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Giant Hogweed

Giant hogweed is a non-native invasive plant, hazardous to people if touched. Learn how to recognize hogweed and safely remove any growth.

You will find giant hogweed in Halton Region along Sixteen Mile Creek. Take caution, as this non-native invasive plant can cause serious burns to skin and temporary or permanent blindness.

What is giant hogweed?

Giant hogweed (Heracleum Mantegazzianum) is an invasive plant known for its enormous size. Characteristics of this plant include:

  • Plants can grow up to 2.5 – 4 metres in height (8-14 feet)
  • Saw-toothed, deeply lobed leaves can grow up to 1 metre (3 feet) across
  • Dark reddish-purple splotches and coarse white hair make up the stems
  • Watery sap contains a chemical that makes skin highly sensitive to the sun
  • A 30 cm (1 foot) umbrella-shaped head holds clusters of small white flowers
  • Oval and flat seeds

Where is giant hogweed found?

  • Roadsides
  • Vacant lots
  • Stream banks

Is giant hogweed a risk to human health?

Yes, the severity of the reaction depends on the sensitivity of the person. Reactions sometimes occur up to 48 hours after contact!

  • Contact with sap can cause a serious skin inflammation known as photodermatitis
  • Sap can cause temporary or permanent blindness if rubbed in the eyes
  • Chemicals in the sap, called furanocoumarins, get absorbed by the skin and create a strong sensitivity to ultraviolet light from the sun
  • Exposed skin can turn red and blister

How can I be exposed to the sap of giant hogweed?

A person can be exposed to the sap by touching or brushing up against the plant. Be careful when handling clothing or pets that may have been in contact with hogweed as the sap is easily transferable from the leaves and stems. If you decide to remove hogweed, take precautions. Pulling, cutting, or mowing can cause airborne sap to come into contact with your eyes.

What do I do if I accidentally come into contact with giant hogweed?

  • Wash affected area immediately with soap and water
  • Stay out of the sun for at least 48 hours
  • Contact a medical professional for advice and/or seek treatment right away

Are there other plants that look like giant hogweed?

The following plants may be mistaken for hogweed:

  • Cow Parsnip
  • Queen Anne's Lace
  • Spottted Waterhemlock
  • Purple-stemmed Angelica
  • Wild Parsnip

Giant hogweed's “giant” size helps to distinguish giant hogweed from other plant species.

What do I do if I think I have giant hogweed growing on my property?

  • DO NOT TOUCH! Keep children and pets away from the area
  • Due to the dangerous sap, we encourage property owners to contact a licensed weed exterminator to remove giant hogweed
  • Dispose of giant hogweed at the Halton Waste Management Site for safety
  • After removal, monitor the area for hogweed regrowth

PLEASE NOTE: If you choose to remove hogweed yourself, use extreme care. For the control of this invasive plant, consult the The Landowner's Guide to Controlling Invasive Woodland Plants.

What do I do if I find giant hogweed on public property?

If you think you have identified hogweed in a park, roadside, or other public area, contact your local municipal office.

Burlington (905) 335-7600
Halton Hills (905) 873-2601
Milton (905) 878-7252
Oakville (905) 845-6601

Does the Ontario Pesticide Ban permit municipalities to control giant hogweed using pesticides?

Yes, municipalities can use pesticides to control this “noxious weed”.

Are there municipal by-laws in place to control giant hogweed?

Yes, municipal by-laws allow property owners to control “noxious weeds” such as hogweed with pesticides. Contact your local municipal office about concerns of hogweed on private property.

Are there other weeds in Halton Region that can cause similar health effects?

Wild parsnip, often mistaken for hogweed, contains sap that can cause severe burns to the skin. A relative of hogweed, wild parsnip usually grows along roadsides, fences and railroad tracks.

More information about wild parsnip is available at wild parsnip, Pastinaca sativa (external link)