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Portrait of Japanese knotweed

Nature · Ferns, grasses & other plants

Japanese knotweed

Neophyte, introduced 1825 · shoots to over 2 m · roadsides, riverbanks, railways and waste ground

Reynoutria japonica (also Fallopia japonica) — Polygonaceae

Japanese knotweed is Britain's most notorious invasive plant, a bamboo-like perennial from East Asia that arrived in British gardens in 1825 and now forms dense thickets on riverbanks, railway sidings and waste ground. Every plant in Britain is female and spreads only from pieces of rhizome, so it moves with contaminated soil and dumped garden waste. It is legally regulated, may affect mortgage lending on affected property, and has been at the centre of a landmark nuisance case.

How to spot them

  • ▸Red-purple, asparagus-like shoots in spring, growing fast into hollow, bamboo-like canes speckled with reddish-purple, with conspicuous nodes
  • ▸Spade- or shield-shaped green leaves with a flat, truncated base, arranged in a zigzag along the stem
  • ▸Sprays of creamy-white flowers in late summer; dead brown canes persist over winter
  • ▸Dense thickets more than 2 m tall; rhizomes underground
  • ▸Look-alikes: giant knotweed has much larger heart-based leaves and the hybrid Bohemian knotweed is intermediate; Russian vine, Himalayan honeysuckle and fish mint are among the plants the RHS lists as look-alikes

How to know it

Bamboo-like hollow canes with red-purple speckles, zigzag spade-shaped leaves, and rhizome fragments of 1 cm that can regrow

Japanese knotweed.

All British plants are female, descended from a single clone, so it spreads only vegetatively

Japanese knotweed — BSBI.

Listed in Schedule 9 of the Wildlife and Countryside Act 1981, making it an offence to plant it or cause it to grow in the wild

Japanese knotweed — and contaminated soil is controlled waste.

The 2022 RICS professional standard replaced the old 'seven-metre rule' with management categories A to D

Japanese knotweed — affecting mortgage lending on affected property.

A 2018 Court of Appeal case found a railway owner liable in private nuisance for encroachment onto neighbours' land, even without structural damage

Japanese knotweed — Network Rail Infrastructure Ltd v Williams and Waistell [2018] EWCA Civ 1514.

A tiny sap-sucking insect, Aphalara itadori, was trialled in Britain as a biological control for this plant

Japanese knotweed — released under licence on trial sites from 2010.

Quick-fire facts

  • •Reached British gardens in 1825, with the first wild escape recorded around 1886 (BSBI)
  • •The BSBI calls it the most aggressive introduced herbaceous plant in the flora of Britain and Ireland
  • •All British and Irish plants are female and reproduce entirely by vegetative means; rhizome fragments as small as 8 g (BSBI) or 1 cm (gov.uk) can regenerate
  • •Law: it is listed on Schedule 9 of the Wildlife and Countryside Act 1981, so it is an offence to plant it or cause it to grow in the wild; removal is not legally required but you can be prosecuted for causing it to spread (gov.uk)
  • •Soil or plant material contaminated with the plant is controlled waste under the Environmental Protection Act 1990 and must not be mixed with other surplus soil
  • •Sellers must declare its presence on the TA6 property information form; misrepresentation can lead to a claim by the buyer
  • •Mortgages: RICS's 2022 professional standard replaced the 2012 seven-metre rule with categories A–D, and many lenders now lend given a management plan for the higher categories; check the position with a lender and surveyor, as criteria vary between lenders
  • •Where it hybridises with giant knotweed the hybrid, Reynoutria x bohemica, can set viable seed
  • •Cutting and burning are ineffective; repeated herbicide treatment over at least three years is the usual approach

Drill it — 7 cards

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