Japanese knotweed Fallopia japonica
This vigorous and dominating foreign plant, known to damage buildings, is notoriously troublesome to eliminate. It is an herbaceous perennial with an appearance similar to bamboo.
Conservation status in Norfolk
Japanese knotweed is highly vigorous and difficult to control, and dense stands of it can dominate natural habitats, preventing native species from growing. It can also cause damage to buildings and hard structures, and is able to grow through walls and tarmac. Under the Wildlife & Countryside Act 1981 it is an offence to “plant or otherwise cause Japanese knotweed to grow” in the wild.
How to help
Japanese knotweed is difficult to eradicate because of its vigorous growth and deep underground rhizomes. Chemical control can be used, but only certain formulations can be used near water. Cutting should be done with extreme caution, as any fragments which are spread around can regenerate into new plants. Plant material should not be composted or removed from a site without a licence. Advice on managing Japanese knotweed can be obtained from the Environment Agency.
As part of its Natural Connections project, Norfolk Wildlife Trust is working in partnership with the Norfolk County Council (Norfolk Biodiversity Partnership) to map the distribution of Himalayan balsam in Norfolk. The aim is to engage the general public in helping to find out where these species are found in Norfolk and help discover what impact they are having on Norfolk’s countryside. The survey will run from end February through to end October. For further information or a copy of the Freepost survey form telephone 01603 598333, or visit our website at
www.norfolkwildlifetrust.org.uk/naturalconnections/surveys where copies of the survey form can be downloaded.
Japanese knotweed
Information on the Japanese knotweed
How to recognise
Japanese knotweed is not native to the UK, and was introduced around 1855 as a garden ornamental because of its exotic appearance. It is an herbaceous perennial, native to Japan and south-east Asia. The plant grows up to 3 m tall and forms dense clumps. It has fleshy green and red hollow stems with swollen red nodes, giving an appearance similar to bamboo. The leaves are green, triangular to heart-shaped, 70 – 150 mm long. Flowers are creamy white, 3 mm wide, in dense clusters borne on the tips of stems. The underground rhizomes can be up to 10 cm in diameter, spreading 7 m from the parent plant and up to a depth of 2 m or more underground. Only female plants are present in the UK.
Where to see
Japanese knotweed is an invasive plant, growing on roadsides, riverbanks and wasteland, and can be found throughout Norfolk. It is often found along watercourses where it has plenty of water for growth and rhizome distribution. It is also able to grow where there is little nitrogen and can tolerate high levels of metallic and other contaminants in the soil. In its native Japan it can survive mountain temperatures of -35ºC and is often the first plant to colonise a volcanic area after an eruption.
When to see
Shoots emerge in spring and grow rapidly up to 3 m tall. Flowering occurs from August to October, but rarely sets seed. Any seeds produced are likely to be from a cross with the closely related giant knotweed. The stems die back in winter, taking several years to decompose fully.
Did you know?
A piece of rhizome weighing no more than 0.7 g, 300 mm below the surface, can generate new plants.
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