Japanese knotweed: What does it look like and where are the UK hotspots for this destructive weed?

·3 min read
Japanese knotweed encroaching into a driveway (Environet)
Japanese knotweed encroaching into a driveway (Environet)

A new heat map has identified the areas of the UK hardest-hit by Japanese knotweed, a notoriously invasive and destructive weed, with north west England, Wales, the Midlands and London among the most overrun locations.

Bolton in Greater Manchester came top in the list of towns infested with the weed, recording 621 outbreaks over the last year, according to data compiled by horticultural consultants Environet, who created the searchable map.

Bristol was second with 465 infestations, followed by St Helens with 440, Cardiff with 419 and Blackburn with 406.

Llanelli and Swansea in South Wales, Rotherham, Shepherd’s Bush in West London and Nottingham rounded out the top ten.

Japanese knotweed is notorious for its habit of growing between brickwork and tarmac, ruining homeowners’ driveways, patios and drains and even lowering house prices by as much as 10 per cent, also representing a scourge to public parks and golf courses.

It is famously difficult to remove, having to be disposed of at landfill sites alongside man-made household refuse.

The plant is currently in its spring growth phase after winter hibernation and is described by the Royal Horticultural Society as “a fast-growing and strong clump-forming perennial, with tall, dense annual stems”.

Gardeners can identify the weed by its purple or red asparagus-like shoots emerging from the ground, which quickly grow into lush green shrubs with heart or shovel-shaped leaves and pink-flecked stems.

Environet’s research suggests the weed knocks £20bn in value from UK house prices every year and £166m is spent every year on removal work, although the British government estimates that £1.5bn would need to be spent to entirely eradicate the species from the UK.

“With the stamp duty holiday extended and lockdown restrictions beginning to ease, the property market is busier than ever - but failing to carry out the appropriate checks for knotweed can turn out to be an expensive mistake,” comments the company’s founder and managing director Nic Seal.

Environet
Environet

“Despite its fearsome reputation, with professional help, the plant can be dealt with and the value of a property largely restored. I’d urge anyone buying or selling a property, or homeowners wishing to preserve the value of their home, to be vigilant for signs of spring growth.”

In his 1996 book Flora Britannica, naturalist Richard Mabey explains that the species was first introduced to Britain by the Victorians in the 1840s when a specimen was imported by Kew Gardens.

Japanese knotweed was briefly in fashion, a craze encouraged by the respected gardening writer William Robinson.

“Its dense sheaves of canes (up to six feet tall), heavy, heart-shaped leaves and spires of tiny white flowers suited the Victorians’ austere taste,” Mr Mabey explains. “[But] when their formidable powers of colonisation were realised they were thrown over the garden wall onto railway embankments and rubbish-tips.”

From here, the plant spread across the country over the coming decades, with botanists reporting sightings in the wild in London in 1900, in Exeter by 1908, in Suffolk by 1924, in West Yorkshire by 1940s and as far north as Northumberland by the 1950s.

Environet
Environet

It was nicknamed “Hancock’s curse” in Cornwall after an unfortunate gardener whose grounds succumbed to it while conspiracy-minded Hampshire residents blamed the British military for its inadvertent spread.

But Japanese knotweed has not always been so hated, however, with Mr Mabey citing a Devon villager who made pan-pipes from its stalks, cooks foraging for it as a vegetable it in the United States and Wales and the plant winning praise for bringing an “Amazonian luxuriance” to the banks of the River Don in industrial areas of Sheffield.

He acknowledges though that it “is now officially regarded as the most pernicious weed in Britain, and it is illegal to plant it deliberately in the wild. Its rampaging spread in the 1970s and 80s is regarded as a parable of the dangers of casually introducing alien plants into the countryside.”

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