Beware the alien invaders

THE British Isles is in the grip of an alien invasion of Triffid-like proportions.

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Across the land there are plants on the move capable of breaking through concrete and forming lethal dense green carpets smothering our rivers, lakes and ponds. This might sound like the stuff of a Malcolm Bradbury novel or an Orson Wells broadcast, but it's very real.

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Over the centuries the great British gardening public's appetite for the new and exotic has led to thousands of plants being introduced from overseas. The vast majority have proved well behaved, but there are a handful, unfettered by the natural pests and diseases that keep them in check in their natural habitat, that have gone on the rampage, causing a major headache for environmentalists and gardeners alike.

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Perhaps the most notorious is Japanese Knotweed (Fallopia japonica). Brought here as an ornamental shrub, it has colonised vast tracts of land and has been known to come up through concrete and even external walls. In Swansea alone the estimated cost of eradication runs to �9million. In summer it has tall canes, approximately five feet, bearing clusters of white flowers in summer. But don't be fooled. It spreads at an alarming rate.

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If you find Knotweed in the garden do not simply try to dig it up, it can regenerate from a piece of its extensive rhizome the size of a thumbnail. And don't, whatever you do, put it on the compost heap or you'll never get rid of it.

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Tests have proved that regular spraying with a glysophate-based weedkiller is the best way to tackle this menace. If you have a serious infestation it is worth contacting your local authority or the Environment Agency for advice.

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Major problems are being caused by imported aquatic plants, some amazingly still legitimately on sale in garden centres.

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Introductions such as Crassula helmsii (Australian Stonecrop), Myriophyllum aquaticum(Parrots Feather), and Floating Pennywort (Hydrocotyle ranunculoides) look perfectly innocent in your garden pond, but will soon take over. If even the smallest fragment escapes into the wild, they quickly form dense mats that de-oxygenate waterways, killing fish and native water plants.

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Avoid these species at all cost. Instead chose well-behaved native aquatics such as Hydrocharis morsus-ranae (Frogbit), Ranunculus aquatilis (Water Crowfoot) or the Callitriche family (Water starwort). Again, contact your local Environment Agency office for guidance if you think you have an aquatic alien problem.

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Worried you are harbouring an invader or need advice on how to get rid of an unwanted plant Click here to ask your question.