As It Happens

'Canada's most dangerous plant' spreading, biologist warns

Conservation biologist Dan Kraus warns about the dangers of giant hogweed, the invasive plant that's been spreading across Canada.
Biologist Dan Kraus says the invasive plant can grow up to four metres tall and cause severe burns. (Darren Calabrese/The Canadian Press)

Story transcript

It was originally brought to Canada to be decorative, but giant hogweed's effects are anything but pretty. 

The plant, native to Asia and Eastern Europe, produces a clear sap that can cause severe burning and blisters if it gets on skin that is met with sunlight.

The Nature Conservancy of Canada is putting out a warning to all Canadians to be on guard for this invasive species.

Dan Kraus is a senior conservation biologist with the organization. He spoke with As It Happens guest host Rosemary Barton to discuss how to spot giant hogweed and what can be done to slow the spread. 

Okay Dan, give us a sense of what giant hogweed looks like.

The two things that people can look for when they're trying to identify giant hogweed is it has a cluster of white flowers that grow in an umbrella shape at the top of the plant and it's actually so big it can be the size of an umbrella.

The other thing is that it is giant. The plants are usually a couple metres tall, but I've seen plants up to four metres in height. So really, it's this big plant that has this giant cluster of white flowers at the top.

If you pick this pretty flower, you could end up with severe blistering burns that last for weeks and scars that last for years. (CBC)

And where is it growing mostly in this country?

In Canada, it's spread to several provinces. It's definitely in Ontario and British Columbia, more recently it has spread to the Maritimes and to Quebec. You can usually find it growing in wet areas, so it can be quite common along the shores of rivers or streams or sometimes along the edges of forests if the soil is damp.

Why are you putting out this warning? It's not just because they're big. What are you warning people about here?

Well, the timing is good now because this plant is starting to flower right now so it makes it very easy for people to identify. So what we'd really like people to do is to make sure, first of all, they're not growing it in their yard. The other thing is to start looking for it in some of the natural places that they're visiting and report it.

And again, it's not just because it's a weed, it's because it's potentially dangerous. Is that right? It could hurt you?

This plant produces a toxic sap, and when you come in contact with that sap and you get it on your skin and then you go into sunshine, it actually can start to form rashes and blisters and even leave permanent scars, so it is a very dangerous plant. And because you don't feel it right away, you can get quite a bit of sap on you until it's activated by the sunlight and you actually start to feel the intense blistering starting to happen on your skin.

Kraus has been a biologist with the Nature Conservancy of Canada for 15 years. (Dan Kraus)

That doesn't even sound believable.

It probably is Canada's most dangerous plant.

We're certainly concerned about the health impacts on people, but it's also impacting nature as well. At the Nature Conservancy of Canada, what we're concerned about is that — because it's non-native — there's nothing that really controls it and it can take over large areas along rivers and streams. 

And because it's so tall, it can shade out all of our native vegetation and actually nothing can grow under it sometimes, so there's exposed soil, which can result in erosion along rivers and streams.

So if people see it, then what should they do?

Definitely don't take a specimen. What would be great is if people can report it. And across Canada a lot of health departments or regional weed inspectors are very interested in knowing where this plant is. Most provinces have invasive species councils that can take records of it.

What I'm starting to use now is an app on my phone called iNaturalist. With iNaturalist, I can take a picture of any species, and there's a whole community out there that will help me to identify it. And the picture will automatically map it as well. I find that really useful because I can go onto that map and see where people are starting to find this plant.

Giant hogweed has spread to British Columbia, Ontario, Quebec and the Maritimes. (PLANTS Database/Natural Resources Conservation Service)

What's the goal? Do you want to kill the plant off — because that doesn't seem like something a biologist would want to do — or do you just want to contain it?

We'd be happy if it was struck from the list of the flora of Canada. It's non-native, it's originally from the Caspian area in Russia. It was brought over here as an ornamental. It would be great it we could get rid of it.

This interview was edited for length and clarity. For more on this story, listen to our full interview with Dan Kraus.