Giant hogweed (Heracleum mantegazzianum) is a close relative to cow parsley, but can reach over 3m (10ft) in height. Although the plant can be attractive in certain situations, most gardeners will want to eradicate it, as it is an invasive species with the sap the potential to severe skin burns. Giant Hogweed is an impressive plant that was once planted in gardens. However, it is highly invasive and has spread throughout the whole of the country, primarily favouring river banks but also other areas such as parks, cemeteries and wasteland.
The sap of Giant Hogweed contains toxic chemicals known as furanocoumarins. When these come into contact with the skin, and in the presence of sunlight, they cause a condition called phyto-photodermatitis: a reddening of the skin, often followed by severe burns and blistering. Which can take as little as 15 minutes to appear! The burns can last for several months and even once they have died down the skin can remain sensitive to light for many years. The comparison to cow parsley can be easily forgiven. It is a tall hollow-stemmed plant in the summer and dies back during the winter. However, cow parsley is not an invasive species!
We have treated this plant in numerous different situations. Timing is crucial as it needs to be treated before the seed head develops. If the seed heads develops it can be removed to prevent the seeds being spread.