Giant Hogweed - Heracleum mantegazzianum

Like many invasive alien plants such as Japanese Knotweed and Himalayan Balsam, Giant Hogweed was introduced into Britain in the 1800s by the Victorians who were impressed by its beauty. From these estate gardens it escaped into water courses and waste ground where, devoid of any natural enemies, it grew into dense colonies and is now one of the most invasive plants in Britain.

Habitat

It can be found in a variety of different places, particularly in areas of wasteland and riverbanks; it grows best in wet areas such as riverbanks, marshes and woodland.

Description

Giant Hogweed can be easily recognised by its sheer size. It is an enormous plant as it can grow up to 5m in height, has 10cm wide stems and 1m long leaves! The thick stems are ridged with sturdy bristles and often with dark purple blotches. Leaves are dark green and deeply notched.

A perennial plant that can take up to 4 years to mature and flower, it flowers in June and July with numerous white flowers clustered in an umbrella-shaped head that reaches up to 50cm in diameter. Once it has flowered it then produces 1,500 to 100,000 seeds.

The shoots die down in the autumn and tall stems mark its locations during winter which can help to identify the plant during winter.

Biology

The seeds produced by Giant Hogweed can remain active in the soil for up to seven years and are spread widely by wind, water and disturbance. Disturbing the soil around it will help to spread the seeds and infest new areas. It particularly rapidly colonises riverbanks, road and rail corridors.

During the summer the plants height and dense foliage is effective in preventing light reaching the ground beneath it. It therefore outcompetes native vegetation and spreads further. As a result, native vegetation can be killed off and whole areas can become covered with Giant Hogweed.

Impacts

It is a very invasive plant that spreads rapidly and is extremely difficult to eliminate once it sets root. By forming dense stands it displaces native plants and reduces wildlife interest.

The hairs on the stems and the undersides of the leaves carry a poisonous sap. The chemicals in the sap are harmful to skin and can cause severe skin blistering and light sensitivity. The watery sap leaves skin sensitive to ultraviolet light. Exposed to sunlight, the sap will cause painful blistering within a few hours of contact. Touching the plant or brushing against it is enough to cause blistering.

Control

Giant Hogweed is very difficult to eradicate once it is established. Some animals can graze upon it without being affected, but it is best to try and prevent the dispersal of the seed by removing the flower spike after it has formed, and before the flowers fade and disperse the seeds. Removing the green growth helps to exhaust the root and will weaken the plant, so digging it out and chopping it is feasible.

Chemical control can be effective and many treatments throughout a year are needed. Once the main plant has gone the numerous seeds left behind can germinate 7 to 15 years later, so ongoing monitoring is required.

Under The Wildlife and Countryside Act 1981, Schedule 9, it is illegal to cause the spread, either deliberately or accidentally, of Giant Hogweed.

The Wiltshire Wildlife Trust's Wessex Chalk Streams Project is taking a lead in recording the presence of non-native invasive plants identified as a significant threat to the characteristic species and habitats of the River Avon. One of the three of the most serious invasive plants species in the River Avon catchment is Giant Hogweed. Click here to find out more about the survey.