Giant hogweed (Heracleum mantegazzianum) is a perennial in the parsley family. It occurs in the Lower Mainland, Fraser Valley, Gulf islands, and central to southern Vancouver Island. A small infestation has also been found in the central Kootenays. There are no confirmed infestations of this plant in the Okanagan or Similkameen Valleys. However, it could be covertly growing somewhere and we just have not discovered it yet. So please inform your neighbours of this invasive plant and keep your eyes peeled. I appreciate each and every possible sighting, so please report any suspect plants.
- Extremely tall, reaching heights of up to 5 m at maturity
- Numerous small white flowers clustered in a large umbrella-shaped head
- Flower heads reach up to 1.5 m in diameter, bloom in mid August
- Dark green leaves are coarsely toothed in 3 large segments with stiff underside hairs
- Lower leaves can exceed 2.5 metres in length
- Stalks have purple blotches, streaks or spots, and stiff bristly hairs.
- Usually found growing on agricultural lands, riverbanks, vacant lots, and along roadways.
Giant hogweed will colonize a wide variety of habitats, but is most common along roadsides, in ditches, along river or creek banks, wetlands, agricultural areas and other disturbed sites. It is a highly competitive plant due to vigorous early-season growth, tolerance of full shade and seasonal flooding, as well as its ability to co-exist with other aggressive invasive plant species. Each plant can produce 50,000 to 100,000 winged seeds that remain viable in the soil for up to 15 years. Interestingly it requires 2-4 years from germination to develop a flowering stem.
The most disturbing feature of giant hogweed is the clear, highly toxic sap that is found in the stem and leaves. When in contact with the skin, this sap can cause a hypersensitivity to sunlight resulting in burns, blisters and scarring. It is so toxic that WorkSafe BC has issued a “toxic plant warning” that requires workers to wear heavy, water-resistant gloves and water-resistant coveralls that completely covers skin while handling giant hogweed.
Similar looking plants in our region include cow parsnip and blue elderberry. Both of these are native plants that closely resemble giant hogweed. Cow parsnip is much smaller in height (1.5 – 2.5 m), has coarse hairs at the base of leaf stalks and hairy leaves. Reddish-purple spots are not present on stems and leaves are not as incised or sharply toothed. Blue elderberry is also shorter (2 – 4 m), with leaves that are opposite and divided into 5-9 leaflets in a feather-like arrangement. It is also lacking the reddish-purple spots on the stems.
Before you call with a sighting, refer to the Invasive Species Council of Metro Vancouver website where you will find a page dedicated to giant hogweed and a video.
An infestation of giant hogweed with plants that have not yet bolted. Note the sharply incised leaves.
Giant hogweed has distinctive reddish-purple spots on the stem.