How A Weed From A Pennsylvania Farm Became A Miracle ‘Cure’ For Roadside Erosion

Crown Top

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.”

1a Crownvetch 1b Crownvetch

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.

2a Crownvetch“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.”

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“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.

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“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.”

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“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.

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“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.

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“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.
-DT]

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:

7a Crownvetchcredit: 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. 

13a Crownvetch

 “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.

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“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.

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“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.

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“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.

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“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.

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“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.

12a Crownvetch

credit Joshua Mayer CC/BYSA via Flickr.com

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41 Responses

    1. 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.

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