Giant Hogweed Threat to Human Health

The Upper Thames River Conservation Authority and Lower Thames Valley Conservation
Authority are warning watershed residents to avoid contact with Giant Hogweed. This nonnative,
invasive plant is spreading very quickly along rivers and streams.

Giant Hogweed is a serious health hazard for humans. Its clear, watery sap contains toxins that
can cause severe inflammation of the skin. You can get severe burns if you get the sap on your
skin and the skin is then exposed to sunlight, as UV radiation activates compounds in the sap.

“If you find what you think is Giant Hogweed, do not go near or touch the plant under any circumstances,” advised Brandon Williamson, Land Management Technician for the Upper Thames River Conservation Authority. “The issue is people getting sap on themselves by touching or breaking the stems, stepping on the plant, or even brushing against the leaves.”

Symptoms occur within 48 hours and consist of painful, burning blisters that can cause severe
irritation, dermatitis, and develop into purplish or blackened scars. Depending on individual
sensitivity, effects can last for months and skin can remain sensitive to UV light for years. Eye
contact with the sap has been reported to cause temporary or permanent blindness.

The Giant Hogweed outbreak began several years ago along the North Thames River and has
been moving rapidly down the river valley.

“Hogweed grows along streams, ditches and roadsides, and is invading old fields and even
woods,” said Williamson. “If you’re out for a hike or canoeing on the river, you need to watch
out for it.”

Description of Giant Hogweed

Giant Hogweed can be much taller than a grown person, reaching up to 5 m (16 ft.) in height. It
has large, flat-topped to slightly dome-shaped white flowers and seed heads and bumpy or bristly
stems. The huge leaves are 1-1.5 m (3-5 ft) wide, and shaped like an exaggerated maple leaf.
For more information please go to and click on the Giant Hogweed picture to download a factsheet.

If you find what you believe is Giant Hogweed on public property, please contact the
– local conservation authority
– local municipality/weed inspector

In the Lower Thames watershed, if you find what you believe is Giant Hogweed on private property, please contact:

Ontario’s Invading Species Awareness Program: giant-hogweed/ to track the location of Giant Hogweed in the watershed and for assistance with plant identification and treatment options. Or, alternatively, contact the Lower Thames Valley Conservation Authority (LTVCA) at 519-354-7310 or email .

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  1. Is there any around st T and if so where.

  2. Wow crazy stuff.

  3. “still they are; immune to your herbicidal battering.”

    (The Return of the Giant Hogweed”, 1970, Genesis – Peter Gabriel, Steve Hackett, Tony Banks, Michael Rutherford, Phil Collins. )