Giant Hogweed in Ontario

If you see Giant Hogweed, DO NOT TOUCH IT. Giant hogweed can be a serious health hazard. Its watery, clear sap contains photosensitizing compounds (furanocoumarins), which, when in contact with human skin and in combination with UV radiation, can cause burning. Content varies depending on plant part, but contact should be avoided at all times. The reaction of the skin depends on individual sensitivity. After 24hrs, reddening and swelling of the skin can be noticed, which is followed by an inflammatory reaction after 3 days. Depending on individual sensitivity, effects can last for months and skin can remain sensitive to UV light for years.

Figure 1.Giant Hogweed

Figure 2. Leaves of Giant Hogweed

Figure 3. Mature seed on Hogweed plant

Giant Hogweed is a short lived perennial weed that reproduces only by seed and is predominantly found along rivers, streams and wet land areas, but can also be found in pastureland. The key to long term management of Giant Hogweed is to limit seed production. Often people will attempt to cut the flowering head off in hopes of limiting seed production. However at this time of year, most flowering heads of giant hogweed have already set mature seed (seed Figure 3). Therefore cutting off seed heads now will only put you at risk of coming into contact with the clear watery sap while helping the plant spread its seed around. Preliminary research conducted in cooperation with OMAFRA, the University of Guelph and John Benham (County of Wellington) has shown that the optimal time to control Giant Hogweed is in late April to Early May since plants are small and easier to manage. A 2% glyphosate solutions spot applied to young plants can be effective, we have also had great success with a product called Milestone (aminopyralid). More information on this species can be found at www.ontarioweeds.com