Japanese knotweed – warning as invasive plant grips UK

The invasive plant, which poses risks to property and infrastructure, has been recorded in 329 verified sightings across the county, according to Environet, which uses a live online tracker to monitor problem areas.

That equates to approximately 0.33 incidents per square mile.

The hotspots identified are Oxford, Banbury, Henley, Bicester and Chipping Norton.

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Emily Grant, director at Environet, said: “Japanese knotweed is now firmly established right across the country, which means homeowners everywhere should know what to look for.

“My advice to anyone living or buying a home in a Japanese knotweed hotspot is to check your garden regularly during the growing season.

“At this time of year, an established knotweed plant can be identified quite easily by its hard bamboo-like canes and distinctive bright green shield-shaped leaves which grow in a zig-zag pattern along the stem.

“The earlier it’s spotted, the easier and cheaper it is to deal with.

“Acting quickly with professional advice will prevent it spreading further and help protect both your property and its value.”

The warning follows new research carried out by Environet in partnership with Censuswide, which found that seven per cent of UK homes are already affected by the plant—amounting to more than 1.58 million properties.

The presence of Japanese knotweed typically reduces property values by around five per cent, or roughly £13,500 per home.

Environet’s Exposed: The Japanese Knotweed Heatmap is built from its own enquiries, public sources and verified sightings submitted by members of the public using the map’s ‘Add Sighting’ feature.

Homeowners and buyers can enter their postcode to see how many sightings have been reported within a five-mile radius.

The top five areas affected in Oxfordshire are Oxford (65 sightings), Banbury (17), Henley-on-Thames (13), Bicester (12), and Chipping Norton (eight).

Japanese knotweed was first introduced to the UK in the Victorian era as an ornamental garden plant and has since spread widely through contaminated soil, waterways, and more than 150 years of industrial activity, construction and infrastructure works.

While the plant cannot reproduce by seed in the UK, its rhizome system can stretch up to seven metres laterally underground, and even tiny fragments can regenerate into new plants.

After ash dieback disease, Japanese knotweed is considered the UK’s second most costly invasive non-native species.

Economic costs are estimated at £246.5 million annually, according to the Centre for Agriculture and Bioscience International (CABI).

Those who suspect knotweed on or near their property can send a photo to expert@environetuk.com for free identification.

If confirmed, a survey will be needed to assess the extent of the infestation, potential risks, and suitable treatment options.

During summer, Japanese knotweed can be identified by tall, upright canes which can reach two to three metres high, dense clusters of stems emerging from a central crown, large green shield-shaped leaves with a pointed tip that grow alternately up the stem, and by late summer, cone-shaped tassels of small creamy-white flowers.

Early identification and action are key to preventing spread, reducing remediation costs, and avoiding potential damage to property and legal disputes between neighbours.