Giant Hogweed (Heracleum mantegazzianum) also known as cartwheel-flower, giant cow parsnip,”hogsbane” or giant cow parsley, is a plant in the family Apiaceae.
In New Zealand, which does not have any native members of the Apiaceae, it is also sometimes called wild parsnip, or wild rhubarb, It typically grows to heights of 2–5 m (6 ft 7 in–16 ft 5 in), sometimes reaching 7 m (23 ft). Except for size, it closely resembles common hogweed (Heracleum sphondylium), Heracleum sosnowskyi or garden angelica (Angelica archangelica).
Giant Hogweed is phototoxic and considered to be a noxious weed in many jurisdictions. Giant hogweed is native to the Caucasus Region and Central Asia. It was introduced to Britain as an ornamental in the 19th century, and it has also spread to Ireland, Finland, Sweden, Germany, France, Belgium, Czech Republic, Latvia, the United States and Canada.
The sap of giant hogweed causes phytophotodermatitis in humans, resulting in blisters, long-lasting scars, and—if it comes in contact with eyes—blindness. These serious reactions are due to the furocoumarin derivatives in the leaves, roots, stems, flowers, and seeds of the plant.
Giant hogweed has a stout, dark reddish-purple stem and spotted leaf stalks that are hollow and produce sturdy bristles. Stems vary from 3–8 cm (1.2–3.1 in) in diameter, occasionally up to 10 cm (3.9 in). The stem shows a purplish-red pigmentation with raised nodules. Each purple spot on the stem surrounds a hair, and large, coarse white hairs occur at the base of the leaf stalk. The plant has deeply incised compound leaves which grow up to 1–1.7 m (3 ft 3 in–5 ft 7 in) in width.
Giant hogweed is a short-lived perennial (lasting typically between five and seven years), with tuberous rootstalks that form perennating buds each year. It flowers in its final year from late spring to mid summer, with numerous white flowers clustered in an umbrella-shaped head that is up to 80 cm (31 in) in diameter across its flat top.
The plant produces 1,500 to 100,000 flattened, 1 cm long, oval dry seeds that have a broadly rounded base and broad marginal ridges. After seeds have set, the individual plant dies. Plants in earlier stages of growth die down in the autumn. Tall dead stems may mark its locations during winter.
Giant hogweed was among many foreign plants introduced to Britain in the 19th century, mainly for ornamental reasons. It is now widespread throughout the British Isles, especially along riverbanks. By forming dense stands, they can displace native plants and reduce wildlife interests.
It has also spread in the northeastern and northwestern United States and southern Canada. It is equally a pernicious invasive species in Germany, France and Belgium, overtaking the local species. It was introduced in France in the 19th century by botanists, where it is much appreciated by beekeepers.
In Canada, the plant has been sighted in British Columbia, Alberta, Ontario, Quebec, New Brunswick and Nova Scotia. It has been seen in Quebec since the early 1990s.
The plant’s spread in Ontario began in the Southwest and was seen in 2010 in the Greater Toronto Area and Renfrew County near Ottawa.
In the song “The Return of the Giant Hogweed” by Genesis, from their 1971 album Nursery Cryme, the history of the plant’s introduction to Britain is humorously recounted, and the dangers of the plant are portrayed facetiously in lines such as: “Turn and run! Nothing can stop them, around every river and canal their power is growing”.
Giant Hogweed Phototoxicity
Giant hogweed is a phototoxic plant. Its sap can cause phytophotodermatitis (severe skin inflammations) when the skin is exposed to sunlight or to ultraviolet rays. Initially, the skin colours red and starts itching. Then blisters form as it burns within 48 hours. They form black or purplish scars that can last several years.
Hospitalisation may be necessary. Presence of minute amounts of sap in the eyes can lead to temporary or even permanent blindness. These reactions are caused by the presence of linear derivatives of furocoumarin in its leaves, roots, stems, flowers and seeds. These chemicals can get into the nucleus of the epithelial cells, forming a bond with the DNA, causing the cells to die. The brown colour is caused by the production of melanin by furocoumarins.
Because of its phototoxicity and invasive nature, giant hogweed is often actively removed. In the UK, the Wildlife and Countryside Act 1981 makes it an offence to plant or cause giant hogweed to grow in the wild. It is also a common plant in marshy areas of Ireland.
Hogweed is regulated as a federal noxious weed by the US government, and is therefore illegal to import into the United States or move interstate without a permit from the Department of Agriculture. The USDA Forest Service states pigs and cattle can eat it without apparent harm.
The New York DEC has had an active program to control giant hogweed since 2008, including reporting, database maintenance, and crews for removal or herbicide control.
In 2011, Maine state horticulturists, describing the plant as “Queen Anne’s lace on steroids”, reported the plant has been reported at 21 different locations in Maine, with the number of plants ranging from one to a hundred.