Japanese knotweed causing problems in Toronto, conservationists warn

Torontonians should be fighting back against an invasive plant species that’s threatening to take over parts of this city, but there’s a problem – it’s kind of pretty.

Invasive species 1st brought to Ontario decades ago for ornamental purposes

If you see this plant in the GTA, conservationists are urging you to report it. (Craig Chivers/CBC)

Torontonians should be fighting back against an invasive plant species that's threatening to take over parts of this city, but there's a problem — it's kind of pretty.

Japanese knotweed was brought to Canada for ornamental purposes as early as 1901, says Colleen Cirillo, director of education at the Toronto Botanical Garden. But while its soft teardrop-shaped green leaves and pink bamboo-like stems are nice to look at, the aggressive plant can quickly overwhelm both home gardens and natural areas, alike.

"Very soon if you had 10 species you will have one: Japanese knotweed. It's basically a bully in the garden," Cirillo said.

That bully is also extremely difficult to fight. Cirillo says you can kill a small knotweed by cutting it down to the roots twice a month for several years in a row — and that's for a small plant.

You won't find Japanese knotweed at the Toronto Botanical Garden, but take a walk into the nearby Wilket Creek Park and you'll find some. (John Rieti/CBC)

Freyja Whitten, of Credit Valley Conservation, says her crews have been battling a 30-metre-by-30-metre thicket of knotweed since 2010, and it's still not dead despite being cut back, covered with a tarp and sprayed with a range of herbicides. 

In August and September, the knotweed, which has been spotted all across the city — Cirillo has seen it pushing through concrete in Parkdale alleys and popping up from the sandy waterfront — will look even nicer as it produces small white flowers.

She says anyone who spots them should try to at least remove the flowers, to keep the plant from spreading any further.

"Everyone needs to help out to control it," she said.

Birds, animals affected by weed's spread

Whitten says while the invasive species isn't dangerous for humans, it could severely disrupt the ecoystems where it digs in. Bugs don't eat the plant, she says, which means less food for birds. Meanwhile it also depletes the food available to deer, forcing them to look elsewhere for a meal.

"The whole food chain is impacted," Whitten said.

She says anyone who spots the knotweed should report it via an online map.

Whitten also has a plea for anyone currently growing it in the backyard. Don't bring it anywhere else. 


John Rieti

Senior producer

John started with CBC News in 2008 as a Peter Gzowski intern in Newfoundland, and holds a master of journalism degree from Toronto Metropolitan University. As a reporter, John has covered everything from the Blue Jays to Toronto city hall. He now leads a CBC Toronto digital team that has won multiple Radio Television Digital News Association awards for overall excellence in online reporting. You can reach him at