Japanese knotweed

by Richard Jones

When we moved into our previous house, in Nunhead, there was some small, but well-established growth of Japanese knotweed in the back garden.

Japanese knotweedWhen we moved into our previous house, in Nunhead, there was some small, but well-established growth of Japanese knotweed in the back garden. It took four years of pulling up stalks and roots to get rid of it … at least I think we got rid of it. 

Such is the persistence of this pernicious weed that I could never, however, feel safe from it. This triffid is widespread in southern London and despite Railtrack's best efforts, it still lines some of the railway embankments from Peckham towards Victoria. 

One of my favoured local spots, Nunhead Cemetery is also infested with a rare hybrid between Japanese knotweed, Fallopia japonica and the giant knotweed, Fallopia sachalensis, often denominated Fallopia x bohemica

Apart from a few flies sunning themselves on the broad leaves, Fallopia is more or less sterile when it comes to wildlife. But this may be about to change. 

I notice that there are rumours of importing a small but pretty Japanese insect, the psyllid bug Aphalara idatori, to try and control the knotweed. This will be fascinating if it comes off. Although the Japanese harlequin ladybird, Harmonia axyridis, has been released in Europe to combat alien aphids, it arrived in Britain under its own steam, or accidentally with horticultural material. It is now (perhaps rather unjustly) regarded with suspicion and might be on the way to becoming a noxious pest in its own right if it out-competes native UK ladybirds. One wonders what might happen to Aphalara

The science is compelling though, and so far the tiny bug is only known to feed on Fallopia. The hope is that it will have the same effect here, controlling Fallopia, as the Mexican moth Cactoblastis had when it was deliberately introduced to Australia to combat the insidious weed spread of prickly pear cactus, originally from central and southern America. That, at least, was a successful biocontrol story. 

Incidentally, students of William Robinson may have noticed that in early editions (1880s) of The English Flower Garden, he promoted Japanese knotweed as 'undoubtedly one of the finest herbaceous plants in cultivation'. However later editions, after about 1905, state 'it is easier to plant than to get rid of in the garden'.

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Gardeners' World Web User 19/08/2009 at 20:32

How many 'biocontrol' successes have there been compared with any failures/disasters?

Gardeners' World Web User 20/08/2009 at 16:49

My brothers garden is full of the stuff and he has tried everything to get rid of it!!! He has now got some weedkiller that you put down the hollow stem to see if that will kill it off, nothing will grown near it.

Gardeners' World Web User 20/08/2009 at 18:17

We have a small patch of Fallopia and I have found that regularly pulling it up is gradually weakening it and I predict that eventually I will win.

Gardeners' World Web User 20/08/2009 at 19:12

Have dug every bit of this beautiful weed up.I THINK , Have tried pouring weedkiller down the hollow stem which has helped but would like to know how long before it rears its head again. I think the only possible solution is to dig all the earth up and replace with fresh topsoil. Not really a job for a weakling . Someone find me a MAN

Gardeners' World Web User 20/08/2009 at 20:29

I don't think this article makes it clear that the control of Japanese Knotweed must be undertaken in consultation with the Environment Agency, as the waste itself is classified due to the plant's ability to regenerate from the smallest fragments. It is really important that it is controlled in the proper manner, as it is people ripping it out with the best intentions who are causing it to spread. (I don't work for the EA by the way, but I am an environmental scientist who has had to do some research into the issues around the plant). See http://www.environment-agency.gov.uk/homeandleisure/wildlife/31364.aspx for more info.

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