The three non-native plants most commonly encountered in Britain that cause concern to homeowners, landowners and developers are Japanese knotweeds, Giant hogweed, and Himalayan balsam.
Japanese knotweed has become increasingly well known in recent years and is a growing commercial problem because of the challenges it causes in the urban environment. The plant, which is native to eastern Asia, was introduced in the early 19th century to adorn the gardens of Victorian England.
As early as the beginning of the 20th century it was widely recognized as an invasive species. Where the plant grows on development sites it can cause damage to hard structures and surfaces. Developers also often need to tackle the plant in order to avoid contravening the Wildlife and Countryside Act 1981.
Giant Hogweed was brought in to the UK as an ornamental plant. It is a native of South-Eastern Europe and is a member of the carrot family. Generally, it grows near watercourses and in damp meadows, though it can be found on waste ground where conditions are right. It is a highly invasive plant that grows vigorously.
Each plant can produce up to 50,000 seeds which can survive for up to 15 years. Giant Hogweed is capable of growing to a height of up to 5 meters.
Introduced to the UK in 1839 from Northern India, Himalayan or Indian balsam is most commonly found on river banks and damp areas, though it is capable of thriving in many other habitats.
The dense stands on river banks to impede the flow in flood conditions, exacerbating flooding. They also shade out native plant species.