Giant Hogweed (Heracleum mantegazzianum) is an invasive biannual known for its enormous size. This species usually grows from 2.5 to 4m (8 to 14ft) high with leaves up to 1m (3 ft) in breadth. It has a thick, 5 to 10cm (2 to 4in) hollow stem. Its stem and the undersides of its leaves are covered in coarse hairs. Its large, umbrella-shaped flowers are white in colour and can be more than 30cm (1ft) in diameter. The seeds of Giant Hogweed are flattened and oval in shape. This plant produces a clear, toxic watery sap that causes a skin reaction known as photosensitivity. Click here for videos on the identification and removal of Giant Hogweed from OMAFRA.

If you have been exposed to the sap of Giant Hogweed you should wash the area immediately with running water and do not expose it to sunlight for 48 hours. If blisters do form contact a medical professional for advice. The reaction is caused by sunlight because the sap causes your skin to be unable to protect itself from the sun's radiation.

Download our Giant Hogweed Factsheet (PDF) here.

 Also available in PDF. (if you know of Hogweed that is not shown on this map please report it!!)

Key Identification Characteristics:

- OVER 2 metres tall when in flower
- dark purple mottling on stem at leaf junction and base of plant
- ring of stiff white hairs on stem at leaf junction

Click here for a document showing Giant Hogweed and some commonly mistaken lookalikes. We encourage the control and removal of Giant Hogweed but the look alikes do not need to be removed.

Click here for a video on the identification of Giant Hogweed from OMAFRA.

Download our Giant Hogweed Factsheet (PDF) here.

Report a sighting!
To report a sighting please fill out the online Invasive Species Reporting Form or contact Brenda Van Ryswyk: brendav@hrca.on.ca or 905-336-1158 ext 282 

Working together: Conservation Halton is working with its watershed partners to raise awareness of the hazards of this invasive species and to work together for an effective control of it in our watershed. A summary of our municipal partners workshop that was held on April 23, 2010 can be viewed here: Giant Hogweed Partners Workshop Summary

A PDF of the "Giant Hogweed introduction" powerpoint given at the workshop can be viewed here.

A PDF of the "Giant Hogweed Legislative Approach, Common Messaging and Outreach" powerpoint can be viewed here.

Removal information:
The best time to remove and control Giant Hogweed is in late April or Early May when it is small and still growing. Attempting removal at a later time in the season puts you at a higher risk for exposure and is less effective for long term control.  We are currently working with the Provincial Weed Inspector and the University of Guelph to find the most effective control method with the least negative effects. The testing will continue in 2011.  The removal of seed heads can help prevent the spread of hogweed but it must be done before they ripen. Do not attempt removal after the seeds are ripe! Movement of dried plant materials (seed heads) will only further disperse the seeds.  An information summary based on the 2010 testing is available from the OMAFRA website.

A list of contractors who are willing to control Giant Hogweed has been compiled by the Ontario Invasive Plant Council.

A Best Management Practice for the removal of Giant Hogweed has also been produced by the OIPC and is available online here.

If you have been exposed to the sap of Giant Hogweed you should wash the area immediately with running water and do not expose it to sunlight for 48 hours. If blisters do form contact a medical professional for advice. The reaction is caused by sunlight because the sap causes your skin to be unable to protect itself from the sun's radiation.