Japanese Knotweed

Species Description

Scientific name: Fallopia japonica

Identification sheet

Native to: Japan, Taiwan, northern China

Habitat: waste land, urban areas and river banks, it will generally grow anywhere its rhizomes are transported to

Probably the most common and best recognised invasive species, it is present through out

Britain

having been introduced as an ornamental by the Victorians. It causes severe structural damage to roads and buildings, but also erodes river banks and out-competes native plants by shading them out. It can grow in excess of 3 metres tall and regenerates from root fragments as small as 10mm. This is a very difficult plant to control due to its hardiness and persistent underground rhizomes.

Further Information

Dispersal and Reproduction

The main method of reproduction is by vegetative means. All the plants in

Britain

are thought to be female so no true-type seeds are produced. Japanese knotweed may however hybridise with other knotweed species such as giant knotweed (F. sachalinensis) to produce viable seeds.

Natural dispersal is by rhizome extension and is relatively slow. Vegetative fragments may also be spread naturally along rivers. Most new colonies are as a result of accidental or deliberate dispersal by humans who transfer stem or rhizome fragments between sites.

Known Predators

A trial release of the plant louse (psyllid), Aphalara itadori is currently underway. This is an insect native to Japan which feeds on knotweed in its home range. It is hoped that this psyllid will help control Japanese knotweed in the UK.

Impact

Its ecological impact is not well studied, but is likely to be fairly low. It tends to occur in degraded urban areas where native flora is already impoverished. It is a major obstacle to habitat restoration and enhancement due to its persistence. This plant has a high economic cost due to the legislative requirement for control and the difficulty of eradication. It is estimated that, as of 2010, Japanese knotweed costs the UK economy £165 million per year*.

Management

Environmental

Due to its hardiness and ability to regenerate it is a difficult plant to eradicate. There are a few options for control, all requiring professional guidance. Herbicide (either spray or stem injection) can be used but treatment will need repeating over several years. Herbicide treatment may be combined with cutting, but all cut plant material must be burned on site or removed by a licensed contractor.

Mechanical

On development sites, digging and removing all soil, dumping in a registered land fill, or burying it over 5 metres deep may be used. The burial can also take place using ‘cells’ however the digging options aren’t suitable for rivers. Flailing should never be used as it will spread fragments of plants far and wide.

* F. Williams, R. Eschen, A. Harris, D. Djeddour, C. Pratt, R.S. Shaw, S. Varia, J. Lamontagne-Godwin, S.E. Thomas, S.T. Murphy 2010.

The Economic Cost of Invasive Non-Native Species to the British Economy. CABI, Wallingford, 198 pp.