Beware of giant hogweed, officials say as battle to kill dangerous
weed drags on
Alessio Donnini
| CBC News | Posted: June 8, 2025 9:00 AM | Last Updated: 2
hours ago
A local trail association says the invasive plant is spreading
Image | hogweed
Caption: Hogweed is prohibited on private properties in most
municipalities. (Getty Images/iStockphoto)
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One of Ontario's most dangerous invasive weeds is spreading
across southwestern Ontario, prompting a warning from a local
conservation authority and extending an on-the-ground battle to
destroy it.
The Upper Thames River Conservation Authority (UTRCA) said it's
spent more than 120 hours so far this year fighting the spread
of giant hogweed — a large invasive plant that has a sap known
to cause reactions ranging from mild to severe.
"It ranges from a mild rash on your skin to heavy purple
blotches and really severe blistering. In the most severe cases
we've heard about ... it can cause temporary blindness," said
Brandon Williamson, the land management coordinator for the
UTRCA.
Officials have been patrolling the upper Thames watershed,
looking for the telltale signs of the dangerous weed and
seeking to destroy it. The plant has been in the area for
roughly 15 years, but its coverage is expanding.
What to watch out for (and avoid)
According to Williamson, giant hogweed is hard to miss.
In its first phase of growth, it has a large, thick stalk with
coarse hairs and purple blotching, "like somebody dipped a
paintbrush in a can of purple paint and splattered it on the
stem," Williamson said.
The plant grows large sugar maple-like leaves.
Image | Giant hogweed leaves
Caption: Giant hogweed leaves. (Brittany Spencer/CBC)
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In its second growth phase, a flower blooms that appear similar
to a Queen Anne's lace, Williamson said. A mature plant can
grow more than five metres tall.
"That massive plant puts out [as many as] 50,000 seeds
annually, depending on the size."
Image | Hogweed
Caption: A giant hogweed plant displaying its telltale flowers
and thick stalk. (Submitted by Ontario Environment and Energy
Ministry)
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Anyone who touches the plant is advised to immediately wash
their hands and monitor the body part that touched the plant.
Williamson said since the poisonous sap reacts to sunlight,
it's best to stay out of the sun.
"Wear long pants and long sleeves if you're going to be going
to areas where the plant appears, especially if you're going to
be wading through long vegetation and grasses."
Williamson said the weed is most commonly found along
waterways, making anglers, kayakers and canoeists especially
prone to running into it, and that hikers also need to watch
out for the plant.
That's something the Thames Valley Trail Association (TVTA)
knows all too well. Volunteers with the association that
maintains over 100 kilometres of trail in the London area have
been having their own battle with giant hogweed for years.
"It's been progressing, I would say, in the last five years for
sure," said Tilman Joosten, a TVTA board member. "It was in the
watershed, north of our trail, and it's slowly working its way
down."
Joosten said the trails that run near Highway 7 in St. Mary's
are a particular problem spot. For their part, Joosten said the
TVTA has been in close contact with conservation officials and
has spent hours spraying herbicide on giant hogweed plants
along the sides of their trails to keep the weed at bay.
The TVTA also works to educate members and new hikers about the
dangers of the plant and is planning to post signs at
trailheads of known problem spots.
Image | Giant hogweed
Caption: Two people in Prince Edward Island remove giant
hogweed while wearing protective equipment. The invasive plant
is found across Canada. (Beth Hoar/P.E.I. Invasive Species
Council)
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Property owners responsible, too
It's also possible for giant hogweed to appear on private
property in the City of London, and property owners are
required to dispose of it.
"Whenever we hear about a giant hogweed, we do take that as a
priority concern," said Orest Katolyk, London's top bylaw
official.
If a property owner fails to remove the plant, city officials
will do it themselves and charge the property owner for the
service.
To remove any prohibited plant from your property, Emily
Williamson, London's environmental planning manager, suggests
knowing what plant you're dealing with.
"In terms of giant hogweed, ensure that you're wearing gloves
and a long sleeve shirt," she said.
"It's important that all of the plant material is bagged and
that, ideally, any of this removal is done before the plant
goes into flower to ensure no seeds are spread into the
surrounding area."
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