The Truth about Japanese Knotweed

As far as I know, there isn’t any Japanese knotweed round our way. Which is good news, as the scare stories about what it can do to your garden and even your house are legion. It’s commonly called the modern-day triffid; it starts out as a rather attractive bamboo-like plant sitting neatly in the corner of the garden and ends up taking over the universe. It’s seriously scary.

Does Japanese knotweed actually come from Japan? Very likely, as it’s native to Japan, China and Korea. According to the Royal Horticultural Society, it was introduced to Britain from Japan by the Victorians in 1825, though they don’t say how they can be so precise about the year.  You can see why the Victorians rather fancied it. It’s got red stems, broad green leaves and pretty creamy tasselled flowers that bloom in late summer and early autumn.

Unfortunately it also has a large network of rhizomes (underground root-like stems) which sprout at a rate of, well, knots, making it one of the most invasive plants in the country. It grows in almost any soil and a wide range of temperatures, and its roots can spread seven metres horizontally and three metres deep.

If you get Japanese knotweed in your garden, you’re actually legally obliged to prevent it spreading into the wild or causing a nuisance, under the Wildlife and Countryside Act 1981, though most of its spread is probably via topsoil movement or construction traffic. There are horror stories of mortgage lenders refusing to lend on houses with Japanese knotweed in the garden, or even in the garden next door, but nowadays they just tend to require that it be eradicated.

It can be tackled with herbicides, but trying to dig it out is unlikely to work as the chances you’ll get it all are slim, and the council won’t take it away in your green waste bag. The RHS suggests letting it grow to about ninety centimetres in May, and spraying then. When it grows back you spray it again in mid-summer. If it grows back you spray it again in September and if that doesn’t do it, repeat the whole process next year.

So what do they do about it in Japan? Well, one thing they do is eat it. If you want to try it, you need to choose new shoots and peel off the outer skin, then soak it in water for half a day before cooking it. Apparently it tastes like sour rhubarb.

There’s also a native form of leafspot fungus which wipes out any plant it gets a hold on. And there’s a kind of psyllid insect (a bit like an aphid) which feeds exclusively on Japanese knotweed. That’s now been introduced into the UK – the first time that biological control of a weed has been allowed by the EU.

10 thoughts on “The Truth about Japanese Knotweed

  1. In the UK, a mature clump of Japanese knotweed has canes arching out that are 10-12 foot high, emanating from the crown that is in the ground, and each clump will have a spread of about 10 foot. Even in an open spot with all day sunshine not shadowed by trees, this takes at least two years. Japanese knotweed only flowers when it has reached this stage of maturity.

    Every autumn, all the leaves and canes above ground die back in readiness for winter, and just like carrots, send all their spare food to be stored in their roots. In winter, you should see nothing but dead stems. If you dig up the crowns, or the rhizomes that run between the crowns, you would find their insides are coloured carrot-orange. Think how much energy is stored in a carrot, because it is this stored energy underground that gives Japanese knotweed one of its best strategic advantages.

    Now the obvious thing here is that if you want to get rid of Japanese Knotweed WITHOUT chemicals, and you only have the time to do it ONCE a year, then the best time is to do it is around September, after it has flowered, but before it dies back and sends more energy to store in its roots and rhizomes. By this time, the stems are very woody. The easiest way is to break each stem one by one AT GROUND LEVEL, starting from the outside of each clump. Do not use a strimmer, or a bush cutter, or anything that will chop up the Japanese Knotweed into little pieces, because each piece can be washed away by the rain or by a stream and become a new plant. Pile up the stems neatly so that their bottoms are resting above the crown from which they just came. If there are several clumps, collect them all into one such pile. Try to do this in dry weather, so that the tops will all die down. Nonetheless if wet weather should descend permanently, because you have lined up the base of the stems above the original crown, the cut stems will only try to put down roots where the original crown already is established!

    The other obvious thing is that if you have some other shrub or plant growing nearby, then treat it as your best ally, because it will compete with the Japanese knotweed provided that you occasionally give it a helping hand by pulling up the knotweed. So always start by pulling up the knotweed that is nearest a growing tree or shrub or even a herbaceous perennial.

    Before dangerous herbicides and weedkillers were invented, ordinary farmers and gardeners were successful in getting rid of whatever they needed to by using such traditional, common sense good gardening practice. Japan has a long history of high density population with rice farming, and clearly they succeeded in doing this by without needing modern dangerous chemicals to get rid of Japanese Knotweed in Japan; they just used common sense gardening practice.

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  2. I went to a supper club once where they served Japanese knotweed crumble for pudding. They admitted they had to pad it out with rhubarb because they couldn’t find enough knotweed – presumably due to people conforming with their legal obligations to get rid of it!

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    • You’re only supposed to pick the spring shoots, just as with asparagus. There is plenty of Japanese Knotweed growing wild in the wet parts of Britain. Trouble is, nowadays everybody is trying to get rid of it, so they are spraying it, or injecting it, with glyphosate (or much more toxic herbicides) in the autumn. I wouldn’t touch the shoots that came up in spring unless I knew for sure that it had never been treated with anything. Better to stick with forced rhubarb… Japanese Knotweed’s main economic use is resveratrol production. Almost all the resveratrol sold in health food shops is extracted from the roots of Japanese knotweed. (You may remember a few years ago that red wine was touted as being good for you because of its resveratrol content? Probably just the usual healthfood fad. )

      Resveratrol seems to be the agent that gives Japanese knotweed roots such resistance to fungal and bacterial attack when the roots are wounded. Most other plants do not have such effective underground disease resistance. The only sure way of killing the roots is to boil them like carrots until they go soft.

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  3. Is this dokudami? The leaves look somewhat similar but not sure as the flowers look different. They keep growing around my building no matter what. Very deep rooted and impossible to extinguish.

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