One day when I was out jogging, I noticed these round purple-ish flowers that seem to be ubiquitous along mid-Atlantic roadsides. Curious about their history and eager for an excuse to take a break, I stopped running and Googled “purple flowers that grow along the road.”
Called crownvetch, it turns out these innocuous flowers have played an enormous role in road construction for the last 50 years, all while beautifying the countryside and saving millions in maintenance costs. But their popularity has also come with some drawbacks. Who knew flowers could be so controversial?
The average driver probably doesn’t think about the road, unless something is wrong with it. The car enthusiast may notice whether a road is fun to drive on and how their car handles it. However, only a botanist or a committed infrastructure nerd regularly thinks about roadside plants. As I learned, there’s a lot going on where the pavement ends.
Erosion Is A Big Problem
“Every time we put any kind of corridor through for a roadway, whether it be an interstate or just a two-lane road, we somehow affect the environment,” said PennDOT Roadway Programs Manager, Luke Crawford.
“A road construction project in North Dakota.” – credit NDDOT
Building a driving surface causes huge changes to the natural environment. Topsoil must be removed, subsoil must be graded and compacted, different layers of binders and aggregates must be applied, and finally the area has to be paved, typically with asphalt or concrete. All this disruption destroys much plant life in the construction zone. Afterward, the finished roadway is often surrounded by open soil that is highly prone to erosion.
“The natural conclusion to erosion, where the dirt is washed away, is sedimentation. Somewhere it has to be deposited, ” explained Crawford. “And if the road is at the bottom of the mountain, there’s no place for the mud and sediment to go. As soon as the water slows down, it leaves that sediment along the roadway.”
“Erosion after a heavy storm. Soil from a nearby cornfield has washed out onto the road. Also, a steep slope gave way, dumping mud onto the road” credit – Joe Ligo
As I noticed on my jog, a recent storm had sloshed mud and gravel onto the pavement, most often when there was loose dirt sloping toward the road with little vegetation to hold it back. Obviously, this created hazards for drivers, as a patch of mud or a scattering of gravel could easily cause a vehicle to lose traction, especially in a curve. More alarming, however, were the places where the ground sloped away from the road, and rainwater had carved soil out from under it. Without intervention, this could eventually cause the pavement to crack and even collapse.
“Deep channels were cut into the soil here where water flowed off the road.” credit Joe Ligo
‘Crownvetch’ To The Rescue
Road crews have several tools to combat these issues, including ditches, culverts, barriers and sediment ponds. But by far the most common solution is plants. During new construction, workers immediately try to cover soil with fast-growing plant matter with root systems that will secure soil and prevent water from moving it. But as Penn State University Horticulture Professor Dr. Jim Sellmer explained to me, all that construction makes it hard to grow anything.
“This makes the soils very tight, dry, and nutrient poor. And the sites are hot. The plants that are successful must be able to grow with little supplements… It helps if their roots can penetrate deeply in what tends to be a rocky limited soil environment.”
“A road construction project in North Dakota. New seedings are visible on the right, along with erosion controls designed to temporarily slow the flow of rainwater until plant growth takes hold” – credit NDDOT
In 1935, Penn State agronomist Dr. Fred V. Grau discovered an unknown weed with lavender flowers growing in a pile of rocky soil on a farm in southeastern PA. He later identified it as crownvetch, a legume native to the Mediterranean regions of Africa, Asia, and Europe. Grau studied the plant, finding that it grew prodigiously, even in poor soils. Eventually, he bred a new variety, which he named “Penngift.”
By 1948, Grau convinced the Pennsylvania Department of Transportation (PennDOT) to plant crownvetch along roadsides, as its sturdy roots, fast growth, and ability to survive in poor soils made it a great solution to roadside erosion. As a legume, crownvetch also had the ability to pull nitrogen from the air and deposit it in the depleted soil, nourishing itself and companion plants.
“A PennDOT employee wades through a roadside patch of crownvetch in this photo from the mid 1970s”
Its use expanded rapidly in the 1950s, with the creation of the U.S. Interstate Highway System. In Pennsylvania and beyond, thousands of acres of crownvetch were planted along miles and miles of beautiful new roads. It proved especially useful for stabilizing steep slopes caused by carving into hills and mountains for roads to pass through.
Demand for seed became so great that the Pennsylvania Department of Transportation specifically asked farmers to grow more crownvetch to supply their needs. Other industries jumped on the bandwagon, as mining companies developed a particular affinity for planting crownvetch as a cheap way to greenify areas ravaged by coal extraction.
“Colorful patches of crownvetch along Pennsylvania highways” credit PennDOT
A 1977 article in American Horticulturist celebrated it as “a landscaping and environmental miracle,” calling crownvetch “the near perfect natural control for erosion, weeds, poor soil, and even litter,” even going so far as to praise how the dense leaves could hide trash that motorists threw from passing cars. (Nevermind whether their garbage was biodegradable or not…)
[Editor’s Note: My brother, Michael, is a senior engineering geologist in Hong Kong who primarily focuses on landslide mitigation. He told me the binding nature of plant roots is an important part of his job; from Michael:
Simply put, landslides typically occur when soil becomes oversaturated with water over a short period of time. Plants play an important role in landslide prevention. Not only do their root systems act as anchors and hold soil in place, but they displace and absorb water, keeping soil from being over saturated.Similar concept for erosion along roadslides, though if the soil is loose like sand, it doesn’t need to be saturated to become displaced. It simply gets picked up by flowing water or wind. Plants are a simple and cost-effective way of mitigating erosion.[…]In the 1950s Hong Kong planted trees over the majority of territory to mitigate erosion and increase air quality, which continues today. Today, after landslides occur, the gov has to quickly react to stabilize the slope through engineering means to keep it from continuously failing. Engineering a slope usually doesn’t involve plants, since it takes too long for plants to take root and secure the soil. But pilot studies are currently being carried out to determine if continuous planting can help reduce further landsliding and keep the hillsides looking natural.
How Transportation Departments Curate Roadside Vegetation To Help Drivers
As I was talking to Luke Crawford and Dr. Jim Sellmer, I came to the realization that road maintenance extends far beyond the edge of the actual road. Every roadside is a purposefully curated area, as state and federal transportation departments work to maximize functionality and driver safety.
Although conditions and terminology vary by location, busy roads and highways try to follow this system, based from the roadside outward:
credit: USDA National Agroforestry Center
The Non-Selective Zone: the gravelly edge of the road where no plants are allowed, so that water can drain off the road and motorists can see.
The Safety Clear Zone: where woody vegetation is prevented, so motorists can see and cars have space to pull off in an emergency.
The Selective Zone: where tall grass and meadow-like plants are encouraged, but trees and problematic weeds are removed.
The Natural Zone: where plants and trees are allowed to grow naturally with only minimal maintenance to prevent problems and damage.
Luke Crawford told me that drivers may not realize that roads can become a lot more dangerous without proper vegetation maintenance.
“If you’re sitting down in a normal car, your eyes are really only about 30 to 36 inches above the road surface. So when you let grass grow up even a little bit, or weeds, you lose that sight distance when you’re maybe pulling out of an intersection, so that’s a big one right there.”
Encroaching vegetation becomes a safety hazard by making it harder to see wildlife, oncoming motorists, and upcoming curves in the road. On top of that, tall plants can damage the pavement itself.
“The more shaded, the wetter the asphalt remains,” said Crawford. “And water is the number one enemy of asphalt. Just simply the fact that the pavement remains wet will degrade the pavement more quickly over time…Potholes form and ice will remain and linger along the edges of that roadway a lot longer.”
And here is where crownvetch proves to be invaluable. Not only will it grow in all kinds of soil, it hardly ever grows over two feet tall, preferring instead to spread sideways. This makes it perfect for steep hillsides that are difficult to mow, and it reduces maintenance in flat areas, too. Crownvetch can happily live for months and never require a trim. Plus, as a perennial it’ll come back every year.
“I spotted the tell-tale purple flowers as far away as Wisconsin, where crownvetch could be seen lining steep hills near highway overpasses that would be tricky to mow.”
According to the ’77 magazine article, these benefits alone helped save PA taxpayers an estimated $1.6 million per year in planting and mowing costs. I can’t imagine what that number would be today.
“Crownvetch lines a hillside below a road” credit Matt Lavin CC/BY/SA via Flickr.com
Growth Like a Weed
If you’re starting to worry that this all sounds too good to be true, you can probably guess where this story is headed. It turns out that crownvetch grows so well that it’s managed to grow in all kinds of places it’s not supposed to. Despite its effusive praise for the plant, American Horticulturalist did issue a warning that sounds more foreboding in hindsight,
“…it is a vicious spreader and should be kept out of the carefully manicured garden where it could easily become a pest.”
Without management, crownvetch can easily take over areas and crowd out native plants, disrupting delicate ecosystems. It’s also toxic to horses, and it can creep into farm fields where it competes with crops. It spreads by branching out with horizontal roots, as well as with hard seeds that are scattered by wildlife. It can also spread through broken root pieces carried in soil that’s moved around during construction.
“Crownvetch can easily become feral and spread by growing in places where other plants won’t, such as this gravel pile at a construction site.” credit – Joe Ligo
While it does crowd out a lot of other vegetation, certain noxious weeds like Canada thistle actually grow better in patches of crownvetch. These weeds may not pose as much of a risk in a field of grass that’s regularly mowed, but if they take root amongst crownvetch, they’ll grow to maturity and spread their destructive seeds elsewhere. Unfortunately, crownvetch is susceptible to many of the same herbicides that kill Canada thistle, so spraying the weeds would kill both plants.
In response, many state governments have declared crownvetch an invasive species, and The Federal Highway Administration now discourages its use. Thankfully, it’s not the most aggressive plant in the world, and it can be controlled with a little bit of diligence. While there are far more destructive weeds out there, humans have been purposely spreading crownvetch since the 1950s, giving it the potential to cause plenty of issues.
“Over time it has been over-deployed in areas that are more manageable and where other vegetation would be better suited,” said Dr. Sellmer.
The Road(side) Forward
Finding exactly what that “other vegetation” should be is what Dr. Sellmer and his colleagues are working on today. Since 1985, Penn State has partnered with the Pennsylvania Department of Transportation to research roadside vegetation management. Recently, that has included finding a replacement for crownvetch.
For all its faults, few plants can check all the boxes that crownvetch does. It’s tough to find something that has cheap seed, will grow anywhere, come back every year, prevent erosion, provide pretty flowers, and not require a lot of maintenance.
Despite the need for regular mowing, different grass varieties remain a popular choice for roadsides, especially in the aforementioned “Non-Selective Zone” where cars can pull off in case of an emergency. In areas with acceptable soil, grasses provide a nice alternative to crownvetch without the risk of spreading.
“A worker mows a grassy highway median to keep the area clear of tall plants.” credit – Joe Ligo
Another foriegn legume with a funny name, birdsfoot trefoil, can also be used. Like crownvetch, it tolerates a wide variety of soil conditions, but is less aggressive. While taking a road trip during a particularly dry spell this summer, I noticed large quantities along roads in Pennsylvania, Ohio, and Wisconsin. Technically, it’s still considered an invasive species in some areas, but unlike crownvetch, birdsfoot trefoil will yield to more competitive plants if the soil quality improves. This makes it somewhat less of a threat to ecosystems. As a result, it is often planted as part of a mix of seeds.
“Birdsfoot trefoil found growing in the dry soil next to a road in Ohio.” credit – Joe Ligo
Looking forward, mixes might actually be the key to replacing crownvetch going forward According to a fact sheet from Penn State,
“The seeding formula that ultimately becomes an alternative… will require several species to provide the same range of site adaptability, rate of establishment, and seeding-time flexibility as crownvetch.”
Increasingly, native plants are being considered as part of that solution. Conveniently, they’re already suited to grow in local soils and climates, but they don’t run the risk of spreading out of control. Planting a wide variety of native species instead of a monoculture of crownvetch also increases biodiversity, which benefits local ecosystems, especially endangered bird and insect populations.
Unfortunately, cost remains a big sticking point, as there aren’t a lot of farmers who specialize in growing native seeds. Plus, more research on exactly what native species to plant still needs to be done. In light of all this, federal and state construction projects still rely on crownvetch to stop erosion, but they’re trying to be more selective about where it’s planted and how often it’s used, especially in areas where other plant varieties will suffice.
During a recent visit with friends who live in PA, I noticed a recently completed highway exit construction zone. The massive project included four ramps, four lanes of highway traffic, four lanes of intersecting traffic, and two sets of stoplights. Given that it was Pennsylvania, natural and manmade hills surrounded the area. Looking closely, I saw that what had once been steep slopes of loose soil were now blanketed with green and dotted with purple and yellow flowers. Sure enough, it was crownvetch, mixed with birdsfoot trefoil and some other plants. I guess PennDOT isn’t quite ready to give it up, yet.
“A recently completed highway exit, surrounded by new growth of crownvetch and birdsfoot trefoil, along with white Dutch clover and some grasses.” – Credit Joe Ligo
“Although it’s an invasive species in Pennsylvania…it’s not very high up the list,” Luke Crawford told me. “And there are still very, very good applications for using it.”
After decades of holding roadsides together, it’s probably for the best that crownvetch’s days as a miracle erosion cure are numbered. It’ll never be fully eradicated from the natural environment, but at least fewer humans will be spreading it on purpose. And, as Crawford explained, its legacy has been largely positive.
“Probably the greatest thing about crownvetch is, it’s a nitrogen fixer… It pulls nitrogen from the air and it puts that nitrogen into the soil. So it actually improves soil over time. So these areas that 40 and 45 years ago were just covered with crownvetch because that’s what they planted… it’s had this secondary benefit of improving the soils, and these areas now grow a wide variety of plants in them because the soil has been improved over time.”
In a way, crownvetch actually helped prepare the ground for its replacements. Eighty-five years later, who could have guessed that a random weed from a Pennsylvania farm could have made such an impact on our modern infrastructure? I guess Dr. Fred V. Grau was really on to something when he named it “Penngift,” after all.
credit Joshua Mayer CC/BYSA via Flickr.com
Why not just use purple clover or rye? Horse and cows at least can eat that. Should be just as easy to use.
I doubt you’ll see this comment because of notifications, but I can offer some insight:
Clover is a great ground cover, but it’s slow to establish (2-3 years oftentimes). A lot of seed mixes will include ~10% red or white clover, with the intent of establishing it in the seeded areas, but it’s not fast enough to prevent erosion.
Rye could be one of several – Cereal rye (the kind we make whiskey from) establishes moderately quickly, but once it goes to seed, it dies out. Annual rye establishes very quickly, but dies out even faster. Perennial ryegrass takes a while to establish, but it makes a great ground cover once it does. Unfortunately, it doesn’t compete well with other more aggressive grasses.
The best mix includes a good native variety. One I can remember using a lot was something like 10% clover, 15% fescue, 10% birdsfoot trefoil, 15%-25% of a mix of other meadow grasses (partridge pea, brown eyed susan, etc.) and the balance of annual rye.
Fascinating article!
Interesting article. If you’ve not already done so, check out Multiflora Rose, and how several countries experimented with it as a highway barrier and living fence. If my memory serves me correct, a car traveling at 55mph could be stopped by around 150feet of rose hedge.
Trust me, the last thing you want in your own private yard is the invasive, woody, perennial weed multiflora rose. We have a 110’ x 50’ area in our Northeast Ohio yard which I’ve endeavored since 2001 to transform from lawn into a forested area. We planted carefully chosen native Ohio trees, and allowed other “scrub” trees to slowly fill the area. Multiflora rose is a nasty plant with lots of thorns and no flowers, which grows out beyond this area and snags me when I’m cutting the grass. I do my best to cut it back and occasionally dose it with some Roundup-adjacent product, yet it persists.
There was a tragic case here in CA of a woman whose car broke down while traveling in the fast lane on Interstate 80 between Sacramento and the Bay Area, and the oleanders where she pulled off the road were so overgrown she couldn’t pull off completely out of the lane and she was rear-ended moments later, killing her. The family sued CalTrans of course, and I’ve noticed the oleanders have been kept trimmed back since then. Fun fact: I used to play in a band with the guy that is now the head of CalTrans. Great drummer and great guy!
Unfortunately, the government has been responsible for introducing a number of invasives for erosion control. Kudzu, Elaeagnus, etc. It’s a shame when we have plenty of native plants that can do the same thing but are actually beneficial to the environment.
Also one of the interesting things about roadsides and power line cuts is that they mimic grasslands which not many people are aware much of the southeastern U.S. used to be covered in before the suppression of fire on the landscape. Some states such as Tennessee have programs where they actively foster them with native seeds and less mowing in order to allow them to take that role.
Thank you for this article. It’s a Saturday treat to see people geeking out over erosion control. I don’t mean that sarcastically either. I do erosion control plans for land development and roadwork, and I had never heard of crownvetch. Our go-tos are generally tackifiers, mulches and a dizzying variety of grass mixes. There’s also the ubiquitous (and ugly and mostly useless) silt fence…
As a bit of a plant nerd, I’ve long been familiar with purple crown vetch. You’ll find it in just about any disturbed area around here, along with other non-natives of varying invasiveness such as lamb’s quarters, garlic mustard, purple loosestrife, japanese knotweed, and, of course, good ol’ native poison ivy. It’s pretty, but it doesn’t belong here. Oh well, it’s not going anywhere, might as well enjoy the flowers.
I’ve noticed though that several major highways near me seem to be planting native wildflower mixes along their margins, which is awesome. Depending on the time of year you might see huge, dense swaths of black-eyed Susans or purple lupines. Ironically, lupines (which are absolutely stunning when in bloom) are an invasive weed in Iceland—where they were introduced as a form of roadside erosion control.
Oh, humans. Our capacity for self-owning through unintended consequences really is limitless, isn’t it?
There is a park near me that just put up a bunch of big signs throughout, bearing the legend “taking ACTION against purple loosestrife!” with a small print explanation of that that is and why it’s good. Hadn’t heard of it before, but nice to see stewardship of native species in action
Purple loosestrife is kind of a hopeless battle. You can eradicate it from a small area like a park if you get enough people to keep pulling it up, but as soon as you let your guard down it’ll move right back in there. There’s no realistic way of completely eradicating it from North America.
It’s not as bad as Japanese Knotweed, though. That stuff, if you cut it down it just stimulates growth—it’ll come back harder and thicker than ever. Don’t bother pulling it up either, its roots will break apart and stay in the ground, and even a 1/4″ section can regrow into a new plant. You can kill it if you go hard enough with the RoundUp (cut it and spray it directly into the open stem ends, then keep coming back for a few years until it stops regrowing) but otherwise you need to cut it down, dig it up, burn the remains, bury them, and then cover them with plastic for ten years if you want to get rid of it. Oh, and its seeds are airborne. Have fun with that.
Great article, and I like the picture of a viceroy butterfly partaking of the crownvetch deliciousness at the end.
Out here in California on our long highways, while not a small plant, they planted zillions of Oleanders. They were frequently planted in the median between the two major traffic directions. Several colors make for a great color break in eh landscape. Now some multiple decades later some of them have grown very large obscuring the metal guard rails they now dominate.
One of my late great uncles was very proud to have been involved with the decision to plant them as part of what is now known as CalTrans.
Another topic that Autopian, and you Joe, could do a wonderful deep dive into.
Oleanders? Now i’m thinking of surfing
I grew up in State College in the sixties and seventies. We were taught all about how crownvetch was “invented” at Penn State in grade school.
On family canoe trips we often had to scramble down roadsides to get to a launching spot. It’s impossible to walk through without tripping.
I learned to HATE crownvetch.
Super interesting. I occasionally wonder about the ubiquity of grass along the road and always like getting a peek behind the curtain of everyday things
IIRC, Lady Bird Johnson worked not just to take down billboards, but also to plant wildflowers along interstate highways. different motivation (aesthetics) but a similar result (lower maintenance costs).
Besides the fact that this is a very well written article, I want to show my appreciation for covering stories about the impact of what vehicles have on our world. It’s not all cool machines, but also the holistic cost of the infrastructure. We have changed our landscape and will continue to as we meet our needs of movement. It may sound silly, but this is right in line with some of those discussions on “what is your favourite road trip snack” and the old 11′ 8″ bridge. Our beloved machines don’t live in a vacuum and this is a perfect article to expose that.
I know I’m no fun at parties, but I wish this would also be part of stop shredding nature because you want to goof off offroad and stop shredding tires because you can’t get laid.
Great article: well written, informative, and makes me pay more attention to the landscape. I know I’ll be looking more at the roadside during my weekend jaunt tomorrow
hy
Can confirm, the CROWN VETCH signs were scattered along the PA roadways when I was growing up. My dad would call out to us whenever we passed a sign just like in the ’70’s photo – his version of a dad joke, lol. The photo brought back fond memories of fun car trips.
Crown what? What’s a ‘vetch’? The kingdom of vetch announcing their crown. Lots of made up, outlandish and crazy stories would ensue about the people of Vetchland.
My wife spent years as a project manager for construction companies that did earth works. We should see if she’d be willing to write some articles on infrastructure.
Interesting stuff. I haven’t heard of this natural erosion control method before, in my line of work we tend to use a filter cloth mesh to achieve the same results on steeper slopes. Grass is also very important in keeping soils in place, at least is less steep circumstances. There are specific seed mixes depending on the roadway application.
This is INCREDIBLY interesting to me. We built a house within the last year and it sits on a literal ravine. One side of the house basically floats above a creek bed, and our front yard rounds a curve around probably a 40 foot slope that drops at about a 35 degree angle.
Our builder was sure to net and immediately plant what they called “highway grass” there as soon as the foundation was poured so that the grass could take root and prevent erosion. It did the trick, but since we moved in this spring, it seems every single kind of weed I’ve ever seen has taken residence there. It’s terrible for allergies, it looks pretty unkempt, and it’s zero fun having to weedwack it to keep things in check.
I had asked several landscapers if there were any sort of wild flowers or nice-ish looking plants we could seed on that hill, but no one had good ideas. Besides the whole part where this weed would apparently overtake our beautiful forested ravine out back… it sounds and looks wonderful.
Our septic mound is hazardous to mow at the best of times. I’d love to be able to just seed it with something like this and let it go, but so far our occasional attempts to do something other than grass have ended up just full of poison ivy and locust trees
We even tried goutweed on ours to no avail, got nettles and thistle
Yeah. The thing about weeds, especially invasive ones, is that they tend to be the best, fastest-growing, hardest-to-kill competitors on the block. The stuff that’s taken over the side of your ravine did so because it was able to elbow out a niche for itself, and you’ll likely have a hard time finding anything more traditionally desirable that has the muscle to shove the weeds out of the way. A lot of weeds are quite pretty though, if you take the time to get to know them.
A lot of places also use the straw craptasically woven into plastic mesh as a bit of groundcover – while the plastic mesh helps keep the straw in place, how does that affect future plant maintenance (like digging)? Is the plastic also craptastic so it breaks easily when you try to dig?
Would runoff be helped if the roadside ditch design had some wiggles to it like a bioswale might have?
The stuff I have seen claims to be biodegradable. So it keeps the soil in place, helps plants get established, and then disappears.
Disappears… into microplastics that are still there.
Microplastics are not biodegradable, that’s why they are micro in the first place.
Way more interesting then I expected it to be.
I appreciate the article. The family business I work at does a lot of erosion control and DOT project seeding. It’s a different animal here in North Dakota since a majority of the state is considered desert prairie. We rely on a more limited range of species that will tolerate the, um…”wide range” of weather patterns we experience here. That said, it’s always interesting to see how other regions approach this challenge.
Mighty informative article! It’d seem like horticulture wouldn’t have much if anything to do with things automotive but this article makes it eminently clear just how important such considerations are in the motoring world, so thanks for that!
Fantastic! Love learning more about the roads we depend on.