Giant hogweed (Heracleum mantegazzianum)


Giant Hogweed was first introduced to North America as a horticultural plant in the early 1900s1. Since then, it has escaped from cultivation and become widespread in northern US and southern Canada. Native to the Caucasus region between south-western Asia and Europe, it is now established in seven Canadian provinces, as well as many US states and European countries2. It is scattered across southern and central Ontario, and can be found within Ontario as north as Kapuskasing3. Giant hogweed is listed under the Noxious Weeds in Ontario list and is listed as a restricted species under the Invasive Species Act. Giant Hogweed is not currently abundant in the Credit River Watershed.


Giant Hogweed is a perennial plant with a long taproot, and a thick stock that can reach heights of up to 6 m when flowering. The stock is hollow and purple spotted, with coarse, white bristles. The leaves are alternate, compound, and deeply divided3. Giant Hogweed is typically found growing in rich, moist soils along ponds, streams, ditches, and forest edges1, 2. During its first year of growth it produces a large rosette of leaves. It flowers between its second and fifth year, after which the plant dies2. Small white flowers are produced in large flat-topped umbels at the top of the stem, usually between July and August. Once it has flowered, the plant dies, but not before producing up to 100,000 seeds, which are spread by wind, water, and human activity1.

Great angelica

Purple-stemmed Angelica with rounded flower clusters

Giant hogweed

Giant Hogweed plants in flower










There are a few other species that are commonly mistaken for Giant Hogweed, including Cow Parsnip (Heracleum lanatum), Wild Parsnip (Pastinaca sativa) and Great Angelica (Angelica atropurpurea). Cow Parsnip is native to Ontario. It has smaller flower heads and is a shorter plant, usually reaching up to 2 m in height. It may have some purple colouration on the stem, but the colouration will not be in distinct spots like Giant Hogweed1. Wild Parsnip is native to Europe. It has yellow flowers, is a shorter plant usually only reaching 1.5 meters in height, and has leaves that are divided into leaflets3. Great Angelica is native to Ontario and is a shorter plant usually only reaching 2 m in height, with a hairless stem. It has flower clusters with a rounded top containing small greenish-white flowers3. It blooms in late summer and has leaves that are divided into leaflets1.

Wild parsnip

Wild Parsnip leaves

Cow parsnip

Cow Parsnip, with green stem (without purple spots)

Ecological Threat

Ecologically, Giant Hogweed is a threat to native biodiversity. It is able to out-compete native species due to its vigorous growth in early spring1. Furthermore, it is tolerant of shade and seasonal flooding. Mature Giant Hogweed plants form a dense canopy that shades out native species2. Along stream banks, roots of Giant Hogweed do not hold the soil as well as native vegetation, which may lead to increased erosion and soil loss2.

In addition to its ecological threat, Giant Hogweed is toxic to humans. The sap contains furocoumarins, which can increase the skin’s sensitivity to sunlight, causing a condition called phytophotodermatitis. The sap is found in all parts of the plant. Transfer of the sap onto skin occurs by brushing against any broken part of the plant or by handling plant material. If the contacted area is exposed to sunlight, severe burns and blistering may result, with scarring lasting for up to six years1. Temporary or permanent blindness may also occur if the sap enters the eyes2. For this reason, removal of the plant is difficult and it is recommended that landowners seek professional assistance.

*CVC is interested in any locations of Giant Hogweed and is tracking the population within our watershed. If you suspect that you have found Giant Hogweed on your property, please contact CVC via email and report sightings of Giant Hogweed to the Ontario Federation for Anglers and Hunters Invasive Species Hotline by calling 1-800-563-7711 and add the sighting to the EDDMaps Ontario website.

Purple spots and hairs on giant hogweed stem

Purple spots and hairs on Giant Hogweed stem

Video: (Ontario invading species awareness program)

Links for Further Information:

Ontario Invasive Plant Council’s “Giant Hogweed (Heracleum mantegazzianum): Best Management Practices in Ontario”:

Ontario Invasive Plant Council’s “Giant Hogweed (Heracleum mantegazzianum) Best Management Practice Technical Document for Land Managers”:


  1. National Invasive Species Working Group. “Giant Hogweed: National Fact Sheet”. Web. 06 June 2012.
  2. Nielsen, C., H. P. Ravn, W. Nentwig, and M. Wade. 2005. The Giant Hogweed Best Practice Manual. Guidelines for the Management and Control of an Invasive Weed in Europe. Forest and Landscape Denmark, Hoersholm, 44 pp. Web. 15 August 2018.
  3. MacDonald, F. and H. Anderson. 2012. Giant Hogweed (Heracleum mantegazzianum): Best Management Practices in Ontario. Ontario Invasive Plant Council, Peterborough, ON. Web. 15 August 2018.



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