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What is Japanese knotweed?

Japanese knotweed can grow more than a metre a month and is famed for pushing through tarmac, concrete and drains. Its effect on native species is often devastating as it out-competes indigenous species covering large tracts of land to the exclusion of the native flora and associated fauna.

Japanese knotweed was brought to Britain from Japan as an ornamental garden plant in the mid-nineteenth century. However, over time it has become widespread in a range of habitats, particularly roadsides, riverbanks and derelict land where it causes serious problems by displacing native flora and causing structural damage.

Due to its vigorous nature and the damage it causes it is one of only two terrestrial plants listed by the UK Wildlife and Countryside Act as illegal to cause it to grow in the wild.

An estimate in 2010 put the cost of Japanese knotweed to the British economy at £165 million (Williams et al.) and would cost £1.56 billion were control to be attempted countrywide. These control methods which rely mainly on chemicals have been deemed unsustainable by many and so a longer-term solution to the problem is required.

About the plant

Japanese knotweed on roadsideNative to Eastern Asia, Japanese knotweed grows to a height of two to three metres in the UK. It has flecked bamboo-like stems, arching branches and large spear shaped green leaves of up to 12cm long. Clusters of creamy white flowers appear at the tips of the stems late in the season between August and September. The rhizomes (underground stems) can stretch up to seven metres from the parent plant and to a depth of three metres.

Reproduction

Male and female plants are required for reproduction to occur. Japanese knotweed however, is an extraordinary example of an invasive plant since almost every plant outside Japan is derived from the same mother plant. In total biomass terms, this clone is probably the biggest female in the world! In Swansea alone the infestation has been estimated to weigh 62,000 tonnes, which equates to 400 blue whales!

How does is spread?

Japanese knotweed thrives on disturbance. Pieces of rhizome or stem can be spread by fly-tipping or carried by waterways, especially after heavy rains or flooding. Thus, clusters of Japanese knotweed are often found on riverbanks, roadsides and redevelopment sites.

What's in a name?

Japanese knotweed belongs to the plant family Polygonaceae: ‘Poly’ means many, and ‘gony’ is from the Greek for ‘knee’, giving 'many jointed'.

The scientific name of Japanese knotweed in current use is Fallopia japonica, although some scientists still use Polygonum cuspidatum (USA) and Reynoutria japonica.

In Japan, the plant is commonly known as 'itadori' which translates as 'take away pain' as it’s used in traditional medicine. In its introduced range, other common names include Sally rhubarb, donkey rhubarb, gypsy rhubarb, Hancock's curse, pysen saethwr, ladir tir, glúineach bhiorach, Mexican bamboo, German sausage plant, Japanese bamboo, Japanese fleece-flower and wild rhubarb.

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