Killing the green monster

Once the super-plant Japanese knotweed takes hold, there's little to stop it. But, thanks to new research, it may soon meet its match, reports Simon Hadlington
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It is one of the monumental lonely hearts stories of our time. Each autumn, the world's largest female organism exposes her sexy bits and lets them waft in the breeze in the desperate hope that a mate will arrive on the scene. So far she has had little success. And this is just as well, because if a compatible male does appear, many parts of Europe and North America could face ecological misery.

It is one of the monumental lonely hearts stories of our time. Each autumn, the world's largest female organism exposes her sexy bits and lets them waft in the breeze in the desperate hope that a mate will arrive on the scene. So far she has had little success. And this is just as well, because if a compatible male does appear, many parts of Europe and North America could face ecological misery.

The lonely lady is the Japanese knotweed, Fallopia japonica, a plant introduced into Europe from Japan in the mid-1800s and that spectacularly fallen from grace. In 1847, F japonica was awarded a gold medal by the Society of Agriculture and Horticulture at Utrecht in Holland for being the "most interesting new ornamental plant of the year".

Its importer, one Philipp von Siebold, proclaimed it as some kind of wonder plant, which, apart from its natural beauty, had medicinal properties, was excellent forage for cattle, sustained bees for the winter, could stabilise sand dunes and could even be used to make matchsticks. Naturally, it carried a hefty price tag.

Now, the plant is a blooming nuisance. Japanese knotweed is herbaceous and propagates via a huge underground network of woody rhizomes. Each spring the stems grow to about three metres tall and in the autumn produce small white flowers. The plant dies back with the first frost to leave a forest of rusty brown stalks. In this country, Japanese knotweed has colonised large areas in Wales and Cornwall, and can often be found along waterways and railway tracks, and in locations such as old cemeteries and on wasteland, although it does occur in more genteel surroundings - including in Buckingham Palace gardens.

The plant is one of only two land-growing species that has been proscribed by the 1981 Wildlife and Countryside Act. It can undermine river defences and push up through roads and pavements. Property developers must treat the plant as a contaminant that needs special and expensive disposal. It is highly vigorous, largely resistant to treatment with conventional herbicides, and a new research programme has just been launched to try to identify the plant's natural enemies in its native Japan to investigate the possibility of finding a specific pest or pathogen that will control the knotweed.

Perhaps the most remarkable feature of the plant is that all the Japanese knotweed found in Europe appears to be one vast female clone - essentially the same plant that von Siebold brought back from Japan in the 19th century.

For the past 20 years, Dr John Bailey, more recently assisted by Michelle Hollingsworth and Catherine Pashley, has been studying Japanese knotweed at the University of Leicester. "In the late Eighties we began looking closely at the chromosomal characteristics of the plant and its morphology," he says. "It soon became clear that we were looking only at female plants. We speculated that given its vigour and its ability to propagate vegetatively - without the need for fertilisation - that it was all one enormous clone." The morphology of the modern plant precisely matched that of contemporary illustrations of the specimens brought over in the 1840s, adding further weight to the idea that it might be a single clone.

However, it was not until the advent of genetic fingerprinting several years later that this could be confirmed. "In the mid-Nineties we gathered many samples from across Europe and used genetic fingerprinting techniques to show that they were indeed identical," says Dr Bailey. Japanese knotweed in Europe is effectively one massive female organism. And it is looking for male company.

"In its native Japan, the plant can reproduce sexually because there are pollen-producing plants there," says Dr Bailey. "If the plant successfully produced seed here, it would spread like wildfire and be an even bigger social and environmental headache than it already is."

In fact, the Japanese knotweed's strategy for finding a compatible pollen has had some limited success. Each autumn the plant flowers and exposes its feathery stigmas to catch any passing pollen. In this way it has hybridised with two other species. The first is another Japanese import, the giant knotweed; the second is the "mile-a-minute" climber the Russian vine.

The Japanese/giant hybrid is more invasive than its parents and, worryingly, produces seed. It is also being found across Europe with increasing frequency.

"Fortunately, the scenario is not as bad as it might be," says Dr Bailey. "While these seeds will germinate and grow like mustard and cress in the greenhouse, they appear not to germinate in the wild." The reasons for this are not clear, but Dr Bailey suspects it is to do with the relatively warm and wet British winter. "These plants originated in the extremely cold mountains of Japan," he says. "It is possible that a really cold winter here might result in some germination, and in other colder parts of Europe or North America these hybrids might have a better chance of surviving."

The hybrid with Russian vine is, thankfully, less vigorous than either of its parents and is sterile. "The Russian vine is a woody perennial that stores its resources in its stem, while the knotweed stores its resources in its rhizomes," Dr Bailey says. "It seems that they have cancelled each other out to make a less vigorous plant." A well-established stand of this hybrid, probably 20 or 30 years old, has been found in an abandoned railway yard in London.

However, once plants have successfully hybridised there always remains the possibility that they can back-cross and produce even hardier, more invasive hybrids capable of producing seeds that can survive and germinate in the wild.

A concerted effort between several government agencies is now under way to identify ways of controlling Japanese knotweed. "If you use a conventional herbicide such as Roundup you will destroy the above-ground growth, but if you are dealing with a very well-established stand of the plant, you will have an enormous underground network of tough, woody rhizomes that can be up to two or three centimetres in diameter, and it is virtually impossible to eradicate it completely," Dr Bailey says.

"Even repetitive spraying over several seasons might not kill the entire plant," he continues. "While the herbicide can penetrate the rhizome to a degree, it would be extremely difficult for it to affect the entire network. Experiments have shown that less than one gram of rhizome is capable of producing a new plant." One radical but costly solution might be to remove the rhizomes from the soil mechanically. But even a small amount left remaining can result in failure.

Now, a multinational project run by CABI Bioscience and funded by a variety of agencies, including Network Rail, the Welsh Development Agency, British Waterways and the Environment Agency, has been launched to investigate the possibility of "natural control" of the rampant Japanese knotweed.

"When an organism becomes established outside its natural environment it is as if it is on holiday," Dr Bailey says. "It has escaped from its natural pests and predators. In Japan it is very hard to find a specimen of knotweed that is not damaged in some way - by aphids or caterpillars or a fungus, and facing stiff competition from other giant herbs. By contrast, over here every plant is virtually pristine. The odd slug might have had a little nibble, but that's about it."

The aim of the project is to identify the source in Japan of the European clone and then to collect samples of any associated predators for culture and assessment as natural control agents.

"The good thing about trying to kill a clone is that once you have found something to knock it out, you are there," says Dr Bailey. "There is no genetic diversity within the wider population to overcome the agent."

The team has identified plants with a good genetic match, but whose morphology is significantly different from the European clone. More recently, some good matches have been located near Nagasaki, which is where von Siebold, the plant's importer, was based in Japan.

"Clearly there would be intensive testing of any non-native agent that might control the plant before it was introduced into this country," Dr Bailey says. "But if we could identify something that attacked Japanese knotweed in a highly specific way and which did not pose any kind of threat to other species, it could be the answer."