Giant Hogweed

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.


Giant Hogweed (Heracleum mantegazzianum) has two major negative impacts:

  1. Due to its invasive nature, it poses a threat to biodiversity.
  2. It is a public health hazard. It produces a noxious sap that sensitizes the skin to ultraviolet light. This is known as photosensitivity, which can result in severe and painful burning and blistering. It is important to avoid any skin contact with this plant.

What does it look like?

Giant HogweedGiant Hogweed is an invasive perennial known for its enormous size. This species usually grows from 2.5 to 4 metres (8 to 14 feet) high with leaves up to 1 metre (3 feet) in breadth. It has a thick—5 to 10 centimetres (2 to 4 inches)—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 30 centimetres (1 foot) in diameter. The seeds of Giant Hogweed are flat and oval in shape. This plant produces a clear, toxic watery sap that causes a skin reaction known as photosensitivity. Click here to see Conservation Halton’s factsheet on Giant Hogweed.

Giant Hogweed

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

The Ontario Ministry of Agriculture and Food has a video on identifying Giant Hogweed, as well as other information on the plant.

Report your sightings!

Plants that look similar

Halton Region is home to a number of native plants that resemble Giant Hogweed but do not pose the same health risk.

The following are native look-alikes in the area:

  • Cow Parsnip (Heracleum lanatum Heracleum maximum)
  • Purple-stemmed Angelica (Angelica atropurpurea)
  • Spotted Waterhemlock (Cicuta maculata)
  • Poison Hemlock (Conium maculatum)
  • Wild Parsnip (Pastinaca sativa)

The University of Pennsylvania has published an informative pamphlet outlining the characteristics of Giant Hogweed and these look-alike plants.

When in doubt, always confirm plant identification with an expert.

Where is it in Halton, and where did it come from?

Portable document format (pdf) version of the Giant Hogweed distribution map.
Portable document format (pdf) version of the Giant Hogweed distribution map.

Giant Hogweed is native to Asia and has been introduced to both Europe and North America, most likely as a garden ornamental plant. Giant Hogweed prefers but is not limited to moist soils; it can be found along roadsides, vacant lots, streams, and rivers. It is often classified as a freshwater weed and is typically found in floodplains. In Halton Region, Giant Hogweed is most widely distributed along Sixteen Mile Creek.


What do I do if I come into contact with it?

  • If you have been exposed to the sap of Giant Hogweed, wash the area immediately with cool, soapy, running water;
  • Since the sap increases the photosensitivity of the skin, it is important to avoid exposure to the sun for at least 48 hours after contact; the sun’s radiation and can cause skin that has been exposed to the sap to burn and blister;
  • If blisters form, contact a medical professional for advice and treatment.

How does it grow and spread?

  • Giant Hogweed will grow 1) from its seeds, and 2) from buds that form on its crown or rootstalk;
  • It takes several years for a Giant Hogweed plant to produce its first flowering stalk. Although Giant Hogweed is a perennial, it is believed to die after its first flowering and seed set; plants may then still produce additional crowns that continue to flower and seed set;
  • Seeds can be viable for more than seven years;
  • Seeds can spread easily since they can float considerable distances down waterways and start new plants at new locations;
  • Seeds can also be distributed far from a plant by birds that eat the plant’s fruit.

Control and removal

We encourage the control and removal of Giant Hogweed but the look-alikes listed above do not need to be removed.

The best control of Giant Hogweed is to not plant it.

Other possible control methods include manual removal and chemical treatment:

  • When removing Giant Hogweed it is very important to wear proper clothing (long sleeves, high shoes, gloves, face and eye protection) to avoid skin contact with the plant;
  • 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 later in the season puts you at a higher risk for exposure and is less effective for long-term control;
  • Removing the seed heads can help control it, but this must be done every year to have a positive effect;
  • Removing seed heads 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;
  • Plants can be manually dug out but it is important to remove the full rootstalk to reduce the risk of it spreading further;
  • When disposing of Giant Hogweed, all parts of the plant should be placed in secure black plastic bag and left in the sun for three to four weeks. This helps to destroy any seeds and roots. These bags should then be sent to the landfill;
  • Mowing Giant Hogweed is not an effective control method since mowing tends to stimulate budding on the rootstalk and risks exposure to the toxic sap;
  • Herbicides can also be effective in controlling the spread of Giant Hogweed, but it is important to use pesticides only in a cautious and responsible manner.

Additional Information