Japanese knotweed (Fallopia japonica)

Lynwood Gardens is currently subject to an invaison by Japanese knotweed. We are working on a control program.

For more information visit http://www.cabi-bioscience.org/html/japanese_knotweed_alliance.htm

The plant & the problem

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 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.

Reproduction is primarily by vegetative regeneration of rhizomes and fresh stems. The rhizome system may extend from a parent plant up to 7 meters laterally and to a depth of 2 metres. Very small fragments of rhizome (as little as 0.7 gram) give rise to new plants. Fresh stems produce shoots and roots when buried in a soil medium or floated in water. Stems in water may produce viable plants within 6 days.

Japanese knotweed thrives on disturbance and has been spread by both natural means and by human activity. In riparian areas, high water flows disperse fragments of the plant downstream where new colonies form. In the past, fly-tipping and transportation of soil containing rhizome fragments have been a major cause of spread, particularly in the urban environment

Problems

Specific problems caused by Japanese knotweed are:

Japanese Knotweed is one of the most extraordinary examples of an invasive plant known. Firstly it is a giant herb, which every Spring grows rapidly to a height of 2 or 3 metres, only to be cut down by the first winter frost and grow afresh the next Spring. It is actually a dioecious plant which means that you need male and female plants for sexual reproduction to occur, yet in Europe, so far, we only have female plants. Not only is it a single sex, it is also a single clone, as work carried out at the University of Leicester has shown. Given that it must occupy many thousands of hectares in Britain alone (the same clone is also known to occur in continental Europe and North America), in total biomass terms, it is probably the biggest female in the world! It is presently widely distributed around the British Isles, but what must be remembered is that it has achieved this distribution solely by vegetative reproduction. That is, it has spread by cuttings or from pieces of rhizome, whether deliberately as cultivated plants or discards from gardens, or inadvertently, spreading downstream along rivers or being spread by road works or site redevelopment involving an infested area.