Although an impressive sight when fully grown, giant hogweed is invasive and potentially harmful. Chemicals in the sap can cause photodermatitis or photosensitivity, where the skin becomes very sensitive to sunlight and may suffer blistering, pigmentation and long-lasting scars.
The giant hogweeds are usually referred to by one name, Heracleum mantegazzianum. RHS research shows that, while this is one of the species, there are as many as four other giant hogweeds at large in Britain some of which are biennial and others perennial. However, when tested all these had high levels of furanocoumarins (the chemicals which cause burning by making the skin sensitive to sunlight) and so all pose a risk to public health.
There is also a native hogweed, Heracleum sphondylium, which will be a familiar plant to gardeners and those who like to go walking in the UK. It can grow to six foot or so when in flower but is nevertheless a much smaller plant than giant hogweed. It can cause rashes and other skin complaints but reactions tend not be as severe as with the larger species.
The giant hogweeds were introduced into Britain and Europe from the Caucasus Mountains in the nineteenth century. The earliest documented reference to their introduction into Britain that has been traced is from the Royal Botanic Gardens Kew Seed List of 1817 where giant hogweed, under the name of Heracleum giganteum was listed among seeds supplied to Kew by the Russian Gorenki Botanic Gardens. They were soon introduced into the horticultural trade and being aesthetically impressive plants, were widely planted in ornamental gardens throughout Britain. Unfortunately they quickly escaped from cultivation with the first naturalised (‘wild’) population recorded in Cambridgeshire in 1828, and are now widely naturalised as invasive species throughout much of Britain and Europe.
Their occurrence is monitored and it appears that they are reported to be continuing to spread and can be found in every part of the British Isles. Details can be found on the Non-native Species Secretariat website.
Protect yourself from any skin contact with the sap, especially your face, when cutting stems, and carry out control measures in overcast weather avoiding sunny periods. Wash off any sap as soon as possible with plenty of cold water.
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