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Giant hogweed

Giant hogweed alongside a bike path
Photo: Daniel Gélinas
Heracleum mantegazzianum
  • Heracleum mantegazzianum
  • Heracleum mantegazzianum
  • Heracleum mantegazzianum
  • Heracleum mantegazzianum
  • Heracleum mantegazzianum
  • Heracleum mantegazzianum
  • Heracleum mantegazzianum

Giant hogweed (Heracleum mantegazzianum) is an invasive exotic plant that is harmful to human health and to the environment. It can cause serious skin reactions similar to burns.

Here is how to recognize and eliminate this plant.

An invasive exotic plant

Giant hogweed (Heracleum mantegazzianum) is a large perennial member of the Umbelliferae (Apiaceae) plant family, the same family as carrots, parsley, dill, etc.

Native to the Caucasus, it was probably introduced to Europe as an ornamental plant. Since then, it has naturalized in several European countries as well as Australia, New Zealand, the United States and Canada.

In Canada, it is widespread in southern British Columbia and Ontario and has also been reported in the Atlantic provinces. It has been identified in Quebec for the first time in the 90 years since it was initially reported in various other regions; however some of these observations have yet to be validated.

Habitats colonized by giant hogweed

Giant hogweed prefers disturbed wet areas (e.g. ditches, the banks of watercourses, roadsides, railways …). It can also be found in other habitats, such as fields and vacant lots.

Effects on health and the environment

It can cause a skin reaction called phytophotodermatitis. Giant hogweed contains furanocoumarins (or furocoumarins) that are activated by light and make the skin very sensitive to sunlight.

Contact with the sap of the giant hogweed, followed by exposure to the sun, can cause localized rash and edema (red, swollen skin), blisters and sometimes severe lesions resembling first or second degree burns. Symptoms can take up to 48 hours after exposure to develop. Contact with the sap may leave brown pigmentation or scars, even when healed.

In addition, it is a massive and vigorous alien invasive species. Each plant can easily produce 20,000 seeds – some have produced up to 100,000! This makes it highly competitive with native flora.

How to identify giant hogweed

It is a large, very impressive herbaceous plant that can hardly go unnoticed!

In its first few years, the giant hogweed produces a rosette of broad leaves. After two to five years, the flowers appear. It blooms just once. The plant can grow to 2-5 m in height and its leaves, which can easily reach 1.5 m in diameter, are composed of three broad, deeply-cut leaflets.

The stems that bear the flowers are thick and hollow and measure up to 8-10 cm in diameter at the base. Stems and leaf petioles have reddish-purple spots. The flat-topped flower heads (umbels), which can reach over a metre in width, are composed of many small white flowers. The fruit is oval and flat, similar to dill seed, but larger.

Be careful not to confuse giant hogweed with common hogweed, or cow-parsnip (Heracleum maximum, also known under the name H. lanatum) which is native to North America and is smaller.

Here are the main characteristics that distinguish giant hogweed from common hogweed:

  • Its great height (2-5 m). Common hogweed rarely grows beyond 2 m.
  • The presence of numerous purple spots on the giant hogweed’s stems.
  • Leaves of giant hogweed are extremely large (up to 3 m long and 1.5 m wide).
  • The flower heads of the giant hogweed are much larger (20-50 cm) and consist of more flowers (between 50 and 150 pedicels) than those of common hogweed.

For more information on the morphological characteristics that help identify giant hogweed, please consult the web page of the Québec Ministry of Environment, and the Fight Against Climate Change (MELCC) (French only).

Reporting giant hogweed 

To report the presence of giant hogweed, please record your observation using the Sentinelle tool or by filling out the MELCC form (French only). It is essential to photograph the plant so that Ministry professionals can confirm your finding.

Please also notify your municipality of the presence of this invasive exotic plant.

How to get rid of it giant hogweed on your land

  • First and foremost: protect yourself well! Cover the entire body with protective non-absorbant clothing that is impervious to the sap to prevent the skin, hands and eyes from coming in contact with the plant (long pants, long sleeves, long-sleeved rubber gloves, safety goggles or a visor, closed-toe shoes, etc.).
  • It is important to limit the spread of giant hogweed by not planting it, allowing it to multiply or transporting it. Wherever possible, you should remove the plant and destroy any regrowth.

Steps for removal

  • For maximum effectiveness, begin control operations in early spring. It is important to keep track during the year and repeat the operations as needed. Interventions must continue until all plants have been completely eliminated.
  • If there are just a few young plants, you can try pulling them out using a round shovel or a spade remove as many roots as possible and prevent them from growing back. For mature or large plants, it is best to sever the root with a sharp shovel, approximately 15 to 20 cm below the soil surface. Be careful not to spread any seeds if some are already present on the plants.
  • If the plants cannot be removed, prevent seed formation by pruning the umbels at the base as early in the season as possible. This will at least limit the spread of the plant.
  • Afterwards, it is a good idea to lay down a piece of geotextile to prevent the germination of any seeds that might be present in the soil. Giant hogweed seeds can remain viable for several years.
  • To get rid of plant cuttings, put them in plastic bags and dry them in the sun for a minimum of one week.
  • Finally, it is very important to wash with water and soap your tools and clothing after they have been in contact with the plant. It is also important to wash your hands and face.

In case of contact with the plant

  • To remove the sap as quickly as possible, pat with an absorbant paper towel. Do not rub.
  • Wash the affected area with soapy water, rinse well and dry.
  • The person who washes the affected area should also wash their hands.
  • Wear clothing that will shield the affected area from light and UV rays for a minimum of 48 hours (or an SPF 30+ sunscreen for 6 months, on areas that can’t be covered).
  • Wash clothing that has been in contact with the plant separately to avoid cross-contamination.
  • If there has been contact with the eyes, rinse thoroughly for at least of 10 minutes, wear dark sunglasses to minimize sun exposure and consult a doctor as soon as possible.
  • In the case of lesions or a severe skin reaction, fever, or whenever a child has been exposed, see a doctor.

At all times, if you think you have been in contact with the plant and are experiencing skin irritation or other symptoms, contact the INFO-SANTÉ helpline at 811.

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