Giant Hogweed (Heracleum mantegazzianum)
Introduced from Central Asia in the nineteenth century as an ornamental plant, it is now widespread throughout the British Isles especially along riverbanks.
It has a dark reddish-purple stem and spotted leaf stalks, it is capable of growing approximately 3 – 5 metres tall and is very dense. It flowers mid-May through July, with numerous white flowers clustered in an umbrella-shaped head containing approximately 1500 seeds which can lie dormant in soil for years.
Giant Hogweed contains a substance within its sap that makes the skin sensitive to ultra violet light resulting in severe burns to the affected areas, producing swelling and severe, painful blistering.
Horsetail (Equisetum arvense)
A perennial plant also called ‘Marestail’ or ‘Pipeweed’. It is an ancient relic of primeval times which is native to the UK, some of the earlier species reached up to 30 metres in height.
Horsetail reproduce by spores rather than seeds and has similar problematic properties to Japanese Knotweed, although not as aggressive.
It is able to push up through hard surfaces such as asphalt, has an extensive rhizome system which once fragmented can re-grow to form new plants.
Himalayan Balsam (Impatiens glandulifera)
Introduced in 1839 from Western Himalaya, Himalayan balsam is now a common weed along most of Britain’s river banks and in ditches. It can grow up to 3 metres annually from a seed bank which can stay viable for up to 2 years.
The seedpods explode when touched or moved by the wind expelling them up to 7 metres from the parent plant. This dense plant out competes the native fauna reducing bio-diversity and the lower level vegetation, then in winter when it dies back it exposes the soil to erosion. Its flowers produce high levels of nectar which makes them attractive to bees and other insects resulting in less pollination of native species.
Ragwort (Senecio jacobaea)
Common Ragwort contains pyrrolizidine alkaloids (complex molecules) which are poisonous to equines and bovines. When consumed, the alkaloids build up over time and cause damage to the liver, sometimes resulting in death.
Ragwort is also harmful to humans as it can enter the bloodstream through contact with the skin. It may cause an allergic reaction called Compositae Dermatitis.