Himalayan balsam is a beautiful plant from the Himalayas in Asia, which can reach up to three metres in height and has large bright pink flowers. It may be pretty, but causes a range of problems in damp habitats across the British Isles.
When was it introduced?
Himalayan balsam was first introduced to the British Isles in 1839 as a garden plant, and by 1855 it had escaped gardens and was growing in the wild. It is now found along riverbanks and in other areas with damp soils, including marsh and woodland. It is widespread in England and Wales, and is also found in Scotland, Northern Ireland and EIRE.
This tall, fast-growing, invader grows in dense clumps that prevents shorter native plants from getting enough light to grow underneath it. As a result Himalayan balsam can take over large areas. As well as causing problems for native species, Himalayan balsam also increases the risk of riverbanks washing away because it stops the more long-lived plants such as grasses, which bind the soil with their roots, from growing. This means that when the balsam plants die in the autumn they leave bare patches of soil, which can be more easily washed away by rain.
Himalayan balsam also causes less obvious problems for native species, by luring pollinating insects away from native flowers. Like many flowering plants, Himalayan balsam produces sugary nectar to attract insects. Its flowers produce more nectar than any native European species, making it a more attractive option to pollinating insects such as bumblebees.
How can you measure such subtle effects and find out if Himalayan balsam does affect the pollination native species? Scientists investigated this by placing potted Himalayan balsam plants in experimental areas where native species were growing. This meant they could compare what happened in areas where balsam was or was not present. They counted the number of bee visits to native species and found that bees made fewer visits to native flowers when Himalayan Balsam was present. Less visits by insects, meant less pollination and the number of seeds produced by native species were reduced in areas where Himalayan balsam was present.
Controlling Himalayan Balsam
Each plant produces around 2500 seeds and exploding seed cases propel them up to five metres from the parent plant. These seeds may spread even further if they fall in rivers and are transported downstream.
Cutting, pulling up plants by hand, or applying herbicides are all good ways of controlling Himalayan balsam. It is important to pick the right time to cut or pull up this invader because if the plants are disturbed when they are fruiting, the explosive pods spread the seeds and increase the problem the following year.
Because the seeds are carried by water, populations of balsam upstream are often prioritised for control to reduce the chance of reinvasion when seeds are washed downstream. Unlike some plant seeds, Himalayan balsam seeds cannot stay dormant in the soil for long, so if a site is managed carefully the population can be dramatically reduced or eliminated in only two to three years.