Prof. Francois Tardif stands close but does not touch giant hogweed.

It’s enormous and toxic and it’s creeping into cities across southwestern Ontario. It’s known as giant hogweed, and Prof. Francois Tardif is leading a research project to battle the plant monster.

For the past year, the plant agriculture professor has tested herbicides and organic products to see which is most effective at taking down the mammoth invasive weed.

Giant hogweed can grow as high as 15 feet tall, with white flowers up to four feet wide. The plant appears innocent enough until you touch its sap. Giant hogweed sap causes skin to become extremely sensitive to sunlight, a condition known as phytophotodermatitis.

“If you get the sap on you and then your skin is exposed to UV rays, the result can be severe burns and blistering,” said Tardif. “It can be quite gruesome and last for weeks and even months.”

Once rooted, the weed quickly multiplies, squeezing out other vegetation. The plant usually appears in April and blooms in June; cities are currently scrambling to get rid of it.

“Most municipalities are dealing with giant hogweed by manually going and digging the plants up,” he said. “The problem is, this can be very labour-intensive, and the workers risk being exposed to the toxic sap the plant produces. There’s also a chance the weed will still come up the following year, because the plant has roots that can be as thick as eight inches in diameter and are very hard to fully remove from the soil.”

Tardif is trying to find a more effective solution. At a research site near Milton, Ont., he and plant agriculture technician Peter Smith are testing currently registered herbicides as well as new products that kill the weed but leave surrounding growth unharmed.

“Other herbicides used on giant hogweed not only killed the plant but killed all the vegetation around it, which allows the seeds still in the soil to thrive and grow. One of the new herbicides we tested leaves the grass, which may help prevent the plant from returning.”

In tests last summer, this herbicide killed the weed. So far, it hasn’t returned.

This year, the researchers are investigating the effectiveness of injecting the herbicide glyphosate into the plant stem rather than spraying the area. One benefit of this approach is that it allows access to plants growing close to water.

They are also testing organic treatments. So far, these products appear to be less effective than conventional herbicides, said Tardif. “They can control the top growth, but the hogweed can regrow from the roots.”

Originally from the Caucasus and central Asia, giant hogweed was introduced into Western Europe in the late 19th century as an ornamental plant. It arrived in Canada in the 1950s and has spread ever since.

“It’s become quite widespread, and people have become more aware of it over the past 10 years.”

Tardif estimates there are at least 100 giant hogweed sites across southwestern Ontario. The plant likes shaded areas near water and is often found along park trails and recreational areas. It has also popped up in people’s backyards.

Tardif hopes to soon identify the best weapons for killing off the towering beast. But he said we also need to investigate how to keep the weed away for good. “We need to start looking into whether it’s possible to manipulate the microenvironment so that the weed doesn’t come back again.”