Himalayan (or Indian) Balsam is a tall flower of riverbanks and damp places, which produces a conspicuous display of various shades of pink flowers in late summer. It has a very effective seed dispersal mechanism: when the ripe pods are touched, the valves rolls up violently inwards and throw out the seeds to some distance.
It was originally introduced from the Himalayas in the 19th century, and spread rapidly throughout Britain. It is now considered a pest in many places, as it grows so profusely that it can clog up streams.

Side-on view of flower		Seed pods, with valves having coiled up after dispersing seeds