Himalayan balsam may look beautiful, but the plant is one of our most invasive species. It erodes riverbanks, silts up rivers thereby contributing to flooding, and kills off native flora wherever it grows. Until now, the only way to tackle it has been ‘balsam bashing’ – pulling it up by hand, but volunteers can only ever have a limited impact. So now scientists have turned to a more high-tech solution -a bio-control called rust fungus.

The fungus will not completely eradicate Himalayan balsam from our countryside, but it should make the plant less vigorous and so less competitive, enabling native wildflowers to re-establish themselves where the nonnative species dominates. Field trials began in 2014, and these were extended to 25 sites in 2015. Now the fungus has been released into the Calder Valley in West Yorkshire.

And because the pathogen co-evolved with Himalayan balsam, scientists say that it has a high degree of specificity to the target plant, which means it is most unlikely to attack our native flowers. “The classical bio-control of weeds is going back to the centre of origin of the weed, finding host-specific co-evolved natural enemies and releasing them in the invasive range,” said Dr Carol Ellison, senior plant pathologist at the Centre for Agriculture and Biosciences International (CABI), the organisation behind the work. “It will be 5-10 years before we see a significant impact,” she added, “because that is how long it takes for the bio-control agent to build up. But the other side of the coin is that it is permanent.”