A breakwater of boulders, installed to prevent erosion of Low Barns Nature Reserve, has gradually back-filled with river gravel and silt, which now supports a forest of Himalayan balsam, one of the fastest spreading alien species. There's been a good deal of angst about this domineering invader because it grows faster and taller that any native annual. This morning I had to force my way through its three metre-tall waxy, succulent stems to reach the riverbank, under a bombardment of seeds from its exploding seed pods. But honeybees love it. The balsam forest is still producing thousands of blooms when few other nectar sources are available. "Himalayan balsam bashing" has become a favourite way for conservation working parties to let off steam but trying to control this invader may be a lost - perhaps even counterproductive - cause, given its easy avenues of opportunity along river corridors, prolific production of dormant seeds and value to hard-pressed honeybees. Its dominance may only be temporary anyway. When I first walked here a quarter of a century ago, the gravel banks were covered in drifts of monkey flower, an invader from Alaska. This afternoon I saw just three plants. Our flora, like our climate, is in a state of flux, which is what makes it so interesting.