The day had been sweltering and, even in the relative cool of early evening, people were picnicking on the river bank and kids splashing in the water. They had no idea that just metres away lurked an alien invader, laying siege to the River Mole, and threatening to take over completely.
Like Scarlet Johannson in Under the Skin, this alien’s danger was disguised by a pretty face. The pink flowers of Himalayan Balsom distract from its invasive nature. The plant can grow up to 2.5m tall, and forms dense swathes. When the seed pods are ready they explode dramatically, firing seeds into the river, where it can be swept for miles downstream, before settling on a new stretch of river bank.
As well as crowding out other species of plant, the shallow roots of Himalayan Balsom can cause erosion of riverbanks. Thick stands of it may also increase the risk of flooding.
Luckily, although it spreads quickly, it’s easy to pull up. Further downstream in Leatherhead volunteers with Surrey Wildlife Trust have done quite a lot of clearance work. But because it spreads downstream so easily, it’s hard to eradicate it permanently from a small stretch of river.
Himalayan Balsom is one of the non-native invasive species I look out for during my Riversearch surveys. And unfortunately I found quite a bit on my latest survey.
Once again the river bank has changed dramatically with the seasons. This time much of the river was inaccessible, as shoulder high nettles formed a barrier I wasn’t willing to test in light trousers and short sleeves.
The river level was low, and apart from the Himalayan Balsom there was little new to report. I did find a rather intriguing stash of fruit between some tree roots. Any idea what could have hoarded that?