Giant Hogweed

Height: Giant hogweed can grow up to 5.5 m (18 ft) in ideal conditions, however, such sizes are rare. The plant typically grows 3-5 m (10-16 ft) in Ontario.

Leaves: Leaves are notably spiked with a jagged appearance.

Stem: Can range from 10-15 cm in diameter with course hairs. The stem can either be completely covered in purple or purple spots.

Flowers: The flowers are whitish in colour and appear in mid-June. The flowers are clustered in umbel-shaped heads and can measure up to 1 m across.

Biology: Seeds may take several years to germinate and are viable in the soil for up to 15 years. During the first year, the plant produces a rosette of leaves up to 1 m high. After 2-5 years, the plant produces flowers. As giant hogweed grows, a large root, thick hollow stems, and large lobed leaves are formed. The stems of the plant are covered with reddish-purple flecks and stiff hairs filled with sap. Sap may also collect in the hollow stem bases. Giant hogweed flowers once in its lifetime, unless the flower clusters are damaged before opening. Once the plant produces seeds it dies. Each plant can produce up to 120,000 winged seeds (typically 50,000). Seeds dropped in streams can float for three days. They can move long distances via water in ditches and streams. Seeds can also be spread up to 10 m by wind.

Giant hogweed has spread across Canada, with populations recorded in Atlantic Canada, Ontario, Quebec, and British Columbia. It often invades along roadsides, in ditches, on riverbanks, fields, and open woodlands.

Giant hogweed has a scattered distribution across southern and central Ontario, south of the line from Manitoulin Island to Ottawa. Confirmed reports of giant hogweed have been made as far north as Kapuskasing. Many counties and municipalities are taking steps to remove giant hogweed from their communities. Established populations with the greatest potential to spread are mostly found south of the line between Ottawa and Parry Sound.

Distribution Maps provided by EDDMapS

Natural resource impacts

There is evidence that giant hogweed can harm Ontario’s biodiversity by shading out native plants, although scientists have not done extensive research on this topic in Ontario or Canada. Established populations of giant hogweed can outcompete native vegetation through rapid growth in the early spring, producing large leaves which shade out other plants before they have an opportunity to grow. This creates stands of low species diversity, typically inhabited by only the hardiest of native grasses and other invasive species such as garlic mustard (Alliara petiolata). Giant hogweed can form dense stands in riparian areas. When the plants die back in the fall, soil is exposed, resulting in increased erosion and siltation of stream banks which can impact fish spawning areas.

Health concerns

The clear watery sap of giant hogweed contains toxins that can cause severe dermatitis (inflammation of the skin). Ultraviolet radiation activates compounds in the sap, resulting in severe burns when exposed to the sun. Symptoms occur within 48 hours and consist of painful blisters. Purplish scars may form that can last for many years. Eye contact with the sap has been reported (in the media and by various web sites) to cause temporary or permanent blindness, though this has not been confirmed. Similar effects result from exposure to the other two cow parsnip species found in Ontario (H. maximum and H. sphondylium) and from the widely introduced wild parsnip (Pastinaca sativa).

Agriculture concerns

Impacts of giant hogweed on agriculture are rare, however, populations have been documented as creeping into agricultural fields and requiring additional management practices from growers to minimize impacts on crop yield. Giant hogweed in pasture and forage fields reduces feed quality and increases health risks to more susceptible grazing animals (Ontario, 2018).

Giant hogweed is commonly confused with native cow parsnip. Cow parsnip is much smaller, reaching heights of 5-8 ft, and does not have the purple blotches along the stem. Leaves of the giant hogweed plant are much larger (up to 5 ft wide), shiny and serrated compared to the smaller (2-2.5 ft wide) leaf of cow parsnip which is not shiny at all. The sap of both giant hogweed and cow parsnip is phototoxic and extreme caution should be taken when near either plant.

Native Cow Parsnip

Giant Hogweed