Aggressive Ornamentals

Giant Hogweed (Heracleum mantegazzianum)

Giant hogweed is a perennial member of the Parsley or Carrot Family native to Asia. It closely resembles our native plant cow parsnip, except the taller giant hogweed grows up to 6 metres or more. It was first introduced to North America as a garden curiosity. Its tenacious and invasive characteristics have made it a garden pest that can readily escape cultivation. In BC it is known from southern Vancouver Island, the Gulf Islands, and Vancouver.

Biology 

Aside from its immense size, giant hogweed is also distinguished by it's stout, dark reddish stem that can grow 5 cm to 10 cm in diameter. Leaf stalks are spotted and produce a compound leaf that can expand to 1.5 metres across. Each leaflet is deeply grooved or divided. Both stems and stalks are hollow and produce coarse hairs around a blister like pustule. Each year tuberous root-stalks form long-lived buds. In summer small, white flowers form together to make up an inflorescence or umbrella-shaped head that can attain a diameter of almost 1 metre.

Reproduction 

Giant hogweed reproduces through seed and perennial buds. A plant takes several years from the time it germinates until it develops a flowering stem. After flowering it sets seed and dies. Additional crowns may form on individual plants that continue to flower and develop seed. Viability of seed can exceed more than seven years. Reproduction is also accomplished when plants develop perennial buds that form on the root stalks or the crown.

Habitat 

Giant hogweed will grow in a variety of habitats but is most frequently found adjacent to streams, creeks, roads, in vacant lots or in rights of ways. It is frequently found in areas that are considered moist to wet.

Concern 

The greatest concern from giant hogweed is human health. The blister like pustules on stems and stalks exude a clear watery sap that sensitizes skin to ultraviolet radiation. Affected areas are subject to severe burns that usually result in blistering and painful dermatitis. Blisters often result in purplish to blackened scars. Giant hogweed's tenacious and invasive nature allows it to readily occupy and crowd out native vegetation. In riparian areas it forms a dense canopy, out-competing native species and causing streambank erosion.

Giant Hogweed Leaf

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