How to - Gardening Resources

Beware of Giant Hogweed

Caution! This poisonous plant could make an appearance in a garden near you

Giant hogweed has been surreptitiously invading Canada since the late 1940s. But in recent years, this dangerous plant’s secret identity has been blown as it has encroached on public parks and gardens. Lately, with more sightings heralding the troubling realization that Giant Hogweed is spreading, it has become a minor news celebrity with warnings to the public to stay away from this poisonous plant.

Municipalities across Canada are paying close attention to the situation. “Ministry staff is aware of the significance of Giant Hogweed from a human health and invasiveness perspective,” explains Mike Cowbrough, weed management field crops program lead with the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA). “OMAFRA is working with the University of Guelph and several municipalities across Ontario to identify best management practices for Giant Hogweed.” Other municipalities, like the Halifax Regional Municipality in Nova Scotia, have set up hotlines to report weed locations.

What is it?
Giant Hogweed (Heracleum mantegazzianum) is a member of the carrot and parsley family and is often mistaken for cow parsnip. Its flowers also closely resemble those of prolific wildflower Queen Anne’s Lace. As its name indicates, Giant Hogweed grows to impressive heights and can reach 15 to 20 feet. A perennial with tuberous roots, the dark reddish-purple stalks and stems of the plant are hollow and quite thick (two to four inches in diameter). Hogweed has large, flat-topped clusters of leaves with white flowers and large, flat fruit. Leaf blades of rosette leaves are very large and deeply incised–the compound leaves can span up to five feet!

Where did it come from?
Hogweed is an extremely invasive species originally from Asia and Eastern Europe. It’s still unclear how the plant found its way into Canada or how long it’s been here. Some believe it was introduced to gardens as an ornamental plant, or the seeds could have been brought by migratory birds or cargo ships. Today, it can be found along roadsides, riverbanks, naturalized areas, and yes, even your own backyard.

Why is it such a problem?
Giant hogweed can pose a serious health hazard for humans. The plant’s watery, clear sap contains photosensitizing compounds called furanocoumarins. When the sap comes into contact with human skin and is then exposed to sunlight, the UV radiation can cause severe burning and weeping blisters. The reaction of the skin depends on the sensitivity of the individual, as well as the amount of sap he or she has been exposed to. After 24 hours, swelling and reddening of the skin will be noticeable. Within two to three days the swelling will develop into painful blisters. Symptoms can last for several months and the skin may remain sensitive to UV light for years after exposure. It can also cause extensive scarring.


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