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tdc's FarmGate
- Giant Hogweed
in Eastern Ontario

Giant hogweed (Heracleum mantegazzianum) is an invasive plant species that has gone viral in more ways than one. It looks like an overgrown Queen Anne's Lace, reaching up to 15 feet tall, but it can cause phytophotodermatitis, a chemical reaction on skin due to sun exposure.

Why is it a problem?

Giant Hogweed presents a serious public health threat, the stems and leaves are covered with small hairs coated with poisonous sap. Even the slightest touch can cause painful blisters and severe skin irritation. The symptoms can take a couple of days to develop, the skin becomes highly photosensitive producing large watery blisters however the effects can be long lasting, with contact resulting in recurrent dermatitis. This is a particular concern for small children who may use the stems as pea-shooters or pretend telescopes resulting in horrific injuries.

It is a threat to biodiversity out competing native species and monopolising local environments. On riverbanks once it has died back in the winter the area is vulnerable to erosion.

Hogweed Burns

Coming into contact with the sap of giant hogweed, followed by exposure to sunlight, can produce painful, burning blisters, also known as photo-dermatitis. Hogweed stems contain a large amount of juice that squirts out when stems are broken or cut. Contact with the toxic sap usually happens when people cut down hogweed plants without taking precautions.

Hogweed Burns on Arms

Hogweed Burns on Ankle

Children are attracted to the large, hollow stems for pretend swords or telescopes. Also, children may run through hogweed patches and brush up against broken stems. Both activities can cause children to be burned by hogweed sap.

When removing hogweed or working near it, wear protective clothing, such as gloves, long sleeves, pants, hats and protective eyewear to prevent skin contact with the sap. If skin comes into contact with sap, wash immediately with soap and water. Burned skin is very sensitive to sunlight, so keep any exposed areas covered when outside.

Contact your healthcare provider as soon as possible after exposure and tell him or her you suspect you have touched giant hogweed. If they are unfamiliar with the plant, have them contact the county or state noxious weed program for more information.

After the burns subside, darkened areas or scars can persist for several years. The affected areas remain sensitive to sunlight so it is important to keep the burned areas away from direct sunlight as much as possible.

Sadly, children are the most likely to get hurt by hogweed because they don't know they should avoid it. It is very important to check parks and greenspace areas where children are playing to make sure there is no hogweed in the area.

What does it look like ?


Very tall plants, 8-14 feet in height
Stems that are 2-4 inches in diameter with rigid hairs, purple blotches and are hollow
Leaves that are unevenly lobed and up to 5 feet long
Flowers clusters up to 2 feet across occurring from mid June to late July
Dead stems that remain upright throughout the winter
Bears a close resemblance to native cow parsley and hogweed
Has a reddish purple stem with fine spines that make it appear furry (like a stinging nettle)
Has spotted leaf stalks
May grow to 3-5m high
Leaves can be up to 1.5m wide
Plants can take up to four years to reach their full height and flower
Flowers in June and July
Flower heads are commonly 500mm (0.5m) wide
Flowers can each produce 50,000 seeds every year
Seeds can stay in the soil for up to 15 years before they develop.

If you must handle this plant be careful not to come into contact with sap from broken stems. You should be well covered, wearing gloves, long sleeves and long pants.

Giant Hogweed is a short lived perennial weed that reproduces only by seed and is predominantly found along rivers, streams and wet land areas, but can also be found in pastureland. The key to long term management of Giant Hogweed is to limit seed production.

Often people will attempt to cut the flowering head off in hopes of limiting seed production. However at this time of year, most flowering heads of giant hogweed have already set mature seed. Therefore cutting off seed heads now will only put you at risk of coming into contact with the clear watery sap while helping the plant spread its seed around.

What You Should Do if You See Giant Hogweed

Certainly Giant Hogweed is an invasive species which could reproduce in an uncontrolled manner if left on its own

Oliver Reichl, a professional naturalist who lives in the Canadian 1000 Islands region say he can identify this invasive species and stop its growth.

To contact Oliver e-Mail

Contact the municipality where you live and they should remove it or kill the weed

According to University of Guelph student Meghan Grguric researching how to eradicate invasive giant hogweed in Ontario.
There is no easy way to prevent this weed from regrowing.

Giant Hogweed Control  .pdf
Coquitlam looking to coral giant hogweed
Giant hogweed creeps around Guelph



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