Giant Hogweed

Man standing next to giant hogweed.
Photos courtesy: Michael Cowbrough, Chief Weed Inspector, Province of Ontario

Giant hogweed is a non-native invasive plant that is most widely distributed in Halton Region along Sixteen Mile Creek. Contact with this plant can cause serious burns to the skin and temporary or permanent blindness.

Man standing next to giant hogweed.
Photos courtesy: Michael Cowbrough, Chief Weed Inspector, Province of Ontario

What is giant hogweed?

Giant hogweed, Heracleum mantegazzianum, is an invasive plant known for its enormous size.

  • The plant can grow from 2.5 to 4 metres high (8 – 14 feet).
  • The saw-toothed leaves are deeply lobed and can grow to 1 metre (3 feet) across.
  • The stems are hollow with dark reddish-purple splotches and coarse white hair.
  • The watery sap produced by the leaves and stems contains a chemical that causes skin to become highly sensitive to the sun.
  • Small white flowers are clustered in an umbrella-shaped head that can grow larger than 30 centimetres (1 foot) in diameter.
  • The seeds are oval and flat.

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Where is giant hogweed found?

  • It can be found along:
    • roadsides
    • vacant lots
    • stream banks

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Is giant hogweed a risk to human health?

Yes.

  • Contact with the sap of giant hogweed can cause photodermatitis, a serious skin inflammation.
  • Temporary or permanent blindness can occur if sap enters the eye.
  • The sap contains chemicals called furanocoumarins that get absorbed by the skin and cause the affected area to become highly sensitive to ultraviolet light from the sun.
  • The exposed skin can turn red and blister, sometimes leaving scars and discolouration that can last for years.
  • The severity of the skin reaction will depend on the sensitivity of the person. Heat and moisture (sweat) can enhance the reaction.
  • Reactions can occur up to 48 hours after contact.

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

  • Since the sap is found in the plant’s leaves and stems, skin contact can occur by touching or brushing up against the plant.
  • A person can be exposed to the sap by touching clothing or pets that have come into contact with the plant.
  • Eye contact can also occur if the sap becomes airborne by pulling, cutting or mowing giant hogweed.

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

  • Wash the affected area immediately with soap and water
  • Keep the affected area out of the sun for at least 48 hours.
  • Contact a medical professional as soon as possible for advice and/or seek treatment.

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Are there other plants that look like giant hogweed?

  • Yes, other plants are often mistaken for giant hogweed.
  • Its “giant” size helps to distinguish giant hogweed from other plant species.
  • Resources are provided below to help you to identify giant hogweed.

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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.
  • Resources are provided below to help you to identify giant hogweed.
  • For safety reasons, property owners are encouraged to contact a licensed weed exterminator to remove giant hogweed from private property. Licensed weed exterminators can be found under “Weed Control Services” in the Yellow Pages.
  • If you choose to remove giant hogweed yourself, USE EXTREME CARE. The Landowner's Guide to Controlling Invasive Woodland Plants (external PDF) provides more information on the control of giant hogweed.
  • DO NOT compost giant hogweed or place it out for curbside collection as garbage, yard waste or in the GreenCart. Giant hogweed must be taken to the Halton Waste Management Site for safe disposal.
  • Following removal or treatment, monitor the area for plant regrowth.

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

If found on Town or City property such as parks, roadsides and public areas, contact the local municipal office:

  • City of Burlington: 905-335-7600
  • Town of Halton Hills: 905-873-2601
  • Town of Milton: 905-878-7252
  • Town of Oakville: 905-845-6601

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

  • Yes, municipalities are permitted to use pesticides to control “noxious weeds”, including giant hogweed.

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Giant hogweed.
Photos courtesy: Michael Cowbrough, Chief Weed Inspector, Province of Ontario

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

Yes.

  • Municipal by-laws are in place to help ensure that property owners control “noxious weeds”, including giant hogweed.
  • If you are concerned about hogweed on private property, contact the local municipal office (listed above).

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

Yes. Wild parsnip, Pastinaca sativa

  • Wild parsnip is a relative of giant hogweed.
  • Like giant hogweed, the sap of this plant can also cause severe burns to the skin.
  • Wild parsnip can be found along roadsides, fence rows and railroad tracks.

More information about wild parsnip is available at:

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Where can I get more information about giant hogweed?

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Resources

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