Giant hogweed: why we can't stop the spread of a toxic plant

Meghan Grguric might have the most dangerous job at the University of Guelph's Plant Sciences Department and she has the scar to prove it. 

"Where I have my burn is on my wrist," the graduate student says as she pulls a thick rubber glove past the scar she earned last summer while working on her thesis . "I don't want any more accidents."

It's easy to see how an accident can happen. Grguric's research is part of a wider war to stop the invasive weed, being waged in test plots, wild areas and even backyards across Southern Ontario. 

It means Grguric is up close and personal with the giant plant, wading through infestations and cutting the plants back, all the while putting herself in potential contact with giant hogweed's toxic sap. 

The clear, watery substance is known to cause severe burns when it reacts with sunlight.

'We don't know that it causes blindness'

More prominent, however, is the weed's reputation for causing blindness, something Grguric notes is widely reported by the media even though, to her knowledge, it has never been proven.  

"We don't know that it causes blindness. There's the potential, but no-one has ever gone blind that we know of," she says. "It has become a false fact." 

Still, Grguric takes no chances when it comes to her own protection. She wears two layers of gloves and a pair of hooded coveralls when she's out in the field. 

Her research aims to understand how the plant's natural reproductive cycle can be disrupted with herbicides and pruning.

Hogweed experiments

At an experimental farm owned by the University of Guelph outside of Woodstock, Grguric has deliberately planted the noxious weed along neat rows, each marked with a stake. That way, she can easily monitor the effect on each individual plant as she either clips them back or exposes them to a specific type of defoliant.

"A lot of people like to use Roundup," she says. "The problem with Roundup is that it doesn't provide any residual control. So, a lot of seedlings come back after you've already sprayed and it kills grasses. Basically you want to go in there and disturb the area the least amount as possible."

"I want regular plants to come back and re-establish where the hogweed was. I don't want to go in and kill everything, I want to leave the area as undisturbed as possible," she says. "It is very surgical." 

The reason for the precision, Grguric says, is partly based on where giant hogweed likes to make its home. 

"The problem with giant hogweed is it likes to grow near streams and rivers," she says. "You're going to run into a lot problems with what you can and can't spray. You need a certain buffer zone between the water and the hogweed." 

"That's where my manual control comes in," she says. "If I can't get in with herbicide then I want an alternative option, which would be cutting it, but there hasn't been a lot of luck with cutting." 

'Hogweed is ridiculously persistent'

"Hogweed is ridiculously persistent," she says. "I go in, I cut it down and a few weeks later there's re-growth from the bottom and it will set flower and set seed." 

Understanding whether the plant's aggressive reproductive capabilities can be curbed might be key to unlocking a potential solution to halt its spread.

Already the plant has infested Ontario in isolated pockets, from Manitoulin Island through Sudbury to Ottawa in the north and in the south, from Muskoka to Tobermory and from Kingston through the GTA to Sarnia.  The  highest concentration is found in the Halton Region.

"What a lot of people don't understand is that you just can't go in and control it one year," she says, noting city crews in Halton often need to return to an infested site for up to four years in a row in order to fully eradicate the weed from one area. 

'The genie is out of the bottle'

"If there's even one plant, you really have to keep on top of it," she says. "There's about 10,000 to 20,000 seeds per plant."

"The way it spreads its seed and the amount of seed it spreads, I don't know whether or not it can be fully controlled," she says. "All you can do is try and contain it to small popultions."

"You can have some control, but I really think the genie is out of the bottle." 

"It's like any invasive species," she says. "Once it's here, it's almost here to stay. It's hard to kick it out once it's already invited itself in."

Giant hogweed native to Russia

Giant hogweed is native to the mountainous Caucasus region of Russia, between the Black and Caspian seas and while it may have overstayed its welcome, it hardly invited itself in. 

Like almost all invasive plants in Canada, it was humans who brought it here, rolling out the red carpet by planting it in a garden, only to have it escape into the wild. 

As for its name, Grguric says giant hogweed might get it from the fact that the odour of the plant is reminiscent of swine when freshly cut, something that was pointed out to her by a colleague, a pig farmer studying at the University of Guelph.  

"I don't know how else to describe their smell," she says. "It's not a horrible smell, but it can be really powerful when you're cutting them."