A highly invasive and hazardous weed is rapidly spreading throughout southwestern Ontario, leading a local conservation authority to issue a warning and intensifying efforts to eliminate the plant from affected areas.
Heracleum mantegazzianum, more widely recognized as giant hogweed, is a monocarpic perennial herb belonging to the carrot family, Apiaceae. It is also referred to by several other names, including cartwheel-flower, giant cow parsley, giant cow parsnip, and hogsbane.
The sap of giant hogweed is highly phototoxic and can trigger severe skin inflammation known as phytophotodermatitis when exposed to sunlight. This reaction, which can lead to painful blisters and long-lasting scars, is caused by furanocoumarin compounds found throughout the plant—including its leaves, roots, stems, flowers, and seeds. As a result, many regions classify giant hogweed as a noxious weed.
In the 20th century, giant hogweed was introduced to the United States and Canada for ornamental use in Victorian-style gardens and arboretums. The earliest known planting in North America occurred in 1917 in garden areas near Highland Park in Rochester, New York.
By the 1950s, the plant had made its way into southern Ontario, and over the next 25 years, it became well-established in the province. It was first documented in Nova Scotia in 1980 and in Quebec by 1990. Remarkably, some Canadian nurseries were still selling giant hogweed as recently as 2005.
In Canada, giant hogweed is found in nearly all provinces, with the exception of the prairie regions. Its presence in Quebec has been documented since the early 1990s. In Ontario, the plant initially spread through the southwestern region and, by 2010, was observed in the Greater Toronto Area as well as Renfrew County, near Ottawa.
Giant hogweed’s sap is phototoxic, meaning it can cause serious skin irritation when exposed to sunlight. This reaction is due to furanocoumarins, chemical compounds found in the plant’s sap. Contact with the sap can trigger phytophotodermatitis—a painful inflammation of the skin—within just 15 minutes. Sensitivity to sunlight usually peaks between 30 minutes and two hours after exposure but may persist for several days.
Health officials strongly recommend avoiding any contact with the plant, particularly for children. When removal or handling is necessary, individuals should wear protective gear, including gloves and eye protection. If sap touches the skin, the area should be thoroughly washed with soap and cold water, and direct sunlight should be avoided for at least 48 hours.
If sap from giant hogweed gets into your eye, rinse it thoroughly with water right away and seek medical attention without delay.
Other species within the Heracleum genus, such as H. maximum, can also cause similar skin reactions, so the same precautions are advised. Because of its resemblance to other, less harmful plants in the same genus, giant hogweed is sometimes misidentified.
If giant hogweed is found on your property, it’s strongly advised to contact a professional removal service. Experts can safely eliminate the plant while minimizing the risk of seed dispersal. Managing a significant infestation typically requires a long-term effort. The ideal time for removal is in late April or early May, when the plant is still under 30 centimeters tall. At this stage, it is easier to uproot and more responsive to herbicide treatments. Additionally, the cooler spring weather makes wearing the necessary protective gear more manageable.
If you suspect giant hogweed is growing on your property or spot it in your area, contact the Invading Species Hotline at 1-800-563-7711 or submit a report online. You may be asked to provide photos to help confirm the plant’s identity. Avoid handling or collecting any part of the plant for identification purposes.
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