***Update: 10 June 2024***

CABI have confirmed the Himalayan Balsam seed collected at both sites near the River Stort are fully susceptible to the pathological rust fungus. This is encouraging news and the Trust will be going ahead with the releases of the rust fungus at the two sites in mid-late June and then monitoring the subsequent infection.

Herts and Middlesex Wildlife Trust is the leading voice for conservation in the region and as part of Invasive Species Week (20-26 May), the charity is unveiling an innovative approach to tackling Himalayan Balsam – a plant that may look attractive but is a menace to the balance of our ecosystem and to the stability of our riverbanks.

Himalayan Balsam was first introduced to this country in 1839 from northern India as a garden plant. With a fast-growing nature and explosive seed pods, the plant quickly escaped and it has gone on to invade wet habitats. With its seeds travelling downstream, riverbanks have become infested and eroded by the plant, which is our largest annual, reaching heights of 2-3m. The plant dominates riverbanks, overshadowing native flora and the stability that their roots provide. In turn, riverbanks become eroded and lose the vital and varied space needed for the survival of many wildlife species.

Hertfordshire is home to 10% of only around 220 chalk rivers found worldwide. These unique river systems are so rare, and support some of our most endangered species – they are the UK’s equivalent to tropical rainforests. Coupled with that, The State of Nature 2023 report found that nearly one in six of the more than ten thousand species assessed are at risk of being lost from Great Britain. Conservation management initiatives and suppressing the growth of Himalayan Balsam, are therefore crucial to nature’s recovery.

Thanks to funding from the Environment Agency, Affinity Water and Lee Valley Regional Park Authority, the Trust is working with CABI, a nonprofit intergovernmental development and information organisation, focusing on agricultural and environmental issues, to trial a novel biological treatment – pathological rust fungus, which is found in the native range of the plant in the foothills of the Himalayas. This is a naturally occurring fungus which grows on the leaves of the plant and stunts its growth, thereby reducing its height and dominance. The fungus has been tested to make sure it doesn’t impact on other species, is able to grow in our climate and is able to reproduce. Seeds taken from the area to be treated are grown on by CABI to see if they are susceptible to the controlling effects of the fungus.

Josh Kalms, People and Wildlife Officer at Herts and Middlesex Trust said:

“It’s early days and we are learning from previous trials but this is very exciting and could be a real breakthrough for the way we manage Himalayan Balsam on our riverbanks. To date, the option has been to hand-pull the plant and although its roots are shallow and come away easily, it’s a major task to undertake – and, with it being an annual it’s one that needs repeating year-on-year.

“We hear a lot about the poor health of our rivers but rarely do we hear about the impact invasive species can have upon them. These can be truly devastating for our natural world. Himalayan Balsam shades out native plants, upon which the survival of invertebrates depends, because many rely on particular wetland species for food. Those native plants with their variety of root structures also uphold the structure of our riverbanks and as they disappear so does the stability around the water’s edge. As well as increasing sedimentation in our rivers, this impacts on the burrows of our most endangered mammal, the Water Vole, as erosion means their homes get washed away.  

“I must express my thanks to all of the region’s river groups and volunteers who work tirelessly to control Himalayan Balsam by ‘balsam bashing’ - a manual process to remove the plants before they set seed. If the trials are successful, we can all look forward to saving time and money but most importantly to seeing wildlife come back, greater biodiversity and the stabilisation of our riverbanks.”

Alexandra Sage, Senior Asset Scientist – Project Manager, Catchment and Biodiversity Team, Water Resources and Environment at Affinity Water said:

“We are thrilled to support innovative projects through our INNS Out scheme. Himalayan Balsam is one of the most prevalent invasive species in our supply area, requiring significant volunteer and contractor efforts for control each year. We're eager to see the trial results, which could greatly benefit us and our partners in combating Himalayan Balsam. If you're also dealing with invasive non-native species and need support, please visit our website for more information on our INNS Out scheme.”

If CABI’s tests are deemed successful, the Trust will be trialling application of the pathological rust fungus in two areas on the River Stort, close to the charity’s Thorley Wash Nature Reserve. Seeds will also be collected in the Ash valley with a view to a further trial there next year.

According to the government, over 2,000 plants and animals have been introduced to Britain from all over the world by people. These are known as non-native species. Most are harmless but around 10-15% spread and become invasive non-native species, which impact the environment, economy or our health and way of life.