Giant hogweed (also known as giant cow parsnip) is a garden ornamental, originating from southwestern Asia that is naturalizing in North America, and becoming more common in southern Ontario. It is a very large perennial member of the parsley family that grows up to 5m (16ft) in height, with a purple-spotted, hollow green bristly stem. It has dark green, coarsely toothed leaves that can exceed 2.5m (8ft) in length at the base of the plant. It also has large, umbrella shaped clusters, called umbels, (up to 0.75m across) of small white flowers. A single plant can produce over 100,000 seeds. The seeds can remain viable in the soil for up to seven years. Giant hogweed looks very similar to our native plant species cow parsnip and Angelica. The best way to tell the difference is by size (both native species are much smaller), and the purple splotches or spots on the stem of giant hogweed. Giant hogweed’s clear, watery sap contains toxins that cause photodermatitis (skin becomes extremely sensitive to sunlight). Skin contact, followed by exposure to sunlight, produces painful, burning blisters that can cause severe irritation, dermatitis, and develop into purplish or blackened scars. It can also cause temporary or permanent blindness following eye contact. If you accidentally come in contact with giant hogweed sap, wash the area immediately with soap and water, avoid sunlight exposure to that area, and seek medical attention.
Control Recommendations
Control methods involve cutting and proper disposal of the plant (taking care to avoid contact with the sap) or the use of herbicide. Use EXTREME CARE when attempting to control this plant. It is suggested that anyone working to control giant hogweed wear eye protection, waterproof gloves, and protective clothing (long sleeves, long pants, closed-toe shoes). A weedwhacker should never be used to control giant hogweed, as the sap may splatter when the stems are cut. A long sharp serrated knife is the best tool to use. It should be noted that mowing does not control established giant hogweed; it simply causes resprouts to appear from the rootstalk. However, mowing may be effective in controlling seedlings, which will sprout after the removal of mature plants.
When controlling giant hogweed, ensure you have several trash bags ready for disposing of the plant; the use of clear bags and "solarization" (leaving the bagged plants in direct sunlight for a week or more) are suggested. First, the flower heads should be removed and placed carefully in the trash bags, making sure not to drop any seeds. The bags should be sealed tightly and left to "solarize" to destroy the seed viability. If the infestation is small, the stems can be dug out (roots and all) and dried thoroughly before disposing them. Another control option for smaller infestations is to cut the stems at ground level and cover them with black plastic to keep them from regrowing, and to discourage new growth. The site should be monitored to ensure that the seedlings are not poking through the plastic. Infestations may require several years of follow-up control to exhaust the seed bank
Credit: WorkSafeBC
How does Giant Hogweed Affect you?
The blister like areas on stems and stalks leak a clear watery sap containing furocoumarins (psoralens), which sensitize skin to ultraviolet radiation (sunlight). Exposure to sunlight can cause severe burns that usually result in blistering and painful dermatitis. Inflamed linear lesions on the skin can sometimes appear where the person has brushed against the plant. Blisters occurring from contact with hogweed sap can develop into purplish or blackened scars sometimes up to 48 hours after exposure. Contact can lead to recurring dermatitis. Contact with the eyes can lead to temporary or possibly permanent blindness.
If you become exposed
Wash the affected areas immediately with soap and water if available
Keep affected areas out of direct sunlight
Seek medical advice as soon as possible
Giant Hogweed (Heracleum mantegazzianum) Mature plant (photo: John Benham)
Giant Hogweed (Heracleum mantegazzianum) Green stem, Flat-topped umbel, spiky leaves
Giant hogweed (also known as giant cow parsnip) is a garden ornamental, originating from southwestern Asia that is naturalizing in North America, and becoming more common in southern Ontario. It is a very large perennial member of the parsley family that grows up to 5m (16ft) in height, with a purple-spotted, hollow green bristly stem. It has dark green, coarsely toothed leaves that can exceed 2.5m (8ft) in length at the base of the plant. It also has large, umbrella shaped clusters, called umbels, (up to 0.75m across) of small white flowers. A single plant can produce over 100,000 seeds. The seeds can remain viable in the soil for up to seven years. Giant hogweed looks very similar to our native plant species cow parsnip and Angelica. The best way to tell the difference is by size (both native species are much smaller), and the purple splotches or spots on the stem of giant hogweed. Giant hogweed’s clear, watery sap contains toxins that cause photodermatitis (skin becomes extremely sensitive to sunlight). Skin contact, followed by exposure to sunlight, produces painful, burning blisters that can cause severe irritation, dermatitis, and develop into purplish or blackened scars. It can also cause temporary or permanent blindness following eye contact. If you accidentally come in contact with giant hogweed sap, wash the area immediately with soap and water, avoid sunlight exposure to that area, and seek medical attention.
