Himalayan Balsom | Wild South

I chose another glorious afternoon for my May Riversearch survey. It’s a joy to be out and about in the May sunshine – the trees wearing their new gowns of leaves, every wildflower bursting with the joy of life.

Butterflies and damesflies were busy making the most of the good weather, and birds were busy with bringing up youngsters.

Brimstone butterfly on a red campion, among cow parsley

The river was back to its normal level, with the Stepping Stones easily crossable (if a little slippery). There were no signs of pollution, and no fresh fly tipping.

Few people were out and about – just a few youngsters enjoying sitting round, listening to music and chatting (enjoying the start of half term, and a brief respite from exams).

I came across an intriguing hole in a tree trunk – not sure who was using it.

So much has grown up since my last survey, it was much harder to see the river. A thick wall of waist-high stinging nettles defeated me in some places – I’d need thicker trousers before I attempt to force may way through them! By the time of my next survey they may well be as tall as me, limiting the thoroughness of my data collection.

Sadly not everything was so positive. When I looked under the bridge to check for signs of otters I came across someone sleeping rough. Not a great place to set up camp for any length of time, as though the river level was ok, any heavy rain and the river would easily cover where the person was sleeping.

The other negative to my survey was signs of Himalayan Balsam returning. I only spotted a few metres of the stuff, but give it a month or two and it’s likely to have engulfed much more of the riverbank.

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?