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Japanese Knotweed
Fallopia japonica
General Description
Japanese knotweed is increasingly being found in patches on farms by ditches and on rough ground. Japanese Knotweed was introduced to the UK as an ornamental garden plant in the early 1800s but by the early 19th century the plants invasive nature was recognised.
In a survey in 2000 it was found in 50% of the National Grid squares 10km squares. Japanese knotweed is now classed as the most invasive plant in Britain and is scheduled under the 1981 Wild life and country side Act. . Under the Environment Protection Act (1990) Japanese knotweed is classed as “controlled waste” and must be disposed off, (the rhizomes), at a controlled landfill site. In Scotland SEPA need to be notified prior to transport of controlled waste and a licence.
Biology
The Japanese knotweed plant produces green shoots in early spring which can reach a height of 3 meters before dying back in autumn. The stems can range between 2-3m in height, are green with red or purple specks and forms dense cane-like clumps. The plant’s shoots and leaves, which can be up to 120mm long, cause very dense coverage of colonized areas. The canes produce long racemes of cream to white flowers which are not unattractive but thankfully are sterile and do not produce seed. The green plant dies back during the winter laving a matt of dead canes.
Japanese knotweed spreads underground by means of rhizomes which can extend 7 meters outwards and reach up to 2 meters deep depending on soil strata. If disturbed for example by building work, flooding or ploughing fragments of rhizome as small as 10mm, about the size of your finger nail, can produce new plants. If not controlled new plants are capable of penetrating tarmac, concrete and often grows up service ducts of buildings. If knotweed is found on ditches or river banks it will smother out natural vegetation increasing the risk of erosion when it dies back over the winter.
Advice
Control of Japanese knotweed depends on where it is found and the size of the infestation. If relatively small populations are found near water courses, ditches etc then it is best to carry out a survey up and down stream to establish if it is an isolated population. If populations are found up stream it is important to get a co-ordinated control policy in place. Glyphosate is the only product that can be used close to water. Check the label before application as not all formulations have aquatic approval. SEPA or the Environment agency should be informed in aquatic situations. For small patches of knotweed stems, close to water, glyphosate can be injected into stems either neat or at a 1:2 ratio. For accurate delivery of glyphosate Micron produce an injector unit called the “Injectordos” which delivers via a needle, a predetermined dose, (2ml/stem) of glyphosate. Other methods of delivery include a weed wiper, 1: 2 ratio of glyphosate or a knapsack sprayer at 5.0lt/ha. For well established populations more than one years control is usually required as the extensive root system is rarely controlled by a single application and often plants are missed.
Often Japanese knotweed is found on rough ground waiting development, railway embankments or areas of fly tipping. This is far more complicated as around as 100% control must be achieved, particularly on proposed building sites. Every site is different requiring a bespoke control measures. Glyphosate is an option but Tordon 22K, (Picloram) is more effective. Note Tordon should not be used close to water courses but unlike glyphosate does not kill surrounding grass. On contaminated building sites it may be necessary to first dig out the rhizomes to an area at least 7m from the edge of the knotweed bush and create a pile of contaminated soil and treat the re-growth on the pile with Tordon. This might have to be repeated more than once. The treated pile can then be back filled into the pit or sent to a specialist landfill site. Before back filling the pit/foundations it should first be lined with a heavy weight gauze, (1000 gauge), to prevent any rhizomes that might have been missed growing through the foundations. The gauze should overlap the sides of the foundations.
If the plan is to develop a site with a population of Japanese knotweed great care should be taken in moving plant material from the site. Rhizomes are often spread by machinery tyres and tracks.