I haven’t talked about invasive species in this space for a while. And not because I got sick of them after writing a popular-science book on the subject back in 2017. In fact, I think about them more than ever, because once you recognize a particular invasive, you literally can’t stop noticing it. Double down on that if it’s ecologically, visually or personally disruptive, triple if it’s all three, and multiply by an order of magnitude if you understand how it wreaks havoc.
That’s me and burdock.
It wasn’t always that way. Like most, I grew up with burdock on the landscape, ignoring it save for learning to carve a wide berth around mature, burr-draped plants in the fall when I was wearing a sweater or walking a dog. Oh, and I knew those nasty burrs were also the inspiration for Velcro. Other than that, meh.
Flash forward a few decades to living in Whistler and volunteering on a Sea to Sky Invasive Species Council (SSISC) burdock pull along the Valley Trail. My eyes are opened. These plants are big and burly, their forearm-length taproots even more so; they’re difficult enough to get out of the ground that the work involves heavy-duty tools. Our crew drags a half-tonne out on tarps. The next year I join a pull in the Alpha Lake dog park; less biomass but I start to notice things—where and how burdock grows, how it excludes some plants but facilitates others. Some of the big ones seem far older than the two years that book descriptions attribute to them—suggesting that if they’re cut out by the Muni’s landscaping machinery and aren’t allowed to mature, they’ll keep coming back for as many years as it takes.
I become intrigued and join the SSISC’s Adopt-a-Trail (since morphed to Banish Burdock ssisc.ca/banish-burdock) program, in which I sign up to keep a section of Valley Trail clear. I pick the messiest one, where burdock forms a wall up and down both sides, and start by removing my own half-tonne. To reduce the seedbank, I also pick up and bag all the burrs I can find. Then I start on the immature plants. I go back every few weeks to see if new ones have sprouted, continuing to remove plants until native vegetation grows back in. It takes me three years and dozens of visits. But like every invasive-species fighter, I’m determined. I learn a ton from the plant about not only its own colonization abilities but those of other invaders.
I expand my removal program to other parts of the trail, the 20 kilometres of highway between Function and Emerald, a dozen neighbourhoods and schools. I remove literal tonnes each year. It’s my daily workout from March to November. Some say I’m obsessed, but it’s something else; I can drive past a burdock in Pemberton (which has a huge problem) with zero urge to dig it up. But if I see a patch in Whistler, it’s immediately on my mental bit-map. I figure if everyone pitched in to reduce the biomass of burdock in Whistler by 50 per cent every year, within five years we’d only be doing spot control. That’s because if I’m obsessed with anything, it’s the science of it.
Occasionally cultivated for food in its home range, burdock is a robust Eurasian biennial classed as a noxious weed across North America and listed in Schedule A of the BC Weed Control Act. The name is inclusive of greater burdock, Arctium lappa, common burdock, A. minus, and a range of hybrids. Burdock in Whistler is probably all three.
Oh, you mean those big leaves that grow kind of like a clump of rhubarb? Yup, that’s burdock. And those head-high plants with the purple flowers that look like small thistles? Yup, burdock, too. The brown, dried-out weepy things you see in the fall? Also burdock.
Burdock is a good invader: it’s highly adaptable and grows in any well-drained substrate, even those of low nutrient value (e.g., gravel—they love snow-dump areas) in full to partial sun (its preference). It got to Whistler in the early days (there are 60-year-old forests growing around patches of it) and became widespread in vacant lots, landfills, right-of-ways, around pumphouses and electrical boxes, and road- and trail-sides. Its sticky seed burrs, each of which contains about 80 seeds, are moved around by walkers, bikers, dogs, bears (a lot—see photo above) and Muni workers going from job to job. As a problematic weed, burdock is understood to impact local plant biodiversity through nutrient and light competition, and creation of dense monocultures, and its seed burrs have demonstrable negative effects on livestock and wildlife like bears and bats (Google this if you want to see dead, dried-out bats caught on burrs).
Observations of burdock playing both nasty and nice with other Whistler plants follows research that shows it to be strongly allelopathic—producing phytochemicals that inhibit growth in some plants but stimulate others; most native plants are strongly inhibited in a cumulative fashion (more burdock + more time = more inhibition), while the growth of fellow Eurasian invasives like bitter dock appear to be facilitated by burdock-induced changes to soil chemistry. This latter relationship suggests a role for burdock in “Invasional Meltdown”—the ability of one invasive to perpetuate invasion by another. Indeed, burdock facilitates a range of invasive Eurasian invertebrates in Whistler, from earthworms (several species) to molluscs (European black slug; chocolate arion slug; brown-lipped snail) and insects (black aphid; Eurasian seed-head moth). Although these organisms contribute to population control by feeding on burdock, the plant represents a reservoir on which these creatures multiply to attack both native and garden species.
My beef with burdock grew as the science showed it to be more problematic than previously thought—but I still don’t like getting burrs on my socks or seeing pets and wildlife compromised. I’m not alone, as others have joined the SSISC’s Banish Burdock program, where you choose your own chunk of Whistler real estate for free and learn how and when to remove plants, deal with burrs, and dispose of all of it.
If the program fills up, I promise to stop writing about invasive plants!
Leslie Anthony is a biologist, writer and author of several popular books on environmental science.
