Wednesday, September 22, 2010

Plant Public Enemy no.1

Japanese knotweed Fallopia japonica tops the table of alien, introduced plants that have become invasive weeds in Britain. Its phenomenal capacity to spread via deep, tough underground rhizomes and its resistance to herbicides, coupled with its tendencies to undermine building foundations, footpaths and roads, have created thriving businesses that specialise in trying to keep it under control. Introducing the plant into the wild is an offence and although it's not illegal to cultivate it in a garden, disposing of it when it begins to overwhelm your plot is a major problem, as Japanese knotweed-contaminated soil is classified as controlled waste and has to be removed by specialist companies. In the past much of the spread of the plant has probably been due to gardeners dumping plant waste, beside roads, rivers and canals. The fact that mortgage lenders are now refusing to lend to buyers of houses with Japanese knotweed in their garden should be sufficient deterrent for anyone contemplating planting it in their plot.


Since our Victorian ancestors first introduced it into their landscaped gardens it has spread around the country via fragments of rhizome in soil, especially in urban waste ground where it often forms four metre tall forests. Apparently, 137,500 tonnes of contaminated soil had to be removed from the London Olympics site alone, before construction could begin and, nationwide, about £150 million is spent every year trying to  control Japanese knotweed. One ray of hope for those seeking to bring it under biological control is that trials of a plant psyllid bug called Aphalara itadori, recently approved for release into infested areas, indicate that this insect could weaken the plant and make other forms of control, like herbicides, more effective



So, with all its destructive tendencies, is there anything positive to say about this aggressive invader? Well - excuse me while I reach for my flak jacket - there is. It's a magnet for honeybees. This fine specimen, that I photographed last weekend growing beside a pavement in Newcastle, near the Ouseburn, was humming with them. It's an ill wind, etc., etc................. 

15 comments:

  1. Hi Phil,

    We have Knotweed here (southern Ontario), but it's not quite so aggressive. Perhaps it's the climate or maybe a better behaved form of the species??

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  2. I didn't realise the problem had got that bad! I followed the link and was surprised to see by the photograph how tall the knotweed can grow. Errr.... um..., in its favour, I've always thought it an attractive plant with its large leaves and pretty flowers. Did I say that? Ssshhhhh.....

    I spent some time last night down by the river Balsam popping. Lol, I love it when you touch the seed pod and it catapults the seeds out. :) I also like the variety of shades of the flower, from the palest pink to dark aubergine.

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  3. On our travels around the British Isles we've found it in Western Ireland the Outer Hebridees and the West coast of Scotland especially on the Ardamurchan Peninsula. Nearer home there are large colonies of it along the Leeds and Liverpool Canal

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  4. I don't think I have ever seen that though now I know what it looks like I'll keep my eyes open.

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  5. In a former house I had a neighbour with a hedge of this or something very like it. Thrushes and blackbirds loved nesting in it, right beneath my window.

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  6. Hi Barry, Strange, isn't it, how some perfectly well behaved plants take on a whole new character when they travel. our purple loosestrife, which is never a problem here, has run amok on N.American wetlands....

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  7. Hi Lesley, There's some in Durham around the peninsula riverbanks that doesn't seem to have become too assertive yet, and there's also some down by the river Wear at Witton-le-Wear, but its biggest impact seems to be in urban areas. Have you ever seen the small yellow flowered balsam Impatiens parviflora that grows around Wolsingham? - its pods pop even more violently than its Himalayan cousin.

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  8. Hi David, I had no idea it had reached that far. Canal banks seem to be a frequent habitat - I guess a lot of garden waste ends up in canals..

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  9. Exactly, lotusleaf, very tempting to cultivate but you'll regret it if you do!

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  10. Hi John, I imagine it's most likely to turn up in your area around canals and riverbanks..

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  11. Hi Mark, there's another closely related species commonly known as Russian vine that's incredibly luxient and clambers through hedgs and up fences and walls, that's also very popular with birds and insects

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  12. Hi Phil. No, I've never seen the yellow-flowered balsam but thanks for telling me about it..... must head up to Wolsingham soon! :D

    There used to be a patch of Japanese Knotweed at Bishop Auckland, going down Newton Cap Bank and just at the beginning of the road leading to the Rugby Club. It might still be there, I'll have to go and have a look.

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  13. I've just noticed your comment to Mark about the Russian vine - there's quite a lot of it on the boundary to our allotments and the birds really do love it! Whenever you walk past all you can hear is frantic chirping - it's like the hedge is alive. :)

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  14. Hi Lesley, That Russian vine is an incredibly rampant plant so I can imagine it makes a great nest site for birds. The yellow-flowered balsam used to grow on the south side of the market square in Wolsingham, around the base of some stone steps...

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