Burns and blind­ness: ‘Very nasty’ gi­ant hog­weed spread­ing in Canada


It can cause third-de­gree burns and even per­ma­nent blind­ness — and it’s spread­ing.

Gi­ant hog­weed is cut­ting a wider swath in B.C. and On­tario, and the Na­ture Con­ser­vancy of Canada is urg­ing peo­ple across the coun­try to doc­u­ment sight­ings of the tow­er­ing, three-me­tre green plant with large um­bels of white flow­ers.

Dan Kraus, a bi­ol­o­gist with the con­ser­vancy, said the in­va­sive Asian species likely ar­rived in Canada in the 1940s and can now be found in ar­eas of the At­lantic provinces and Que­bec, and has been spread­ing in south­ern On­tario and south­ern B.C.

“No­body’s re­ally sure when it ar­rived here. It was prob­a­bly in­tro­duced as an or­na­men­tal plant and it is start­ing to slowly spread,” said Kraus from Guelph, Ont.

“It’s pos­si­ble peo­ple are mov­ing it from gar­den to gar­den. They see it in their aunt’s gar­den and they think it’s this won­der­ful plant, and they’re col­lect­ing seeds and mov­ing it to an­other lo­ca­tion, which is some­thing we def­i­nitely don’t want peo­ple to do.”

In 2015, five chil­dren in Eng­land were re­port­edly burned in two sep­a­rate in­ci­dents af­ter com­ing into con­tact with gi­ant hog­weed in pub­lic parks.

Often mis­taken for the sim­i­lar­look­ing cow parsnip, it can be seen grow­ing in gar­dens, along road­sides, in ditches and on the shores of rivers and streams. Its clear sap can cause blis­ter­ing third-de­gree burns and even per­ma­nent blind­ness if it touches the body and is then ex­posed to the sun, through a pho­to­toxic re­ac­tion.

“It’s very nasty. It can cause huge wa­ter blis­ters — al­most like boils — that erupt on your skin,” said Todd Boland, a re­search hor­ti­cul­tur­ist at Me­mo­rial Univer­sity’s Botan­i­cal Gar­den in St. John’s, N.L.

“It may be the next day be­fore you start to see the ef­fects. That’s the funny thing about this. It’s not like it’s an in­stant thing. It takes awhile and you have to have re­peated ex­po­sure to the sun.”

But sim­ply touch­ing the plant is not dan­ger­ous, Boland stressed. It’s the sap that is prob­lem­atic and wash­ing your body and clothes af­ter ex­po­sure can pre­vent the pho­to­toxic re­ac­tion.

“If you get it in your eye, it can lead to per­ma­nent blind­ness, but that’s pretty rare. You’d be hard­pressed to get it in your eye un­less you were rolling around in the plant,” said Boland, adding that gi­ant hog­weed can be found in the St. John’s area.

The plant has prompted com­mu­ni­ties across Canada to is­sue warn­ings to res­i­dents in re­cent years.

Guelph, Ont., has been deal­ing with gi­ant hog­weed for about two years and although it is now con­tained in two lo­ca­tions, erad­i­cat­ing the plant has proven dif­fi­cult.

“In 2015 we re­moved some plants from one lo­ca­tion and the next year we re­turned to the site and there were no plants, but this year we re­turned to find plants,” said Timea Filer, an ur­ban forester with the city. “So there ap­pears to be a seed bank and we’ll have to mon­i­tor it con­tin­u­ally.”

Kraus said there is also a con­cern about a loss of na­tive bio­di­ver­sity, as gi­ant hog­weed is an ag­gres­sive plant that can out­com­pete na­tive plants and spread — es­pe­cially when it grows near wa­ter­ways and its seeds are car­ried down­stream. One plant can pro­duce thou­sands of seeds and they can stay in the ground for years be­fore ger­mi­nat­ing.

The con­ser­vancy is ask­ing peo­ple to doc­u­ment sight­ings of the in­va­sive plant through apps such as iNat­u­ral­ist, which helps sci­en­tists un­der­stand how the plants are spread­ing and iden­ti­fies ar­eas in which they need to be erad­i­cated, he said.


A patch of gi­ant hog­weed in Terra Cotta, Ont., is seen in 2009. The plant can cause third de­gree burns and even per­ma­nent blind­ness — and it’s spread­ing.

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