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 similar-looking 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 hard-pressed 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.

"We also want to make people aware that they may have a plant in their garden which at some point could spread into a natural area and impact on biodiversity... or have public health impacts," said Kraus.

"Ideally you want to keep invasive plants out, but the next best thing is to detect them early and to remove them before they take over large areas."

Kraus said Canadians who spot giant hogweed should contact local parks officials.