Special forces for tackling aliens

When plants lose their proper place in the world, usually by human agency, they can seem possessed by a berserk and exponential vigour. One thinks of the mattresses of water hyacinth choking the lakes of Africa; the impenetrable thickets of kudzu vine in the American south - or simply of the beautiful but rampant rhododendron that would, if we let it, choke the few old oakwoods we have left.

The opportunism of invaders such as these is held in check on its home ground by competition from other plants, or by insect appetites. In mixed broadleaf woods of the Pontic Mountains of northern Turkey, Rhododendron ponticum is a well-behaved member of the plant community. In Ireland, it has seized its niche at the expense of native trees, woodland shrubs and herbs, and even our birds and insects.

Rhododendron makes copious seed. What gave the shrub its chance here was the heavy grazing of our woods in modern times and the ease with which it could smother heather on adjoining open bog. As if expecting opposition, it renders itself distasteful to grazing animals (and also, probably, most insects) with a substance in its sap called andromedo-toxin. And burning just encourages it to put out more shoots.

A second plant invader, never more luxuriant than now along the highways of the west, has something of the same, unnerving ambition. Japanese knotweed, Reynoutria japonica, makes tall, dense thickets afresh every year, like a slender bamboo with big floppy leaves and spires of attractive, creamy flowers. Its expansion is at the stage of seeming "remarkable"; ecological concern may come later.

In Japan, it is one of the first plants to colonise the raw lava of volcanoes. Brought to the UK in 1825, it escaped on to cinder tips in Wales and then started an explosive spread across Britain. In Ireland it escaped from demesnes, settling and thickening in sharply drained corners along the roads, and in quarries. In my corner of west Mayo, it has reached the nearest county council gravel pile, and drifts of gravel and rubble around farmhouses are other favourite places.

In these islands, the plant spreads mainly through rooting bits and pieces: just one centimetre of root can start a whole new thicket. West Glamorgan, in Wales, now has almost 50 hectares of it, often on banks along waterways where herbicides cannot be used safely. In Japan, the plant's pioneering foothold on the fumaroles of volcanoes, along with the mosses and lichens, gives way to other herb species within half a century. It also grows commonly in some kinds of Japanese grassland, but there it has a host of natural control species, including an especially voracious leaf-feeding beetle and at least one disease. In Ireland and Britain, only a handful of beetles or caterpillars show any interest in tasting its leaves.

All the plants in the UK are thought to be clones of a single individual, which should make them uniquely vulnerable to the sort of strategy devised by the International Institute of Biological Control at Silwood Park, near Ascot, in England. I described here recently the IIBC's proposal for the release of two South African moths, whose caterpillars feed exclusively on bracken. Now, they have turned to Japanese knotweed as an even more ideal target for control.

The institute is offering a five-year programme, a collaborating Japanese professor included, for some £335,000 - a bit less than it would cost to give West Glamorgan's 50 hectares a single year of spraying. As with the South African bracken moths, it would, of course, first demonstrate the exclusive knotweed diet of the aforesaid knotweed-hungry beetle, Gallerucida nigromaculata. Some 250 alien plants have now established themselves in Ireland, and a good many of them enrich the landscape of the west without exciting paranoia. This is the month of scarlet fuchsia hedgerow, and of flaming trumpets of montbretia, the glorious South African hybrid. What would Achill be without its statuesque clumps of "giant rhubarb", Gunnera tinctoria (not a rhubarb at all) or farm hedges of escallonia, the seaside shrub from South America?

An Antrim reader, Philip Jordan, in Connemara recently, was surprised to be shown big, sunflower-yellow daisies springing up among brambles at the edge of a meadow near the shores of Mannin Bay. He recognised elecampane, Inula helenium, a plant from old herbals and modern garden catalogues. It is a garden escape and described as "certainly quite naturalised" in the north of Ireland exactly a century ago. It has gone from its old, wild stations there, and now seems to wander the island as a rarity, springing up and fading out in one place after another.

There are other old, introduced, garden plants which have this transient habit in the wild. Tree lupins come and go as seaside thickets in the south-east. Michaelmas daisies, wallflowers, antirrhinums seed themselves into walls and waste-ground. One of my cherished weeds is dame's violet, Hesperis matronalis, rescued from a clifftop by a reader in mistake for the extinct native sea stock, which it resembles. It seeds itself everywhere, and scents the paths in summer with an intense, stock-like perfume. Would you have thought the two-metre tree-mallow, Lavatera arborea, with its rosy, purple-veined spikes of flowers, was a native or a "garden" plant? The answer is both. It grows wild and native along the cliffs of Aran, near Dun Aonghusa, and has been taken into the island gardens as well. Up in Co Down, it is frequent on the upper seashore, but many of the plants are probably (to quote The Flora of North-East Ireland) "escapes . . . derived ultimately from native stock brought into local cultivation". So the wheel turns full circle, confusing us all.