The battle to remove Himalayan Balsam from riverbanks by hand has become a staple activity of river conservation management. This invasive non-native species returns annually – and spreads profusely.
However, a biological method of tackling it could eventually rid our rivers of it completely. Nicky Scott, our Volunteer and Engagement Officer, reports the initial results of a trial on the River Hogsmill.
Himalayan Balsam (Latin name Impatiens glandulifera) was introduced to the UK from India and Pakistan in the mid-1800s, when Victorian plant hunters were especially prolific. It is now the most common non-native plant colonising the UK river systems.
Much like its close relative the popular Busy Lizzie, the species is an annual plant. It dies at the end of its yearly cycle.
So what’s the problem…it’s very pretty and the bees love it, right? Yes! In fact the bees love it so much (balsam honey is now very much a thing) that they are favouring it over native plants, meaning that pollination of the latter is being significantly compromised.
In addition to competition for pollinators, native plants must also compete for light, nutrients and space, leading to an overall reduction in biodiversity. Moreover, the fact that the balsam dies back in the winter means that it leaves river banks bare and susceptible to erosion, and the dead leaves and stems can also cause blockages, which lead to flooding.
What’s more, each plant can produce up to 800 seeds per year – and one plant can propel copious amounts of that seed a distance of up to seven metres. This seed can spread considerably further if carried by the river, making it certain – if unchecked – to be more widespread year on year.
As with other introduced species, Himalayan Balsam has no natural enemies in the UK and therefore grows more aggressively than it would in its original habitat in the sub-continent.
It is for this reason that the Centre for Agriculture and Bioscience International (CABI) was tasked by DEFRA and the Environment Agency with finding a biological control. After surveys for natural enemies attacking the plant in its native range, the rust fungus Puccinia komarovii was selected.
A multi-year research programme ensued in order to assess the safety and suitability of this pathogen (effectively giving the plant a virus). In 2015 a strain of the rust fungus originating from the Kullu Valley in India was approved for release.
Fast forward to 2021 and, having learned about CABI’s work, the South East Rivers Trust approached the organisation about using this form of treatment on the River Hogsmill.
Samples from the plants were collected and sent to the CABI lab for analysis to check the strain of Himalayan Balsam was suitably susceptible to rust fungus. We were delighted to hear in early 2022 that we had a match.
In June, we received the first batch of spores from CABI, to release on the Hogsmill.
Using this biological control is an element of a strategy to tackle invasive non-native species along the whole catchment, which rises in Ewell and stretches up to Kingston, taking in Tolworth, New Malden and Chessington. Two further releases were conducted in July and August.
At the end of September, we returned to see if the spores had “taken”. The evidence would be if there were rust-like spots on the underside of the leaves.
The outcome was uncertain, because of the unusually prolonged dry spell we experienced this summer. However, we were delighted to find immediate evidence of the fungus within the test plot.
It is still early days, but if the rust takes hold properly (a further release will be carried out next year) it will cause significant damage to the balsam, reducing seed production and thus the spread of the plant in subsequent years.
The rust spores are also able to survive over winter in the leaf litter and infect seedlings again in the spring, meaning that the rust should cause a substantial reduction in seedling survival.
The competitive advantage of the balsam is therefore impacted, meaning a fighting chance for native species to colonise and, pertinently, less of it to pull!
One day, the so-called “balsam bashing” events – pulling up and cutting the plants – could become a thing of the past.
However, for the foreseeable future, fighting balsam on our rivers will require a multi-pronged approach, including continuing with a sustained contribution from our committed volunteers via those ‘balsam bashing’ events.