Invaders of the riverbank
Study on curbing nuisance shrub
Research by academics at Stirling University could help improve the management of one of the area’s most invasive plants.
Himalayan balsam, which has pink flowers and can grow to head height, is now a feature along the banks of the Forth in places such as Riverside, Forthside and Kildean.
It spreads rapidly and like giant hogweed has been blamed for crowding out native species and reducing the stability of riverbanks, enabling silt to enter the water.
Stirling Council is taking steps to tackle the spread but only on land owned by them.
Staff at the university embarked on a study to find out why the proliferation of the plant, which originates in the Himalayas and was introduced to the UK in the 1830s, varies dramatically from place to place.
Dr Zarah Pattison, of the university’s faculty of natural sciences. led the research team which conducted field surveys along 20 rivers across the Central Belt .
She explained their research found that the shrub dislikes conditions that are too wet, unlike native plants such as nettles, butterbur and canary grass which dominate lowland riverbanks.
According to Dr Pattison, balsam prefers drier, steeper riverbanks where it can compete more effectively with the native plants.
She added: “Over the last couple of years, the erratic temperature changes have had odd effects on balsam.
“For example, this year we had a late frost which impacted balsam growth, then an extended period of dry conditions. So any rainfall would have been welcomed by the plant.
“The balsam is a lot shorter this year and has flowered quickly.
“In our study we describe conditions at sites which have been saturated by continuous rainfall and/ or extensive flooding. This reduced the abundance of balsam.”
She believes their findings offer a gateway to managing Himalayan balsam indirectly, by manipulating conditions on riverbanks.”
“River engineering often involves straightening and over-deepening rivers and, combined with the abstraction of water, this leads to drier riverbanks during the summer, benefitting Himalayan balsam growth,” she added.
“This effect of riverbank drying may also be exacerbated with future climate change and drought conditions, as seen this summer across the UK.
“In contrast, the restoration of rivers often results in gently sloped banks, meaning water is retained and riverbanks are therefore moister, favouring native species.”
The authors also found that riverbanks with a large abundance of native plants are more likely to resist invasion by Himalayan balsam.
Dr Pattison believes the findings will aid river management by helping to pinpoint resources in attempt to control Himalayan balsam.
“The UK spends an estimated £1.7million on managing invasive species, including thousands of man-hours manually removing or spraying species such as Himalayan balsam,” she said. “Therefore, understanding the conditions which benefit the growth and spread of this species will enable better management and use of resources, in order to control the amount and spread of Himalayan balsam.”
Research conducted by the team was funded by Scottish National Heritage, Scottish Environment Protection Agency and University of Stirling.
In a project run by Forth Fisheries Trust, volunteers have for six years been battling to keep giant hogweed from spreading along the banks of the Forth in Cornton and the Allan.
A spokesperson for Stirling Council admitted the priority for them this year was dealing with giant hogweed but said they were aware of the spread of Himalayan balsam.
“The warm summer coupled with the recent wet weather has led to the proliferation of a variety of plant species along the Forth. Our officers focus on treating council land along the river, which runs from the confluence of the Forth and Teith downstream towards St Modan’s High School.
“We have no power to treat the spread of this plant on private land.”