Beware of giant hogweed, officials say as battle to kill dangerous weed drags on

One of Ontario's most dangerous invasive weeds is spreading across southwestern Ontario, prompting a warning from a local conservation authority and extending an on-the-ground battle to destroy it.

The Upper Thames River Conservation Authority (UTRCA) said it's spent more than 120 hours so far this year fighting the spread of giant hogweed — a large invasive plant that has a sap known to cause reactions ranging from mild to severe.

"It ranges from a mild rash on your skin to heavy purple blotches and really severe blistering. In the most severe cases we've heard about ... it can cause temporary blindness," said Brandon Williamson, the land management coordinator for the UTRCA.

Officials have been patrolling the upper Thames watershed, looking for the telltale signs of the dangerous weed and seeking to destroy it. The plant has been in the area for roughly 15 years, but its coverage is expanding.

According to Williamson, giant hogweed is hard to miss.

In its first phase of growth, it has a large, thick stalk with coarse hairs and purple blotching, "like somebody dipped a paintbrush in a can of purple paint and splattered it on the stem," Williamson said.

The plant grows large sugar maple-like leaves.

In its second growth phase, a flower blooms that appear similar to a Queen Anne's lace, Williamson said. A mature plant can grow more than five metres tall.

"That massive plant puts out [as many as] 50,000 seeds annually, depending on the size."

Anyone who touches the plant is advised to immediately wash their hands and monitor the body part that touched the plant. Williamson said since the poisonous sap reacts to sunlight, it's best to stay out of the sun.

"Wear long pants and long sleeves if you're going to be going to areas where the plant appears, especially if you're going to be wading through long vegetation and grasses."

Williamson said the weed is most commonly found along waterways, making anglers, kayakers and canoeists especially prone to running into it, and that hikers also need to watch out for the plant.

That's something the Thames Valley Trail Association (TVTA) knows all too well. Volunteers with the association that maintains over 100 kilometres of trail in the London area have been having their own battle with giant hogweed for years.

"It's been progressing, I would say, in the last five years for sure," said Tilman Joosten, a TVTA board member. "It was in the watershed, north of our trail, and it's slowly working its way down."

Joosten said the trails that run near Highway 7 in St. Mary's are a particular problem spot. For their part, Joosten said the TVTA has been in close contact with conservation officials and has spent hours spraying herbicide on giant hogweed plants along the sides of their trails to keep the weed at bay.

The TVTA also works to educate members and new hikers about the dangers of the plant and is planning to post signs at trailheads of known problem spots.

It's also possible for giant hogweed to appear on private property in the City of London, and property owners are required to dispose of it.

"Whenever we hear about a giant hogweed, we do take that as a priority concern," said Orest Katolyk, London's top bylaw official.

If a property owner fails to remove the plant, city officials will do it themselves and charge the property owner for the service.

To remove any prohibited plant from your property, Emily Williamson, London's environmental planning manager, suggests knowing what plant you're dealing with.

"In terms of giant hogweed, ensure that you're wearing gloves and a long sleeve shirt," she said.

"It's important that all of the plant material is bagged and that, ideally, any of this removal is done before the plant goes into flower to ensure no seeds are spread into the surrounding area."