This very attractive plant can frequently now be seen flowering beside rivers and increasingly along roadsides.   The Policeman’s Helmet or Himalayan balsam (Impatiens glandulifera) is a relative of the busy Lizzie, but can grow over 3 metres or ten feet tall in one season. It was introduced in 1839 but quickly escaped from gardens and has become very invasive and a major problem smothering the native vegetation. Each plant can produce 800 seeds a year and their explosive pods can shoot seeds up to 7 m or 22ft away, which then stick to shoes and car tyres so facilitating their spread. Bees are also attracted to the flowers and can spread the seeds widely.
An interesting article about the need to eradicate the plant is here.