Heracleum mantegazzianum, commonly known as giant hogweed, is a monocarpic perennial herbaceous plant in the carrot family Apiaceae. H. mantegazzianum is also known as cartwheel-flower, giant cow parsnip, or hogsbane. In New Zealand, it is also sometimes called wild parsnip (not to be confused with Pastinaca sativa) or wild rhubarb.

Description

thumb|upright=.75|Green, red-spotted stem with white hairs

Giant hogweed typically grows to heights of . The leaves are incised and deeply lobed. A mature plant has very large leaves, wide, sometimes even up to wide and up to long and a stout, bright green stem with extensive dark reddish-purple splotches and prominent coarse white hairs, especially at the base of the leaf stalk. The hollow, ridged stems are in diameter, occasionally reaching diameter, and can grow to more than high. Dark red spots on the stem each surround a single hair. The umbrella-shaped inflorescence, known as a compound umbel, can reach a diameter of up to across its flat top. The flowers are white or greenish white and may be radially symmetrical or strongly bilaterally symmetrical (zygomorphic).

  1. Pre-flowering plants: In the first year, leaves sprout from the seed. In subsequent years, leaves grow from the overwintering roots, producing a rosette of leaves at ground level with no above-ground stem. This pre-flowering phase can last for several years.
  2. Flowering plants (midsummer): After several years of growth, the plant flowers, sending up an erect stem from the rootstock.
  3. Seeds (late summer/early autumn): A flowering plant can produce 20,000 seeds.
  4. Dead stems (late autumn/winter): After producing seeds, the plant dies, leaving dried stems and seed heads standing; the seeds slowly fall and are dispersed by wind and animals.

Emerging seedlings tend to be morphologically diverse, with jagged pinnate forms and more rounded leaf shapes being sighted almost always occurring within the same seed bank. During the first few years of growth, the leaves and stem of a pre-flowering plant die back over winter. In spring, the plant grows back from its root. In other words, the giant hogweed is a herbaceous perennial.

A giant hogweed plant usually produces a flowering stalk in 3–5 years, but plants may take up to eight years to flower if conditions are unfavourable. In the Czech Republic, a single plant reached twelve years old before flowering. In any case, when the plant finally flowers, it does so between June and July (in the northern hemisphere).

Seeds are typically produced in August. On average, a single flowering plant will produce 20,000 seeds It has spread in the northeastern and northwestern United States and southern Canada, and is an invasive species across western Europe; The plant's spread in Ontario began in the southwest and it was first observed in the Greater Toronto Area and Renfrew County near Ottawa in 2010.

In the US, giant hogweed occurs in Maine, Wisconsin, and south to Indiana, Michigan, Maryland, and New Jersey. In June 2018, it was reported growing in Virginia and North Carolina. The plant is federally listed as a noxious weed in the US.

The spread of H. mantegazzianum throughout Europe continued unabated until the mid-20th century, by which time the dangers posed by giant hogweed had become better known. Despite these warnings, however, the plant continued to be used by gardeners, beekeepers, and farmers (for cattle fodder) for another 50 years. Heracleum mantegazzianum was finally delisted by the Royal Horticultural Society of Great Britain in 2002.

Introduction to North America

During the 20th century, giant hogweed was brought to the US and Canada to be displayed in arboretums and Victorian gardens. The earliest recorded instance of its planting in North America was in 1917 in gardens near Highland Park in the city of Rochester, New York.

Giant hogweed first appeared in southern Ontario by 1950, and within a quarter of a century the plant had become firmly established there. The first collections were made in Nova Scotia in 1980 and in Quebec in 1990. Giant hogweed remained on sale in Canadian nurseries until 2005.

On the west coast of North America, H. mantegazzianum appeared in Oregon, Washington, and southwestern Canada, but it is unclear how the species arrived in this region. The first reports of giant hogweed in British Columbia date back to the 1930s.

Russia

Heracleum mantegazzianum has not spread in Russia. Another similar very large hogweed species, H. sosnowskyi, also native to the Caucasus region, was introduced into Russian agriculture starting from 1947 as a fodder plant and later spread extensively on its own throughout Russia and some other countries of eastern Europe. On 2 August 2017, it added the species to its List of invasive alien species of Union concern, thus placing restrictions on keeping, importing, selling, breeding and growing it, and requiring governments to detect and eradicate it throughout the EU. In the United Kingdom, the Wildlife and Countryside Act 1981 makes it an offence to plant or cause giant hogweed to grow in the wild.

In the US, hogweed is regulated as a federal noxious weed, and it is illegal to import it into the US or move it across state borders without a permit from the Department of Agriculture (USDA). The New York State Department of Environmental Conservation has had an active program to control giant hogweed since 2008. In 2011, Maine state horticulturists reported that it has been found at 21 locations in Maine, with the number of plants ranging from one to a hundred.

Hybrids

Hybrids have been reported between invasive specimens of H. mantegazzianum and native populations of H. sphondylium in Britain; characteristics of such plants tend to be intermediate between the two species. Hybrids only tend to survive where H. sphondylium is the maternal plant.

Other specimens have been recorded which have intermediate morphology, but whose hybrid status is uncertain.

Phototoxicity of sap

thumb|right|Giant hogweed flower head

The sap of the giant hogweed plant is phototoxic. The sap contains furanocoumarins, which leads to phytophotodermatitis, a serious skin inflammation. A phototoxic reaction can begin as soon as 15 minutes after contact with the sap. Photosensitivity peaks between 30 minutes and two hours after contact but can last for several days. Protective clothing, including eye protection, should be worn when handling the plant. Parts of the body that come into contact with the sap of giant hogweed should be immediately washed with soap and cold water, and further exposure to sunlight should be avoided for at least 48 hours.

Poisoning via ingestion by humans is not known to occur, discounting phototoxicity from contact with external tissues, and consumption of small amounts of the plant is not deemed to be harmful. However, it should still be avoided due to the obvious extreme hazards created by proximity to the plant. Consumption by other grazing animals does not appear to cause them harm, and sheep have been trialled as a giant hogweed control agent in Scotland. The USDA Forest Service states that pigs and cattle can eat it without apparent harm.

The 1971 album Nursery Cryme by the progressive rock group Genesis contains the song "The Return of the Giant Hogweed". The darkly humorous lyrics describe an attack on the human race by the hogweed, long after the plant was first 'captured' and brought to England by a Victorian explorer.

In her 1985 novel Curse of the Giant Hogweed, popular mystery author Charlotte MacLeod places her established character Peter Shandy and his colleagues in a fantasy version of Wales to investigate giant hogweed endangering Britain's hedgerows.

The plant is featured in season 10, episode 3, of the AMC television series The Walking Dead, growing from zombies which a character encounters, rendering him unable to see properly.

See also

  • List of poisonous plants

References

Bibliography

  • Giant Hogweed (Heracleum mantegazzianum): A Federal Noxious Weed U.S. Department of Agriculture
  • Photo of blisters caused by the plant (graphic) from the Finnish Environment Institute (archived 2012-02-06)
  • Surveys for natural enemies of giant hogweed (Heracleum mantegazzianum) in the Caucasus region and assessment for their classical biological control potential in Europe
  • "Beware Giant Hogweed!" brochure from New York State Department of Environmental Conservation
  • "Giant Hogweed in Germany"
  • Species Profile – Giant Hogweed (Heracleum mantegazzianum), National Invasive Species Information Center, United States National Agricultural Library.
  • Antonova, Maria, "A Toxic Alien Is Taking Over Russia", The New York Times, 2020-10-03
  • The Day of the Giant Hogweed! John Harrison