Tied in knots

In July and August, I try not to go anywhere, at least by road. One reason is that it can destroy, within half-an-hour, whatever good opinion of our species I have managed to restore since last summer. The other is that I shall annoy the driver with phrases such as, "would you look at that!" and, "there's a whole new stretch!" uttered just as she is poised to overtake the camper we have been following all the way from Maam Cross.

My syndrome will be readily recognised by the reader in Kinsale, Co Cork, who wrote this month: "On a recent visit to Clare and Galway we were horrified to see small clumps of JK all over these two counties. Yesterday, on a drive from Kinsale to Bantry, the same story. But along this road there are quite long stretches of verge where JK has become dominant and presumably it will spread steadily throughout the country unless measures are taken to stop it."

The words he could not bring himself to write were "Japanese knotweed", Reynoutria japonica, (or Polygonum cuspidatum if you happen to be American). Either way, it's a plant to arouse the utmost paranoia. What Rhododendron ponticum has been to our oakwoods, JK is fast becoming to our roadsides.

In the US, Philadelphia is spending more than $1 million to eradicate it from the city parks. In Wales, some local authorities have special knotweed officers, and the total smothering of a two-acre Swansea cemetery is a showpiece of its colonisation. In Ireland, the plant is hitch-hiking along the roads in gravel and soil: just one centimetre of root can start a whole new thicket.

JK is like a rosy-shanked, skinny bamboo (but actually in the dock family), two metres high or more, with bright-green, heart-shaped leaves and spires of attractive, creamy flowers. It was introduced to Britain from Japan as an ornamental shrub in 1825 and went from there to the US, before the century was out. In Wales, it escaped on to cinder tips - the nearest thing to the raw, volcanic slopes in Japan, where it acts as a pioneer species - and now it is everywhere in Britain and especially rampant throughout the moister western counties.

Escaping from Irish demesnes, it has settled and thickened in sharply-drained niches of roadsides and quarries (an undisturbed gravel pile is perfect). Left to run, it will stretch out rhizomes 15 or 20 metres long and grow an annual thicket so dense and tall that no other plant can survive. Such is its vitality that a rhizome will regenerate even buried a metre deep and may even force a shoot through five centimetres of asphalt.

So far, eradicating an established stand of JK by any other than herbicidal means has been a largely futile affair: so long as a few rhizomes are left in the soil, it will be back. The British Nature Conservancy Council recommends cutting it back in May or June and then spraying the new growth with glyphosate (Roundup) in August. Repeated for two or three years, this should, at length, prevail.

But the cost of spraying, say, 50 hectares (which West Glamorgan, for example, has to reckon with), and reluctance to use herbicide in nature reserves or wetlands, have encouraged the proponents of biological control. As all the knotweed plants in these islands are believed to be clones of a single individual, their genetic uniformity should make them uniquely and swiftly vulnerable to the beetles, caterpillars and pathogenic fungi that control the species in Japan.

A researcher from the International Institute of Biological Control (at Silwood Park, near Ascot in England) is off to Japan this autumn to gather knotweed-eating insects and test just how exclusive is their appetite for the plant. But biologists in these islands are exceptionally wary of introducing one alien species to control another: there have been too many misadventures in other corners of the world.

The US feels prepared to take more chances, so seriously is it concerned about alien weeds. A multi-agency report in 1998, Invasive Plants: Changing the Landscape of America, assessed their threat to native plants as second only to loss of habitat. "Their invasion," said Interior Secretary Bruce Babbitt arrestingly, "has created a level of destruction to America's environment and economy that is matched only by the damage caused by floods, earthquakes, wildfire, hurricanes and mudslides."

Among the US invaders is Connacht's own, lovely, purple loosestrife, which, carried across the Atlantic, ran like a purple tide through wetlands on the eastern coast of the US and Canada, displacing sedges, bulrushes and other plants vital to native wildlife. Four species of European leaf-eating beetles have now been introduced for its control, with dramatic results in southern Canada.

The spread of Japanese knotweed in the US had produced notably large stands along rivers and on river-islands in western Pennsylvania. When the plant reaches riverbanks, fragments of its roots are carried off by floods and lodged downstream to form new thickets, and its control by herbicide can threaten fish-life.

Its occurrence along Irish roadside banks and gravels simply reflects the way it has been spread so far - by lorry and dumpster. In fact, it will grow in any kind of soil, acid or limey. Research in Wales showed no correlation between soil type and plant size or the vigour of the thicket.

JK has certainly taken to Connacht and points south, in counties already well-adapted to energetic naturalised aliens: fuchsia, escallonia, montbretia. Even gunnera, the "giant rhubarb", is establishing new bridgeheads in disturbed soil on the margins of widened roads. But in habit, spread and persistence, Japanese knotweed is in a class of its own. Perhaps now, before it really takes off, is the time to tackle the triffid.