Himalayan balsam is a non-native invasive terrestrial plant species. The species is particularly frequent along the banks of watercourses, where it often forms continuous stands. Individual plants reach 2m in height, have translucent fleshy stems, pink-purple slipper-shaped flowers and large oval pointed leaves. Plants produce large numbers of flowers which are followed by ‘seed pods’ about 25mm long. When mature and dry, they split open explosively, dispersing the seeds a considerable distance from the parent plant. Each plant can produce about 2,500 seeds which fall to the ground, and with several parent plants close together, seeds can occur at a density of between 5000-6000 seeds per square metre, with plants quickly outcompeting the native flora. The seeds also float, making watercourses a prime route for dispersal of the species.
The fact that seeds are easily transported along watercourses makes control of the species particularly difficult. This is because although the landowner may be controlling plants on their property, if the same doesn’t happen upstream then recontamination of the site is almost inevitable. Seeds can remain viable for 18 months, so a three year herbicide treatment programme is usually required as a minimum starting point to control the established plants on the site, with annual monitoring following on from that.