Giant hogweed (Heracleum mantegazzianum)

Giant hogweed was first introduced to North America as a horticultural plant in the early 1900s. Since then, it has escaped from cultivation and become widespread in the northern US and southern Canada. Native to the Caucasus region between south-western Asia and Europe, it is now an invasive species in seven Canadian provinces as well as many US states and European countries.

Giant hogweed is a biennial or short lived perennial herbaceous plant with a long taproot, and a thick stock that can reach heights of 6 m when flowering. The stock is hollow and purple spotted, with coarse, white hairs. The leaves are alternate, compound, and deeply divided. Giant hogweed is typically found growing in rich, moist soil along ponds, streams and ditches, or on forest edges. During its first year of growth, it produces a large rosette of leaves. It then flowers between it’s second and fifth year, after which the plant dies. Small white flowers are produced in large umbels at the top of the stem, usually between July and August. Once it has flowered the plant dies, but not before producing up to 100,000 seeds, which are spread by wind, water, and human activity.

There are a few native species that are commonly mistaken for giant hogweed, including cow parsnip (Heracleum lanatum), wild parsnip (Pastinaca sativa) and great angelica (Angelica atropurpurea). Cow parsnip has smaller flower heads and is a shorter plant usually only reaching 2 m in height. It may have some purple colouration on the stem but not in distinct spots like giant hogweed. Wild parsnip has yellow flowers, is a shorter plant usually only reaching 1.5 meters in height, and has leaves that are divided into leaflets. Great angelica is a shorter plant, with a hairless stem. It has rounded topped flower clusters, blooms in late summer and has leaves that are divided into leaflets.

Ecologically, giant hogweed is a threat to native biodiversity. It is able to outcompete native species due to its vigorous growth early in spring. Furthermore it is tolerant of full shade and seasonal flooding. The large plants form a dense canopy that shades out native species. Along stream banks, roots of giant hogweed do not hold the soil as well as native vegetation, which may lead to increased erosion and soil loss.

In addition to its ecological threat, giant hogweed is toxic to humans. The sap contains toxins called furocoumarins, which can increase the skin’s sensitivity to sunlight, causing a condition called phytophotodermatitis. The sap is found in all parts of the plant, and contact occurs by brushing against any broken part of the plant or by handling plant material. If the contacted area is exposed to sunlight, severe burns and blistering may result. Temporary or permanent blindness may occur if the sap enters the eyes. For this reason, removal of the plant is difficult and it is recommended that landowners seek professional assistance.

*CVC is interested in any locations of giant hogweed and is tracking the population within our watershed. Contact us at 905-670-1615.