Giant Hogweed

Identification & Biology
Photo By: Invasive Species Council of Metro Vancouver
Giant hogweed patch
Photo By: Lisa Scott
Giant hogweed
Photo By: Don Hare
Giant hogweed height comparison
Photo By: Lisa Scott
Giant hogweed stem
Photo By: Lisa Scott
Giant hogweed along path
Alias' : Cartwheel Flowers, Giant Cow Parsley, Giant Cow Parsnip, Hogsbane
Latin Name : Heracleum mantegazzianum
Category : Terrestrial Plants
Description :
  • Perennial that can grow 2 – 5 m tall 
  • Hollow, flowering stalk is up to 10 centimetres in diameter, usually with bristles and purple spots 
  • Dark green leaves, up to 3 metres long, divided into 2 to 3 lobes, with highly jagged edges and with stiff underside hairs 
  • Large, umbrella-shaped clusters of white flowers, 20 – 50 centimetres across 

 

Giant hogweed is a highly competitive plant that can tolerate full shade and seasonal flooding. Plants reproduce by seed and can produce up to 100,000 winged seeds that remain viable in the soil for up to 15 years. Seeds can be dispersed by wind, water, animals, equipment and vehicles. Above-ground portions of the plant die back in winter. Giant hogweed is of particular concern for its toxic sap. Worksafe BC has issued a toxic plant warning for giant hogweed that requires workers to wear heavy, water-resistant gloves and water-resistant coveralls that cover the skin when working near this plant. 

In the Okanagan, giant hogweed can be easily confused with cow parsnip (Heracleum maximum). Cow parsnip blooms in July, while giant hogweed blooms in August. Cow parsnip flower heads are much smaller than giant hogweed (0.2 m compared to up to 1.5m, respectively). Cow parsnip stems are smooth and have soft hairs, whereas giant hogweed stems have purple streaks, blotches, lines, and spots with stiff hairs and a bristly feel. Giant hogweed can also be confused with blue elderberry (Sambucus cerulea), which also has large clusters of white flowers. However, blue elderberry stems lack the purple blotches and hairs that are characteristic of giant hogweed, and do not nearly grow as tall as giant hogweed. 

Look Alikes
Habitat

Giant hogweed can grow under a variety of conditions, but prefers to grow in wet areas in parks, forest edges, gardens and along streams. It is distributed throughout the Lower Mainland, Fraser Valley, Gulf Islands, and central to southern Vancouver Island. There have been no reported cases of giant hogweed in the Okanagan as of yet. 

Impact & Risks
  • Giant hogweed is TOXIC; its sap can cause significant burns to the skin after exposure to sunlight; blistering, scarring and skin irritation can re-occur for several years; sap may cause blindness if rubbed in the eyes  
  • It is a highly aggressive invasive plant; it outcompetes native vegetation and displaces desirable forage species 
  • Giant hogweed can quickly dominate ravines and stream banks and cause erosion
Prevention & Mitigation

Giant hogweed is on the “Watch For” list in the Okanagan and sightings should be reported via the BC Report Invasives phone app or online. For more information, go to: www.gov.bc.ca/invasive-species.

The most effective way to ensure that your lands do not become infested with giant hogweed is by prevention. Here are some recommendations to prevent invasion: 

  • Do not purchase, trade or grow giant hogweed 
  • Maintain your crops and natural lands in a healthy, vigorous condition to ensure a competitive plant community 
  • Regularly patrol your property for giant hogweed plants and immediately control or remove infestations before seed set  
  • Immediately re-vegetate disturbed, bare soils with a suitable seed mixture that provides dense, early colonization to prevent weed invasion  
  • Do not move contaminated soils to a new area  
Treatment & Disposal
  • It is best to hire a professional to manage giant hogweed infestations; protective gear (gloves, goggles or a face shield, a suit and boots) should be used, as all parts of giant hogweed contain toxic sap that can cause significant burns 
  • In spring use a shovel or loppers to cut the main root 5-15 centimetres below the soil surface; this is suitable for individual plants, sites with less than 100 plants 
  • From May to July, cut off the flower clusters to prevent seeds from forming; this will not kill the plant and new flowers may form after cutting 
  • For disposal, cut plants into smaller pieces, double-bag in heavy garbage bags, seal tightly and dispose as garbage waste
  • Monitor sites each year in the spring to identify and remove new seedlings
  • Restore the area with native or non-invasive plants only after the giant hogweed is no longer present 
  • Chemical control is also an option; before applying herbicides, read the label for full use and precautionary instructions  
  • For further information on the selection and application of chemicals to protect your crop, contact AgriService BC at 1-888-221-7141 or email AgriServiceBC@gov.bc.ca  
  • Plant material should be placed in a large, sealed heavy-duty plastic bag and disposed of at the landfill; do not compost this plant  
  • There are currently no biocontrol agents (natural insect enemies) available for use in B.C.

 

*The manual control information in this section was adapted from Tackling Giant Hogweed (Dec 2020) produced by MetroVancouver and available online. 

Okanagan Distribution

Priority Level Definitions

Watch For - Poses a significant threat (very high risk) and does NOT presently occur in the region OR is relatively new to the region and is very limited in extent.
High - High risk/impact; limited population with significant potential to spread in the region.
Medium - Medium risk/impact; limited distribution – broader population distribution with potential to spread further in a region.
Low - Low risk/impact; may be widespread or not, may be of concern in specific situations with certain high values – e.g. specific agriculture crops. Some species may be treated primarily with biological control agents.

Commodities

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