Written by: Brenda Van Ryswyck, Natural Heritage Ecologist
There are many invasive species that can threaten the native plants in your garden, but probably one of the most well-known is Giant Hogweed (Heracleum mantegazzianum). This fame is due to its tremendous height and the dangers from its sap which has attracted media attention over the past few summers.
Giant Hogweed has two major negative impacts. Firstly, due to its invasive nature, it poses a threat to native biodiversity. Secondly, Giant Hogweed is a public health hazard. It produces a sap that sensitizes the skin to ultraviolet light so when that skin is then exposed to sunlight it can result in severe and painful burning and blistering. This is known as photosensitivity, which is not an immediate burning sensation but is like getting a very severe sunburn. It is important to avoid any skin contact with the sap of this plant. If you come across this plant, you are best to stay away from it. If you have been exposed to the sap wash the area immediately and do NOT expose it to sunlight for at least 48 hours! (Contact a health professional if you do develop blistering).
What is Giant Hogweed?
Giant Hogweed is known for its enormous size, reaching heights of 2.5 to 4m (8 to 14ft), with leaves up to 1m (3 ft) in breadth. It has a thick, 5 to 10cm (2 to 4in) hollow stem. Its stem and the undersides of its leaves are covered in coarse hairs. Its large, umbrella-shaped flowers are white in colour and can be more than 30cm (1ft) in diameter.
Halton Region is home to a number of native plants that resemble Giant Hogweed but most do not pose the same health risk. The following are look-alikes in the area:
- Cow Parsnip (Heracleum maximum)
- Purple-stemmed Angelica (Angelica atropurpurea)
- Spotted Waterhemlock (Cicuta maculata)
- Poison Hemlock (Conium maculatum)
Wild Parsnip (Pastinaca sativa) does pose the same risk as Giant Hogweed, but is much easier to overlook.
The University of Pennsylvania has published an informative pamphlet outlining the characteristics of Giant Hogweed and these look-alike plants. When in doubt, always conﬁrm plant identiﬁcation with an expert.
The OMAFRA (Ontario Ministry of Agriculture, Food and Rural Affairs) also has some good informative identification and removal videos: http://www.omafra.gov.on.ca/english/crops/facts/ontweeds/giant_hogweed.htm
What if I see Giant Hogweed (or think I see it)?
If you see Giant Hogweed, we at Conservation Halton want to know. We are tracking the location of this noxious weed as part of our environmental monitoring. To report a sighting of Giant Hogweed please complete an invasive species reporting form, or send an e-mail to me, or call me at 905-336-1158 ext. 2282.
Okay so how do I get rid of it?
At this time of the year Giant Hogweed is generally at its full height, and is a giant plant with big white flowers. It is easy to spot, but it is not necessarily the ideal time to try and control it. In fact, you could be putting yourself at significant risk from the sap in the plant’s stem and leaves. In the spring, when the plants are just starting to grow, is the best time to try and control it, and prevent it from producing seeds.