The City of Guelph has issued a public warning to residents, telling people to keep pets and children away from giant hogweed, an invasive, noxious weed with a sap that can cause painful blisters, even blindness, if it comes in contact with the eyes.

1. How to identify giant hogweed:

Giant hogweed certainly lives up to its name, growing 3 to 4 metres in height and when in bloom, carries numerous small, white flowers that form an umbrella shape.

It looks like a colossal version of Queen Anne's lace, or wild carrot and is often mistaken for angelica or cow parsnip because of its similar leaf pattern.

Giant hogweed is most commonly recognized by its flowering stalks, the plant blooms in mid-May through July.  

2. Where does giant hogweed grow?

Giant hogweed grows throughout the Grand River watershed and tends to grow in undisturbed sites with abundant light, but it also grows on the forest's edge or partially shaded areas.


Giant hogweed is commonly mistaken for Queen Anne's lace or wild carrot, but the invasive weed is far larger, growing to 3 to 4 metres in height. (Wikimedia Commons)

The plant is a perennial, meaning it returns every spring. It is originally from Asia and was brought to Ontario as an ornamental plant.

Since arriving in Ontario, the plant has spread quickly, taking advantage of local waterways, including those in the Grand River watershed, to carry its particularly buoyant seeds.

3. How common is giant hogweed?

Experts say the plant is becoming increasingly common throughout Ontario, growing at the edges of forest, rivers and even backyards, but it is difficult to say how widespread the weed has become.


Giant hogweed can be identified by its purplish, hollow stalk, with small spines sticking out of it. The plant also carries a sap that can blister the skin and eyes when it reacts with light. (Wikimedia Commons)

"We're certainly seeing an increase," Cam Linwood, spokesman for the Grand River Conservation Authority said.

"Whether people are becoming more aware of the plant, or whether it is actually getting more of a stronghold along some of the river banks," he said.  

4. What to do if you spot giant hogweed:

If you find the plant in your backyard, the City of Guelph recommends removing it. The weed is a public health hazard and is dangerous to both children and pets.

The city advises people to wear protective clothing, including gloves and eye protection to avoid getting the sap on their skin or in their eyes.

People are told to remove the flowering head in order to stop the spread of the plant's seeds and sever the plant's roots, 8 to 12 centimetres below the surface of the soil.

When disposing of the plant, the City of Guelph recommends not mixing it with regular compost and instead sealing what's left of the weed in a double-bag.

The city warns that any seeds left behind can germinate for up to 15 years after the parent plant has been killed.

5. How to treat giant hogweed burns:

Giant hogweed, if encountered in the wild, should be left alone, according to Cam Linwood, with the GRCA.

"Stay away from it," he said. "The sap of the giant hogweed plant is photoreactive, so it does react with the sunlight. It can cause an intense burn right away or an intense blistering. Also, if you get it in your eyes, it can cause blindness temporarily or permanent."

If your skin comes in contact with the weed, the City of Guelph recommends washing any affected areas immediately with soap and water.

Authorities also advise keeping any affected areas of skin out of direct sunlight and seeking medical advice as soon as possible.