Occurrence
Himalayan balsam is an introduced annual naturalised along riverbanks and ditches. It prefers moist soils but will grow anywhere. Himalayan balsam grows up to 3 m tall and is reputed to be the tallest annual plant found in the UK. A native of the Western Himalaya, it was introduced in 1839 and is now recorded throughout Britain. It grows rapidly, spreads easily, out-competes other vegetation and readily colonises new areas. Himalayan balsam is said to be relatively shade tolerant. When the plants die down in winter they leave large bare areas that are sensitive to erosion.
Biology
Himalayan balsam flowers from June to October. It is pollinated by bees. Seeds are set from August to October. There are 4-16 seeds per pod and each plant can produce 800 seeds. The seeds have a chilling requirement for germination to occur. The entire seed population germinates synchronously in spring to form a dense stand.
Persistence and Spread
Although it is currently believed that the seeds are not viable for more than 2-3 years, they are in fact very resilient to all types of conditions and therefore without committed and continuous control, the seed bank is extremely persistent.
The seedpods are dehiscent and explode when touched or shaken. The seeds are expelled up to 7m from the parent plant. The seed is transported by water but can also be carried in mud by animals and man. Himalayan balsam has spread at the rate of 645 km2 per year in the UK.