Giant Hogweed: large, hairy perennial herb in the Parsley Family with stout taproot of fleshy, fibrous roots; hollow green stems with purple spots grows to 5 metres tall; dark green, coarsely toothed leaves divided into 3 large segments; lower leaves can exceed 2.5 metres in length; small white flowers are produced in large flat-top umbrella-like terminal clusters up to 0.8 metres across

Giant Hogweed is a phototoxic plant. Its sap can cause photodermatitis (severe 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 that can last several years. See this short video.

Hospitalisation may be necessary.[1] 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.

Children should be kept away from Giant Hogweed. Protective clothing, including eye protection, should be worn when handling or digging it. If skin is exposed, the affected area should be washed thoroughly with soap and water and the exposed skin protected from the sun for several days.

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Countermeasures

Because of its phototoxicity and its 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.[1][2]

It is also a common plant in marshy areas of Ireland although the extent of its dangers is not commonly known there.

Hogweed is regulated as a federal noxious weed by the U.S. Government, and is therefore illegal to import into the United States or move interstate without a permit from the Department of Agriculture.[3]