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Giant hogweed can cause temporary and permanent blindness. (Darren Calabrese/The Canadian Press)
Giant hogweed can cause temporary and permanent blindness. (Darren Calabrese/The Canadian Press)

Poisonous hogweed infests Toronto's Don Valley Add to ...

A poisonous plant that can cause boils and blindness is growing near running trails in Toronto's Don Valley.

The Toronto and Region Conservation Authority says there are infestations of giant hogweed throughout the ravine in the Finch and Bayview area.

No one is sure how it began growing in the Don Valley, but it's possible some hogweed growing in the Markham area travelled downstream, said the conservation authority's restoration services project manager, Karen McDonald. The Humber River Watershed is also home to the weed, she said.

"When they populate into an area frequented by people, it becomes a real human health concern," she said.

Serious blisters develop when the plant's sap touches human skin, Ms. McDonald said.

"I wouldn't mess around with it just because of the severity of how it infects you," Ms. McDonald said.

The sap is photosensitive, which means it becomes even more toxic when exposed to sunlight. People should immediately wash the area with soap and cool water and go to the hospital if the sap touches their skin, she said. Hogweed can also cause temporary or permanent blindness if the sap comes into contact with eyes.

Giant hogweed is not native to Canada. The plant comes from the Caucasus Mountains region, at the borders of Europe and Asia, Ms. McDonald said.

Infestations have also been spotted recently in the Ottawa and Halton regions, she said.

Toronto conservation workers will either dig up the plants, use a herbicide or mow over them repeatedly to kill them.

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