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.