County council declares giant hogweed to be a noxious weed

It’s formal name is Heracleum mantegaz­zianum.

It is also known as Giant Cow-parsley. Giant Hogweed is native to the Caucasus region and Cen­tral Asia. It may reach up to 12 to 15 tall, and, in some cases, even higher.

And county council declar­ed it a noxious weed at the council meeting on June 26.

That step came, according to planner Gary Cousins, after Well­ington-Dufferin-Guelph Pub­lic Health sent a letter to county officials warning that the plant has spread into Well­ington. It can be found throughout southern Ontario. Iso­lated plants have even been reported as far north as Hali­burton County. Giant hogweed can grow in a wide variety of habitats, however it is most often found along roadsides, streams and riverbanks.

Planning committee chair­man Walter Trachsel said, “Ap­parently, it’s a very dangerous weed.”

Cousins said it is dan­gerous to the touch, and the Ontario Chief Weed Inspector believes it is appropriate for Well­ington County to declare it a noxious weed.

Information from the inter­net stated that except for its size, it closely resembles com­mon hogweed or Garden Angelica.

It is further distinguished by a stout, dark reddish-purple stem and spotted leaf stalks that are hollow and produce sturdy bristles. Stems vary from 3 to 8cm in diameter, occasionally up to 10cm. The stem shows a purplish-red pigmentation with raised nodules. Each purple spot on the stem surrounds a hair, and there are large, coarse white hairs at the base of the leaf stalk. The plant has deeply incised compound leaves that grow up to 1 to 1.7 metres in width.

Giant Hogweed is a photo­toxic plant. Its sap can cause phytophotodermatitis (severe skin inflammations) when the skin is exposed to sunlight or to ultra-violet rays. Initially the skin colours red and starts itch­ing. Then blisters form as in burns within 48 hours. They form black or purplish scars, that can last several years. Hospitalization may become necessary for victims. Presence of tiny amounts of the sap in the eyes, can lead to temporary or even permanent blindness.

Those reactions are caused by the presence of linear deri­vatives of furocoumarin in its leaves, roots, stems, flowers, and seeds. Those chemicals can get into the nucleus of the epithelial cells, forming a bond with the DNA, causing the cells to die. The brown colour is caused by the production of melanin by furocoumarins. In Germany, where the plant has become a real nuisance, there were about 16,000 victims in 2003.

Giant Hogweed is a peren­nial. It flowers from late spring to mid summer, with numerous white flowers clustered in an umbrella-shaped head that is up to 2.5 feet in diameter across its flat top. Shoots die down in the fall. Tall stems mark its locations during winter.

The plant came to Great Britain in the 19th century, mainly for ornamental reasons. It has spread from there to Eur­ope and North America.

It is particularly a problem along riverbanks. By forming dense stands they can displace native plants and reduce wild­life interests. In Canada, it is is southern and Western Ontario.

Keep kids away

Children should be kept away from Giant Hogweed, and protective clothing (includ­ing eye protection) should be worn when handling it or digging it. If skin is exposed, the affected area should be washed thoroughly with soap and water and the exposed skin protected from the sun for several days.

The Ontario Federation of Ang­lers and Hunters has posted information about invasive species. It states that for effec­tive control, the plant should not be mowed, but rather spray­ed with an herbicide at inter­vals until it is gone. Even after the parent plant is completely removed, the numerous seeds left behind can germinate seven to 15 years later, and ongoing monitoring is required.

Herbicides such as 2,4-D, TBA, MCPA and dicamba will kill above ground parts but are reportedly not particularly ef­fec­tive on persistent rootstalks. Roundup is considered the most effective herbicide but it is nonselective.


Those exposed to giant hogweed should:

– wash the affected areas immediately, with soap and water if available;

– keep affected areas out of direct sunlight;

– seek medical advice as soon as possible.

Council was unanimous in naming the weed noxious.