A stroll along the riverside at this time of the year is likely to be enhanced by a blaze of beautiful pink flowers. These are the blossoms of the Himalayan Balsam which grows in dense stands up to 6ft (2m) high.
It may be beautiful but it is also an undesirable weed. The plant, which is native to the Himalyas, was introduced to UK gardens in 1839. It soon escaped and spread along the banks of rivers and streams until it has now become our most invasive species.
After flowering, the plant forms seed pods which explode when disturbed, scattering its seeds up to 23ft (7 metres).
This efficient method of seed dispersal allows it to outcompete and suppress attractive native plants such as Marsh Marigolds and Water Mint.
Another problem is that the plants die in winter leaving the river banks bare and vulnerable to erosion. Himalayan Balsam thrives where the seepage of fertilizers and sewage raises the levels of chemicals such as nitrates and phosphates.
This artificial enrichment accounts for the plant blooming so successfully along the banks of the Ouse and Adur rivers where regular sampling and analysis by the Ouse and Adur Rivers Trust (OART)shows that pollution is a serious ongoing problem.
Control of this persistent weed is difficult because while cutting and pulling out the plants on a regular basis for three years can clear a site, on river banks seeds can be washed downstream to re-colonise the cleared area.
Chemical control is not an option as this would only add another pollutant to the water.
Research suggests that the best way to control the spread of Himalayan Balsam is to decrease artificial enrichment thereby permitting the better-adapted local vegetation to rebound naturally.
For this purpose OART is working closely with farmers to promote sensitive land management and thus reduce the input of nutrients from agriculture. OART is also lobbying the water companies to improve the quality of discharged sewage effluents.