Himalayan balsam Impatiens glandulifera is extremely invasive. A native of the Western Himalaya, it was introduced in 1839 and rapidly spread throughout Britain.
It is an annual, growing up to 2.5m tall and is reputed to be the tallest annual plant found in the UK. It thrives on moist soils, especially along water-courses, and is moderately shade tolerant, so it can invade most woodlands.
Himalayan balsam flowers from June to October. Seeds are set from August until the first frosts. Each plant can produce 800 seeds. The seed-pods explode when touched or shaken, and the seeds thrown up to 5 metres. Seeds are transported by water, enabling it to rapidly colonise new sites downstream and throughout floodplains.
It is best controlled by repeated cutting early in the season, and pulling once flowers start to set seed.