sites.

Daisy fleabane (erigeron strigosus).  Lincoln Creek Prairie.  Aurora, Nebraska.
Daisy fleabane reaches toward the sky. Lincoln Creek Prairie. Aurora, Nebraska.

Last Friday evening, I took my camera for a walk in a small prairie here in town and found quite a few daisy fleabane plants growing along the trail.  I wasn’t the only one enjoying them – I saw numerous small bees and flies feeding on the pollen, and a few crab spiders hoping one of those pollinators waiting to ambush those same small pollinators.

Daisy fleabane (erigeron strigosus).  Lincoln Creek Prairie.  Aurora, Nebraska.
Daisy fleabane flowers and small fly.
Fly on Daisy fleabane (erigeron strigosus).  Lincoln Creek Prairie.  Aurora, Nebraska.
A closer look at the fly.

The first crab spider I noticed slipped over the edge of the flower to hide when it spotted me coming toward it.  I turned away to photograph something else nearby.  When I looked back, the spider was back on the flower.  I adjusted my position very slightly and the spider slipped back to its hiding place.  Argh.  Stubbornly, I decided I was going to photograph that spider if I had to wait all evening to do so.  I didn’t have to wait quite that long, but it felt like it.  I got my tripod positioned so that I could take the photo when/if the spider reappeared.  Holding perfectly still, (with sweat running down my nose and mosquitoes feeding on my neck) I stayed in position for at least 5-10 minutes until the spider finally showed itself again.  Got it!

Crab spider on daisy fleabane.  Lincoln Creek Prairie.  Aurora, Nebraska.
This spider photo is nice enough, but will always be memorable to me because of the effort it took to get it.  I hate to think how many mosquitoes got a free meal while I sat still waiting for my little spider buddy to make itself available for a photo…

A little further up the trail, I saw another crab spider that had caught a fly.  I figured it too would make a run for cover when I got close, so I came in low and slow.  I’m not sure it would have mattered – this spider showed none of the anxiety of the first one, and sat very still while I set up the tripod and waited for the breeze to pause long enough to get a good shot.  Maybe this spider was too distracted by its meal to care about me (though that’s not been my experience in the past).   I wasn’t sure whether to be grateful to the second spider for its cooperation or mad at the first one for all the mosquito bites on my neck.

Crab spider on daisy fleabane.  Lincoln Creek Prairie.  Aurora, Nebraska.
This crab spider seemed happy to have its photo taken with its hunting trophy.

I can understand why people might think of daisy fleabane as a weedy little plant, but its just filling an important role.  When the grasses are weak, something has to take advantage of the temporarily available resources around and between them.  There are numerous species that can do that, including a few that can cause real problems if they become established.  Given the choices, I’m always happy to see the pretty little daisy flowers and the diverse tiny creatures they attract.

Photo of the Week – May 8, 2014

In my last post, I mentioned that I didn’t mind having dandelions in my prairies.  Here is a further celebration of this beautiful, tough little plant.

Dandelions - pollinator heaven.
These dandelions were blooming near our shop building earlier this week.

While dandelions (Taraxacum officinale) are kind of weedy, they are not invasive – at least not in our prairies.  They essentially fill spaces left when perennial plants are either absent or weakened.  Typically, they come and go from the plant community pretty quickly, except in places (like around our shop) where frequent mowing and/or poor soil conditions prevent more competitive plants from establishing.

The dandelion’s status as a non-native plant doesn’t bother me in the least.  It is an attractive species and is great for pollinators – especially in the early part of the season when few other plants are blooming in our prairies.  Until relatively recent history, dandelions were seen as a useful and attractive garden plant around the world.  We’ve made the social decision to call it a weed, but that doesn’t change it’s ecological value.  You can read more about the history and uses of dandelions here.

Dandelions and henbit
Dandelions and henbit (another pretty and innocuous non-native flower) offer a nice counterpoint to each other, don’t they?

If you can get past the social aesthetic of dandelions as weeds and look at them as just a flower, they’re really very pretty.  Kids, who haven’t yet been pressured to label dandelions as a nuisance, can see that beauty – why can’t we?

Dandelion seeds
Dandelion seeds at the Helzer Prairie last week.

 

Ragwort – Prettier (and More Valuable) than its Name Might Suggest

One of my favorite spring flowers is prairie ragwort (Senecio plattensis, aka Packera plattensis).  Its bright yellow flowers add welcome color to prairies every May, especially when it appears in high numbers.  We always try to harvest as much seed from the species as we can when we’re doing prairie restoration projects – partly because it fills some important ecological roles, and partly because I just like it.

Prairie ragwort along one of the hiking trails at The Nature Conservancy's Platte River Prairies, Nebraska.
Prairie ragwort was blooming along our hiking trails last week.  The Nature Conservancy’s Platte River Prairies, Nebraska.

Ragwort is typically a biennial, so it germinates and forms a rosette (a few basal leaves) its first season, then blooms and dies the next year.  I’ve read that the rosettes can survive more than one year before blooming, but I’ve never watched closely enough to confirm that at our sites.  Regardless, it’s one of a suite of opportunistic plants that can take advantage of open space created by drought, fire, and/or grazing.  I have a soft spot in my heart for those species because their presence and abundance helps me read what’s happening in our prairies.  When I see lots of ragwort and other opportunistic species in our sites, it tells me that the vigor of dominant grasses has been suppressed (because of weather, management, or both).  Suppressing dominant grasses is a major focus of our management work because that suppression facilitates the establishment and spread of many other plant species, not all of which are “weedy”.  While a number of opportunistic species (ragweeds, annual grasses, and many others) can indicate grass suppression, ragwort is nice because it also happens to be attractive – and because it flowers at a time of year when few other showy plants are blooming.

An eastern-tailed blue butterfly on ragwort at the Helzer family prairie by Stockham, Nebraska.
This eastern-tailed blue butterfly was feeding on ragwort at the Helzer family prairie a couple weeks ago.  Near Stockham, Nebraska.
A sweat bee on ragwort at the Helzer family prairie.
A sweat bee on ragwort at the Helzer family prairie.

Prairie ragwort attracts a large number of pollinators, including bees, flies, moths, and butterflies.  It is an equal opportunity resource for pollinators because it doesn’t hide or restrict access to its pollen or nectar with funny shaped flowers that require long tongues or other specialized body parts.  Everything is right there – available to any insect that lands on it.  In years like this one, when ragwort appears in big numbers, it might be the most important species available to pollinators during its blooming period.

I enjoy watching the ebb and flow of ragwort populations in response to our management, but I also like to monitor its establishment and spread in our restored (reconstructed) prairies.  Although its seedhead is fluffy like a dandelion, and the individual seeds can travel long distances, most end up falling near the parent plant.  As a result, new populations tend to radiate outward from the initial colonizing individual, and the size of ragwort patches can be an indication of the age of a restored prairie.  However, that pattern falls apart when a prairie isn’t managed with frequent disturbances because the populations can quickly shrink during years when thatch and vigorous grasses prevent seed germination and establishment.

A common checkered skipper butterfly feeds on ragwort in one of our restored Platte River Prairies.
A common checkered skipper butterfly feeds on ragwort in one of our restored Platte River Prairies last week.  The short heights of the grasses surrounding the ragwort plants in this photo are a result of both drought and grazing last year.

Ragwort is certainly not a rare plant in Nebraska, or one that is of conservation concern.  The droughty nature of our state helps keep populations strong, as does the prevalance of grazing in many prairies.  However, I think it’s important not to judge the value of plants by whether or not they are rare.  Ragwort, along with hoary vervain (Verbena stricta), common and show milkweeds (Aclepias syriaca and A. speciosa), and many other “weedy” wildflower species serve as great indicators of ecological events, step up to fill holes in weakened plant communities, and are among the more important wildflowers for pollinators in our prairies.

What’s not to like?

Blowing Against the Wind?

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.

Sweet Clover: Ugly but Harmless? Or Dangerous Invasive Species?

Why is sweet clover the target of aggressive control by some prairie managers and largely ignored by others?  After talking to a number of people across the Midwest and Great Plains, I think there are a couple of things happening.  First, the usually biennial sweet clover can be very abundant and showy in the years it blooms, but is harder to find in other years.  I think some prairie managers see those big flushes and mistake abundance for aggressiveness.  However, I also think that some soil/precipitation/latitude(?) conditions may lead to real negative impacts from sweet clover on plant diversity.

One of the lessons that’s been strongly reinforced for me this summer is that it can be difficult to extrapolate successful prairie management/restoration strategies from one region to another.  Just during the last several months, I’ve visited prairie managers in Nebraska, Indiana, Missouri, and South Dakota and I’ve seen tremendous variation between (and even within) those states in terms of which species are invasive and which are not.  It’s dangerous to assume that just because a species like sweet clover isn’t causing problems in one prairie, it won’t cause problems in another.  I hope we’ll eventually learn enough to accurately predict when to worry and when not to, but in the meantime, it behooves prairie managers to carefully evaluate species at their own sites.

Yellow sweet clover. This exotic species is still planted in some wildlife and ground cover grassland plantings because of its purported wildlife value and cheap seed. However, it appears to be invasive in some places and/or situations.

I’ve been working with prairies along Nebraska’s Platte River for nearly 20 years now, and my observations have led me to conclude that sweet clover is more of a big ugly plant than a true invasive species in those prairies.  Years of data collection on my plant communities support those observations.  That annual monitoring work entails listing the plant species I find in each of about 100 1m2 plots across a prairie.  Those plots are stratified across the prairie so the site is evenly sampled.  Once I have those plotwise species lists, I calculate the floristic quality  (FQI) inside each plot, a calculation that takes into account both the number of species present and the average “conservatism” value of those species.  I can then look at changes in mean floristic quality over time to help me see how the plant community changes over time.  I monitor a few prairies annually, and others on a periodic basis.

Those data show the same thing I’ve seen observationally – sweet clover changes in abundance from year to year (though not as much as it appears visually), but the species doesn’t increase in abundance over the long term and doesn’t appear to negatively impact floristic quality.  Below are graphs from three sites that show both sweet clover frequency (% of plots occupied by sweet clover) and mean floristic quality.  Two of those sites were annually grazed during the data collection period, and the other was only grazed once – toward the end of the sampling period.  Cattle grazing almost certainly helps control sweet clover because it is one of their favorite plants to eat, but I don’t think sweet clover is causing me problems where I don’t graze either.

What my data don’t show is the flush of tall blooming plants that happens every other year or so.  I’m just counting whether at least once sweet clover plant is present in each of my small plots – not how big it is, or whether or not it’s blooming.  Nevertheless, sweet clover frequency changes from year to year but doesn’t appear to correlate at all with changes in mean floristic quality.

Nine of years of annual data from a 1995 prairie restoration seeding. The site has been under patch-burn grazing during each of the nine years of data collection. Sweet clover is never abundant at this site, but has also not increased over time, even through years of drought and heavy grazing. Error bars for floristic quality indicate 95% confidence intervals.

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Sweet clover was present in between 22% and 42% of 1m plots in this restored crop field, planted in 2002. Though the sweet clover frequency varied from year to year, the mean floristic quality of the plant community increased between 2004 and 2008 before leveling off after that - apparently independent of sweet clover. Sweet clover data from 2008 was eliminated from this graph because I later questioned whether I'd confused black medick and sweet clover in some plots. This site was not grazed except in 2009.

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These data are from a remnant mesic prairie under patch-burn grazing. Instead of being stratified across the entire prairie, these data are from a single patch that was burned in 2008, and that same patch was sampled again in 2009 and 2010. It was grazed very hard in 2008. In 2009, no new patch was burned, so it was grazed hard again, but then fenced out in early June. In 2010, the site was not fenced out, but received only light grazing while cattle focused on another portion of the site that was burned. These data are from only about 30 plots per year.

I feel pretty good about ignoring sweet clover and focusing on more invasive species on our prairies.  Both my observations and data support that strategy.  However, as I said earlier, just because the species doesn’t appear to be problematic for me doesn’t mean it isn’t an invasive species in other prairies.  It’d be great if we could compare data similar to what I’m presenting here from a number of sites to see if sweet clover is acting differently in different places.  Without data, it’s hard to know whether or not people are just interpreting the “invasiveness” of sweet clover in different ways.  For now, my answer to the question,  “Is sweet clover really invasive?” is still the same…

Maybe.