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Giant Hogweed Factsheet

Heracleum mantegazzianum

› Click here to download the Giant Hogweed Factsheet

CAUTION: This plant is poisonous. Do NOT touch it!


Giant HogweedWhat is it?

Giant Hogweed is a perennial and biennial flowering plant in the carrot family. Native to eastern Europe and western Asia, it was likely introduced to North America as an ornamental plant, and arrived in Ontario in 1949. It has since spread into natural areas and is now classified as an invasive species. It competes with native species for light, and can change the composition and reduce the diversity of native plant communities. Giant Hogweed grows in moist environments, generally near streams, lakes and ponds.
Identification

When mature, Giant Hogweed is a very large plant. It can reach a height of 4-5 metres when in flower, and have a stem diameter of 5-10 centimetres. When young, it can be much shorter and without flowers. The hairy stem is either purple in colour, or has purple spots. The alternating leaves are dark green, and have coarse teeth and three deep lobes. Leaves near the base of the plant can be over 2 metres long. Giant Hogweed has small white flowers that are arranged in an umbrella-shaped head. These flower heads can measure 80 centimetres across. A single plant can contain more than 80,000 flowers.

 

Ecology

Giant Hogweed grows for 3-5 years before flowering. Flowering occurs from June to August, at which point the seeds are released. Once the seeds are released the plant dies. Seeds are dispersed by wind, water and humans, and can survive in the soil for 7 years. Each plant can release between 20,000 to 100,000 seeds, resulting in high rates of spread.

 

Toxicity

Like some other members of the carrot family, Giant Hogweed contains a clear, watery, toxic sap. When this sap contacts skin it causes sensitivity to sunlight resulting in a painful burn within 15 minutes of exposure. The extent of reaction depends on the individual, and sensitivity can last for months or years.

 

What you should do if you find Giant Hogweed...

...In the Wild

  • DO NOT touch the plant
  • Contact the Invading Species Awareness Program hotline 1-800-563-7711 to report the infestation
  • Before leaving the natural area, remove seeds and fluff from clothes, pet fur, bike tires, and shoes

...In your Backyard

  • DO NOT touch the plant or try to control it yourself
  • Contact a pest control expert for removal • Keep children and pets away from the plant

What you should NOT do

  • NEVER touch ANY plant if you are not completely certain of its identity
  • Do not cut or pick fresh or dried flower heads because you are spreading the seeds

What TRCA is doing

Toronto and Region Conservation (TRCA) is developing a Terrestrial Invasive Species Strategy that will help prioritize and manage exotic invasive plants like Giant Hogweed. TRCA is working with our partner municipalities to identify infestations and effectively manage them. If Giant Hogweed is identified in public areas or areas near trails, the plants will be removed.

Similar Plants

Giant Hogweed is often mistaken for native species Cow Parsnip (Heracleum maximum) and Angelica (Angelica atropurpurea). Cow Parsnip is a shorter plant (only 1-3 metres), has smaller flowers and a green stem without any purple. Angelica is also much smaller (1.2-2.7 metres) has a rounded-topped flower cluster and leaves divided into many leaflets.

What to do if you touch Giant Hogweed

Should you touch Giant Hogweed, wash the affected area immediately with soap and cool water. Avoid sunlight and seek medical attention

 

Additional Resources

Invading Species: Giant Hogweed http://www.invadingspecies.com/invaders/plants-terrestrial/giant-hogweed/
Ontario Weeds: Giant Hogweed http://www.weedinfo.ca/en/weed-index/view/id/HERMZ
Ministry of Agriculture Food & Rural Affairs http://www.omafra.gov.on.ca/english/crops/facts/ontweeds/giant_hogweed.htm