Brought to Britain because of its beauty, Japanese knotweed is now causing a headache across the country – here is your ultimate guide to the problematic plant.
With its red stems and deep green leaves, it is a pretty enough plant.
But Japanese knotweed’s beauty belies the fact it has become the scourge of British homeowners.
It grows at a ridiculous rate, is near-impossible to get rid of and has ruined house sales – wiping thousands off property prices.
Just this week, a woman told how nearly half the value of her aunt’s home had been wiped off by the plant growing on an adjoining piece of land.
Elizabeth Abraham’s Swansea home should fetch around £80,000 – but now the 91-year-old has been told it will not sell for more than £45,000 because of the untamed wild weed.
So what can be done about Japanese knotweed?
Here we tell you everything you ever wanted to know about it.
How did it get here?
Japanese knotweed, or Fallopia Japonica, was brought to Europe from Japan in the mid-19C by German-born botanist Phillipp von Siebold who found it growing on the sides of volcanoes.
Initially lauded for its beauty and potential as animal feed, and it was so celebrated that in 1847 it was named the “most interesting new ornamental plant of the year” by the Society of Agriculture and Horticulture at Utrecht in Holland.
In 1850, the Royal Botanic Gardens at Kew received a shipment from Siebold of various plants from his travels, including a sample of knotweed.
By 1854 the plant had been sent to the Royal Botanic Gardens in Edinburgh and was then sold commercially by nurseries.
The rest, as they say, is history.
How did it take over Britain?
Knotweed’s spread – through purposeful planting and it escaping – went undetected for years.
According to researchers at the University of Leicestershire, people sharing cuttings or disposing of unwanted plants was the “primary pattern of distribution” .
It was also spread through watercourses, and through the movement of soil for construction and road-building.
Knotweed expert Ann Connolly, who died in 2010, found one of the earliest examples of it being planted purposefully outside gardens was in Welsh coal-mining valleys in the 1960s and 70s as it was good for stabilising loose soil.
How do I know if I’ve got it?
Japanese knotweed starts growing from early spring and can reach 1.5m by May and 3m by June, before dying back between September and November.
- fleshy red tinged shoots when it first breaks through the ground
- large, heart or spade-shaped green leaves
- leaves arranged in a zig-zag pattern along the stem
- a hollow stem, like bamboo
- dense clumps that can be several metres deep
- clusters of cream flowers towards the end of July that attract bees
- dies back between September and November, leaving brown stems
Why is it so problematic?
In its native Japanese volcanic landscape, the climate and regular deposits of ash would keep knotweed plants small, while the plant survived thanks to energy stores in its deep root system.
But in Britain, without these impediments, it grows unabated.
And at its most prolific it can grow up to 20cm EVERY DAY.
It can even grow through concrete and tarmac and its roots can go down up to 3m deep.
There are no natural predators either meaning the weed can grow unabated, swamping other plants and preventing them from getting any light.
And while it does not produce seeds it can grow from minuscule fragments of rhizomes – the underground network of stems and roots – meaning it spreads easily.
What has been the cost of the problem?
Knotweed costs the UK economy £166 million per year for treatment and in home devaluations.
Last year, homeowners Matthew and Suzie Jones were told it would be cheaper to knock down and rebuild their £300,000 London home rather than try and treat their knotweed problem – that saw the familiar red bamboo-like plant grow through their floor.
And earlier this year, a man who murdered his wife before killing himself cited the weed that had blighted their West Midlands home as the cause for his mental distress.
In a suicide note, lab technician Kenneth McRae, 52, wrote: “I believe I was not an evil man, until the balance of my mind was disturbed by the fact there is a patch of Japanese Knotweed which has been growing over our boundary fence on the Rowley Regis Golf Course.”
What can I do about it?
1. Dig it out.
Digging out Japanese knotweed is a possibility but leave any trace of its deep root system at your peril – it takes just 0.8g of root for a new plant to grow again.
And be careful about getting rid of it once it’s dug up – Japanese knotweed is classified as ‘controlled waste’ under the Environmental Protection Act 1990 and can only be disposed of at licensed landfill sites.
Or you could dry it out, and then burn it, or even bury it 5m deep – though that’s not practical for most gardeners.
2. Feed it to bugs
In 2010, experts introduced a Japanese bug, aphalara itadori, to the UK that feasts almost exclusively on knotweed.
It’s hoped this will become available to gardeners if it works.
3. Kill it with chemicals
You can turn to chemicals, especially treatments containing glyphosate, but beware: it can take up to five years’ treatment to finally be rid of the pesky plant.
Professional treatments can set you back thousands of pounds.
4. Eat it
Or, you could eat the problem and cook your Japanese knotweed – though you’d need to eat A LOT to even come close to eradicating the problem.
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