Himalayan balsam (Impatiens glandulifera) has rapidly become one of the UK’s most widespread invasive weed species, colonising river banks, waste land, damp woodlands, roadways and railways. It reaches well over head height, and is a major weed problem. Growing and spreading rapidly, it successfully competes with native plant species for space, light, nutrients and pollinators, and excludes other plant growth (through shading and smothering), thereby reducing native biodiversity. As an annual, Himalayan balsam dies back in the winter, and where the plant grows in riparian systems this can leave river banks bare of vegetation and liable to erosion. Dead plant material can also enter the river, increasing the risk of flooding.
Like most introduced plant species Himalayan balsam arrived in the UK (in 1839) without any of the natural enemies that help keep the plant in check in its native range (the foothills of the Indian and Pakistani Himalayas). Without these natural enemies, Himalayan balsam is able to grow more aggressively and has a greater ability to reproduce, giving it an advantage over native species. Traditional control methods are currently inadequate in controlling Himalayan balsam in the UK. This is often because the plant grows along river banks and for any control measures to be successful, control needs to be undertaken on a catchment scale. Many of the areas colonised by Himalayan balsam are inaccessible areas or sites of high conservation status where chemical and/or manual control is deemed to not be an option. Economically, using existing measures, the UK’s Environment Agency estimates that it would cost £300 million to eradicate I. glandulifera from the UK.