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Giant Hogweed

Giant Hogweed (Heracleum mantegazzianum), or Giant Cow-parsley, is a member of the family Apiaceae, native to the Caucasus Region and Central Asia.

As its name indicates, it is characterized by its size and may grow 2-5m (rarely to 7 m) tall. Except for size, it closely resembles Common Hogweed (Heracleum sphondylium), Heracleum sosnowskyi or Garden Angelica (Angelica archangelica).

 

It is further distinguished by a stout, dark reddish-purple stem and spotted leaf stalks that are hollow and produce sturdy bristles. Stems vary from 3-8 cm in diameter, occasionally up to 10 cm. The stem shows a purplish-red pigmentation with raised nodules. Each purple spot on the stem surrounds a hair, and there are large, coarse white hairs at the base of the leaf stalk. The plant has deeply incised compound leaves which grow up to 1-1.7 m in width.

Giant Hogweed is a perennial with tuberous rootstalks which form perennating buds each year. It flowers from late spring to mid summer, with numerous white flowers clustered in an umbrella-shaped head that is up to 80 cm (2.5 ft) in diameter across its flat top.

The plant produces flattened, 1cm long, oval dry seeds that have a broadly rounded base, and broad marginal ridges.

The Giant Hogweed flowers from late spring to mid summer, and then produces numerous, large flattened elliptic dry seeds (between 1,500-100,000). Shoots die down in the fall. Tall stems mark its locations during winter.

Many foreign plants were introduced to Britain in the 19th century, mainly for ornamental reasons. A few have become aggressively dominant, creating serious problems in some areas. It is now widespread throughout the British Isles especially along riverbanks. By forming dense strands they can displace native plants and reduce wildlife interests. It has also spread in the northeastern and northwestern United States. 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, much appreciated by beekeepers.

In the UK the Wildlife and Countryside Act 1982 makes it an offence to plant or cause Giant Hogweed to grow in the wild.

Toxic

Giant Hogweed is a phototoxic plant. Its sap can cause photodermatitis, skin inflammations when the skin is exposed to sunlight or to UV-rays. Initially the skin colours red and starts itching. Then blisters form as in burns within 48 hours. They form black or purplish scars, which can last several years. Hospitalisation may become 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. In Germany, where this plant has become a real nuisance, there were about 16,000 victims in 2003.

Keep children away from this plant. Wear protective clothing when handling it if you dig plants; consider wearing eye protection. Chopping out the root is feasible, but may have to be done several times as the plant re-grows. Wash off exposed skin thoroughly with soap and water and protect the exposed skin from the sun for several days.

Keep the plant mowed down during the summer to prevent seeds from maturing. Don't allow pieces of it to land on bare skin. Even after the parent plant is completely removed, the numerous seeds left behind can come up, 7 to 15 years later. Ongoing monitoring is required. Removing the green growth will help to exhaust the root and will weaken the plant, so digging it out (as far as 60 cm deep) and chopping it is feasible.

2,4-D, TBA, MCPA and dicamba will kill above ground parts but are reportedly not particularly effective on persistent rootstalks. Glyphosate (Roundup) is considered the most effective herbicide and should be used cautiously around desirable species since it is nonselective. Application during bud stage and while the plant is actively growing is recommended by New York Cooperative Extension.

Close up Hogweed

Source.

Close up view of Hogweed

Musical reference

"Return of the Giant Hogweed" by Genesis, on the album Nursery Cryme (1971) tells the story of "regal hogweed" being brought from Russia by a Victorian explorer to the Royal Gardens at Kew and how it was spread throughout England by "fashionable country gentlemen". The hogweed then "prepares for an onslaught, threatening the human race" and people must run for shelter and attack it at night, as it cannot photosensitize its venom.

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