WARNING: If you come in contact with this plant you could experience severe burns to your skin and risk temporary or permanent blindness.
Giant Hogweed is a non-native invasive plant that City of Hamilton Public
Works staff have found along Spencer Creek and roadways in Dundas and have observed some plants on private property in Dundas and Flamborough.
Reaching heights of up to 5 metres (15 feet), Giant Hogweed poses a serious threat to human health and the natural ecosystem.
Photo courtesy of Credit Valley Conservation
How to identify Giant Hogweed
Giant Hogweed, Heracleum mantegazzianum , is an invasive plant that looks similar to cow parsnip, but poses a serious threat to human health and natural ecosystems. The Giant Hogweed can reach an impressive height of 5 metres when flowering. The purple-spotted hollow green bristly stems can be 5 to 10 centimetres (2 to 4 inches) in diameter. Leaves are deeply lobed and toothed. The large blossom consists of numerous white flowers clustered in an umbrella-shaped head that is more than 120 centimetres across. Most plant parts are covered in stiff white hairs that are filled with sap.
The sap found in the Giant Hogweed stem and stem hairs causes phytophotodermatitis, a serious skin inflammation activated by exposure to the sun. Symptoms typically consist of painful blisters that form within 48 hours of exposure, and pigmented scars that can last up to 6 years.
Health Hazard
The plant's leaves and stem produce a watery sap containing a chemical, which makes human skin highly sensitive to the sun and other sources of ultraviolet light. Touching the plant or brushing against it is enough to cause quite serious injury. Exposed to sunlight, the sap will cause painful blistering within a few hours of contact. The healing process can be slow, and your skin may remain sensitive to sunlight for the rest of your life.
Contact can also produce permanent discolouration of the affected areas of skin and blindness can result from contact with the eyes. If any sap reaches your skin, you must wash it off immediately with soap and water while avoiding direct sunlight. You should also seek medical advice as quickly as possible.When the clear watery giant hogweed sap, which contains several photosensitizing furanocoumarins, contacts the human skin in conjunction with sunlight (specifically ultraviolet radiation), it can cause phytophotodermatitis. This is a serious skin inflammation that may lead to painful blisters that form within 48 hours and become dark pigmentation or scars that can last up to 6 years, though typically only last a few months. Long-term sensitivity to sunlight often occurs. Blindness may occur if the sap gets into the eye.
The skin reaction will depend on the sensitivity of the individual. Heat and moisture (sweat or dew) can enhance the skin reaction. The phototoxic reaction can be activated by ultraviolet radiation 15 minutes after contact, with a sensitivity peak between 30 minutes and two hours. The toxic furanocoumarins are present in all parts of the plant: the lower parts of the hollow stems and petioles may be partly filled with fluid; the hollow hairs on the plant also contain the sap.
If you are exposed to Giant Hogweed
- Wash the affected area immediately.
- Keep the affected area out of the sun.
- See a physician if you have a reaction.
For general questions call TeleHealth Ontario at 1-866-797-0000 or TTY 1-866-797-0007.
How to identify Giant Hogweed
Height: 1 to 5.5 metres (3 ft to 15 ft)
Leaves: Large, deeply cut with sharp coarse teeth, reaching widths in excess of 1 metre (3 ft).
Stems: Covered in coarse whisker-like hairs, with red-purple spots.
Flowers: White, umbrella shaped, up to 1.2 metres across (4 ft).
If you find Giant Hogweed on your property, please report it by calling 905-546-CITY (2489).
Removing these plants from your property
For your own safety, the City of Hamilton strongly encourages property owners to contact a licensed weed exterminator to remove Giant Hogweed from private property. To find a licensed weed exterminator, look under "Weed Control Service" in the Yellow Pages of your telephone directory.
DO NOT compost the plants. DO NOT place out for curbside collection either as Leaf & Yard Waste, Green Cart or waste material.
Giant Hogweed on trails and roadways
City of Hamilton Public Works staff have reported the discovery of Giant Hogweed along Spencer Creek and roadways in Dundas and have observed some plants on private property in Dundas and Flamborough.
Hamilton Public Works has received approval from the Ontario Ministry of the Environment to eradicate Giant Hogweed on road allowances and City maintained trails using a spray herbicide. Red signs warning of herbicide use will be posted in the application areas of Dundas.
If you find Giant Hogweed in area parks or on trails, please report it by calling 905-546-CITY (2489).