7.2.6 Japanese knotweed
Japanese knotweed, a rhizomatous perennial, was introduced from Asia to Europe in the mid-nineteenth century as an ornamental and fodder plant. It is an impressive species, which grows to a height of 2-3 meters, with flecked bamboo-like stems, arching branches and clusters of creamy white flowers appearing late in the season. The orange to brown coloured, woody, dead stems persist erect throughout the winter and new shoots, produced from the extensive rhizome system, grow up amongst these the following spring to form dense thickets. The dead stems and leaf litter decompose very slowly and form a deep organic layer, which prevents native seeds from germinating. Once present at a site, Japanese knotweed increases in area very rapidly and soon forms monoculture stands. Specific problems caused by Japanese knotweed are:
* Damage to paving and tarmac areas
* Damage to flood defence structures
* Damage to archaeological sites
* Reduction of biodiversity through out-shading native vegetation
* Restriction of access to riverbanks for anglers, bank inspection and amenity use
* Reduction in land values
* Increased risk of flooding through dead stems washed into river and stream channels
* Increased risk of soil erosion and bank instability following removal of established stands in riparian areas
* Accumulation of litter in well established stands
* Aesthetically displeasing
* Expensive to treat