Himalayan balsam (Impatiens glandulifera) is an introduced summer annual that has naturalised in the UK, mainly along riverbanks and ditches. It prefers moist soils but will grow pretty much anywhere. Plants can grow up to 3m tall, making this the tallest annual species growing wild in the UK.
A native of the Western Himalaya, it was introduced in 1839 to Kew Gardens as a greenhouse exotic. It escaped into the wild and is now recorded throughout the UK, particularly along the banks of watercourses. It is locally common in Wales and England. It is less frequent in the Highlands of Scotland and western parts of Northern Ireland, but it is rapidly increasing its range. It grows quickly, spreads easily, readily colonises new areas and out-competes other vegetation. When the plants die back each winter they leave large areas of bare ground that can be sensitive to erosion.