Ausable Bayfield Conservation Authority

Giant Hogweed Plant

Giant Hogweed

Heracleum mantegazzianum

For fact sheet on invasive, harmful Giant Hogweed click on 'Publications and Downloads' and go to fact sheets.

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Fact sheet on Giant Hogweed - Invasive Species

Health Hazard

Giant Hogweed can be a serious health hazard for humans.

Its watery, clear sap contains photosensitizing compounds, which, when in contact with human skin and in combination with UV radiation, can cause burning.

The reaction of the skin depends on individual sensitivity.

After 24 hours, reddening and swelling of the skin may be noticed. This may be followed by an inflammatory reaction after three days.

Depending on individual sensitivity, effects can last for months and skin can remain sensitive to UV light for years.

The plant also has the potential to blind, temporarily or permanently.

If you become exposed:

1. Wash the affected areas immediately with soap and water if available.

2. Keep affected areas out of direct sunlight.

3. Seek medical advice as soon as possible.

Control and Removal

Here are some guidelines suggested, by various agencies, for control and removal of Giant Hogweed. Ausable Bayfield Conservation Authority (ABCA) does not assume liability for any health concerns that may arise from coming into contact with the plant.

• Suit up with protective clothing, including waterproof gloves, long-sleeved shirts and pants, disposable spray suit coverall over top of clothes, safety glasses and face shield. Wash gloves with soap and water before removing spray suit. Wash rubber gloves before taking them off and remove protective eyewear last

• Removal or control of Giant Hogweed should not be done at the brightest time of day

• For smaller plants, in early May, cover the area with black plastic to smother growth or use glyphosate (e.g., Roundup). Cover area in mulch 10-14 days after, to reduce germination of other seeds

• For flowering plants, cut white flower head in early July before turning green and producing seeds. You may need to cut flowers a month later. Carefully place flower heads from stems into black plastic bags. Tightly seal the bags and leave in direct sunlight for one week. Do not burn or compost If in doubt about appropriate removal, please contact a professional. For more information see list of references at If you see Giant Hogweed:

• Report it to your local municipal weed inspector

• Send location, when confirmed, to the invasive species tracking system website at

• If it is on the property of Ausable Bayfield Conservation Authority, call 519-235-2610, or toll-free 1-888-286-2610 or e-mail: info(at) or click on our staff contact list.

• Please provide digital photos of the leaf, stem, and flower (taken at a large distance from the plant using a zoom lens)

• Please remember to be very careful and DO NOT TOUCH THE PLANT unless you have proper protection

Often mistaken for:

Giant Hogweed has been mistaken for other species, especially others in the carrot family such as wild carrot, water parsnip, and angelica. The most distinguishing feature of Giant Hogweed is its giant size – as a mature plant it towers above these other species.

Ausable Bayfield Conservation Authority (ABCA) 71108 Morrison Line, RR 3 Exeter, ON N0M 1S5 519-235-2610 • 1-888-286-2610 •


Giant Hogweed has large, flat-topped to slightly dome-shaped flower and seed head and a bumpy or bristly stem


Mature plant grows taller than a grown adult, eight feet to 14 feet tall (up to five metres in height), with green to reddish-purple stem, stem and leaf stocks, not smooth but with raised reddish purple nodules, forming bumps or bristles


Numerous white flowers clustered in an umbrella-shaped head up to 2.5 feet in diameter. It blooms spring – mid summer


Large, compound, deeply incised, and three to five feet wide, leaf edges – bristle tipped or spiky Habitat: Roadsides, stream banks, waste areas, yards Status: Introduced as an ornamental from Asia

For more information visit or or or

Report presence of plant to your local municipal weed inspector and Invading Species Hotline at or 1-800-563-7711 or Photos of plants on Ausable Bayfield Conservation Authority property can be sent to info(at) or click on our staff contact list but DO NOT get near plant.

The content provided in this publication is intended for local educational and information purposes only. Every effort has been made to ensure the correctness of information as at the publication date (June 2015).

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