Giant Hogweed (Heracleum mantegazzianum) is the next best know invasive plant in the UK after Japanese Knotweed. Both were introduced as spectacular horticultural specimens and spread to all the best Victorian gardens during the 19th Century Giant Hogweed also managed to jump the wall and it is now widespread throughout the British Isles where it frequently colonises riverbanks.
However this is where the similarity ends, Giant Hogweed is a perennial plant that can grow for up to 4 years before flowering. The flowering spike can extend to 5 metres, producing a white umbrella like flower which can be 0.5 metres across. Each plant can produce 50,000 seeds, which can remain viable for up to 15 years. These seeds are spread locally and large distances in water as well as by human intervention.
Giant Hogweed presents a serious public health threat, the stems and leaves are covered with small hairs coated with poisonous sap. Even the slightest touch can cause painful blisters and severe skin irritation. The symptoms can take a couple of days to develop, the skin becomes highly photosensitive producing large watery blisters however the effects can be long lasting, with contact resulting in recurrent dermatitis. This is a particular concern for small children who may use the stems as pea-shooters or pretend telescopes resulting in horrific injuries.
It is a threat to biodiversity out competing native species and monopolising local environments. On riverbanks once it has died back in the winter the area is vulnerable to erosion.
It is scheduled in the Wildlife & Countryside Act 1981 and the Environmental Protection Act 1990.