Whilst we were down in Cornwall, I was struck by the number of alien plant species that are thriving along the coast. The steep slopes above Sennen Cove are covered in dense thickets of Japanese knotweed, with an understorey of montbretia (originally from France) and three-cornered garlic (from the Mediterranean). Damp flushes on the hillside are often dominated by gunnera (from South America). Carpets of Hottentot-fig (from South Africa) are spreading along south-facing cliffs and shorelines.
The spread of invasive species seems to be a problem all along the Celtic Fringe of Europe. The hedgerows of County Kerry are over-run by fuchsia (from Argentina and Chile). A few years ago, we noticed dense stands of Himalayan knotweed around Poolewe in western Scotland. I assume that the problem stems from the exceptionally mild climate and the absence of winter frosts.
However, all is not lost. Cornwall County Council has made a lot of progress in tackling Japanese knotweed. Ten years ago, the Cot Valley was a dense jungle of knotweed - now there are just a few rather sickly looking plants which should be easily eradicated. There’s a useful account of how it was tackled here.
Interestingly, some of the areas formerly dominated by Japanese knotweed have been invaded by hemlock water dropwort. As far as I know, this species is indigenous to the British Isles... but the downside is that it's highly toxic. It's said to be responsible for more deaths by poisoning than any other plant in the UK!
Japensese knotweed = Fallopia japonica
Montbretia = Crocosmia x crocosmiiflora
Three-cornered garlic = Allium triquetrum
Gunnera = Gunnera manicata & G. tinctoria
Fuchsia = Fuchsia magellanica
Hottentot-fig = Carpobrotus edulis
Himalayan knotweed = Persicaria wallichii
Hemlock water dropwort = Oenanthe crocata