Weed blooms big - but wait until the male arrives

It is one of the great 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.

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, introduced into Europe from Japan in the mid-1800s.

The plant is also found in New Zealand.

In Europe, its fall from grace has been spectacular.

In 1847, F japonica was awarded a Society of Agriculture and Horticulture gold medal at Utrecht in Holland for being the "most interesting new ornamental plant of the year".

Its importer, Philipp von Siebold, proclaimed it as a 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.

Now, it is a blooming nuisance.

Japanese knotweed is herbaceous and propagates through a huge underground network of woody rhizomes.

Each spring the stems grow to about 3m and in the autumn produce small white flowers. The plant dies back with the first frost, leaving a forest of rusty brown stalks.

In Britain, Japanese knotweed has colonised large areas in Wales and Cornwall, and can often be found along waterways and railway tracks, in 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 banned by the UK's Wildlife and Countryside Act. It can undermine river defences and push up through roads and pavements.

It is highly vigorous and largely resistant to conventional herbicides.

A research programme is trying to identify the knotweed's natural enemies in its native Japan to find a specific pest or pathogen that will control it.

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 von Siebold brought back from Japan in the 19th century.

For the past 20 years, Dr John Bailey has been studying Japanese knotweed at the University of Leicester.

He started by looking at the chromosomes and structure of the plant.

"It soon became clear that we were looking only at female plants. We speculated that given its vigour and its ability to propagate without the need for fertilisation that it was all one enormous clone."

The structure of the modern plant precisely matched that of illustrations of the specimens brought over in the 1840s, adding further weight to the idea that it might be a single clone.

But it was not until the advent of genetic fingerprinting that this could be confirmed.

"In the mid-90s 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 produced seed here, it would spread like wildfire and be an even bigger headache than it already is."

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."

A concerted effort between several government agencies is seeking ways of controlling Japanese knotweed.

Says Dr Bailey: "If you use a conventional herbicide 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."

- INDEPENDENT