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The aliens (might be) landing !

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Rhododendron x superponticum dominating waterside habitat in Yorkshire. Maybe the otters like it for cover but I can’t imagine much else does.

I wrote the following for Pro-Landscaper magazine, last year – i.e. for a British Isles audience. So please realise that issues may well be very different elsewhere. . . . .

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The press love a good invasive alien story. Shades of martians landing and/or man-eating triffids on the prowl. There is often a hint of racism too, invasive plants almost inevitably come with national labels: Japanese knotweed, Spanish bluebells, and the language used to describe them is not unlike that used to discuss immigration issues in certain quarters.
The landscape industry is very much in the front line here, both in preventing the use and spread of invasive aliens and sometimes in their control too. But how much of a problem do we really face?
I would argue that the invasive alien story is in danger of being grossly exaggerated, and those of us who work in horticulture and landscape need to keep a cool head. First of all, we need to realise how lucky we are. While some countries battle enormously damaging invasive species, Britain faces relatively few real problems. We have an amazingly aggressive natural grass flora, which has evolved to benefit from the exceptionally long growing season we have – the result of our being on the north-west fringes of Europe, facing the warm waters of the Atlantic. Our wild grasses have an incredible ability to spread, propagate and suffocate most of what comes in their way. They may create problems in establishing garden or landscape plants but they are a great defence against invaders.
So what problems do we face? The obvious answer is Japanese knotweed (Fallopia japonica); the press love stories about it, and the government has responded by enacting legislation that potentially adds considerable costs to landscaping and construction projects. However, it does not seed, it is suffocated by trees, is easily killed with herbicide, eliminated by mowing and makes little headway against grasses. Neither does it kill small dogs (unlike, we are told, seagulls). It is a big problem in a very small number of localities. The main reason for its spread has been the moving of infected soil, something entirely preventable. It is important to realise it is not going to engulf the country.
‘Perspective’ is one thing which those who get very excited by invasive aliens find difficult to maintain. Particularly important is to recognise the difference between the spread of a species and it being problematic. Buddleia is a good example. Its appearance on buildings worries property owners (rightfully) but its extensive seeding into waste ground creates an impression that it has capacity to spread. This is liable to alarm those with a dogmatic understanding of ecology, who believe that only native species have a right to be here. Given time, buddleia gets suppressed by native grasses and in particular by our native brambles and shrubs. However even at its most vigorous it grows alongside other plants (and of course butterflies love it).
The plants we need to worry about are those that 1) do not get suppressed by our native vegetation, and 2) get the better of it, even though these may only be problems in particular places. Rhododendron x superponticumis a good example of something that does both of these; spreading even in the shade of woodland and suppressing almost anything which grows in its dense shade. Certain cotoneasters might be another, but only with regard to very localised habitats (cliff faces). With time many seemingly aggressive species decline, as local infective agents and pests discover them – there is evidence that this has happened with Himalayan balsam (Impatiens glandulifera) in continental Europe. Given the costs and difficulties of eradication, keeping a cool head and focussing on identifying real problems, not headline-generating ones, is vital.
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