Invasive of the month: Japanese Knotweed

Japanese knotweed Fallopia japonica (Houtt.) Ronse Decraene [Polygonaceae], is arguably the most troublesome invasive alien plant in Europe and North America.  Since its introduction a popular new horticultural plant promoted for the Victorian garden, it has increased its range very rapidly and moved from prize-winner to pariah. 

Much of the spread has been without the benefit of sexual reproduction which normally contributes to long-distance dispersal and Japanese knotweed, often with giant knotweed Fallopia sachalinensis, and their hybrid F x bohemica, can now be found across much of Europe and in 41 US states and 8 Canadian provinces as well as parts of New Zealand and Australia..

The plant is most famous for its ability to break through tarmac, drains and concrete adding substantial costs to infrastructure projects such as the 2012 London Olympics.  However, it is its impact as an environmental weed of watercourses where the threat to biodiversity is most obvious.  Japanese knotweed grows extremely quickly, reaching 3m in as many months and excluding all other native species in the process thanks to its thick canopy.  This can have detrimental impacts not only on flora but also on associated invertebrates and vertebrates (Gerber et al, 2008; Maerz et al, 2005).  The plant can reliably regenerate from as little as 0.7g of rhizome material, so its removal from contaminated soil is very challenging and the risk of spreading the weed in topsoil or on machinery is very high.  Current control measures rely mainly on the use of chemicals, where permitted, often in association with physical removal.  However, these are often felt to be ineffective, requiring repeated follow-up visits and are very costly.  A nationwide control programme, were it to be attempted in the UK for example, using current control methods, has been estimated to cost in excess of £1.5 billion. 

Biological control may provide a more sustainable solution to this problem and an application has been made to release a specialist psyllid in the UK

http://www.cabi.org/japaneseknotweedalliance
http://www.cornwall.gov.uk/media/pdf/c/n/knotweed_booklet_10-08.pdf

Reference: Dr Richard Shaw, CABI Europe-UK