Conservation officials are warning about giant hogweed present in the watershed covering parts of Huron, Lambton, Perth, and Middlesex Counties.
The Ausable Bayfield Conservation Authority says it has received reports of the noxious plant on its properties, as well as others.
Giant hogweed releases a toxin that can cause serious burns and blisters when the substance is exposed to UV radiation from the sun.
In rare cases, blindness can occur.
If you come into contact with the sap from the giant hogweed, health officials recommend you wash all affected areas immediately with soap and water, keep affected areas out of direct sunlight, and seek medical advice immediately.