Japanese Knotweed (Fallopia japonica) is an invasive, non-native species that was introduced into the UK by botanists at the end of the Victorian era.
Japanese knotweed is a tall, vigorous rhizomatous (produces underground stems) perennial plant with distinctive, branching, hollow, bamboo-like stems, covered in purple speckles, often reaching 2-3 m high. The leaves of the mature plant are up to 120 mm in length with a flattened base and pointed tip and are arranged on arching stems in a zig-zag pattern.
The plant flowers late in the season, September to October, with small creamy-white flowers hanging in clusters from the leaf axils (point at which the leaf joins with the stem). The underground rhizomes are thick and woody with a knotty appearance and when broken reveal a bright orange-coloured centre. The rhizome system may extend to, and beyond, a depth of at least 2m and extend 7m laterally from a parent plant. During winter, the leaves die back to reveal orange/brown coloured woody stems which may stay erect for several years, and although it appears to be dead above ground it is still alive underground in hibernation. Stem and leaf material decomposes slowly, leaving a deep layer of plant litter. During March to April, the plant sends up new shoots, red/purple in colour with rolled back leaves. These shoots which can look like asparagus, grow rapidly due to stored nutrients in the extensive rhizome system. Growth rates of up to 40 mm a day have been recorded.
Method of Spread
Rhizomes (underground stems which put out lateral shoots and adventitious roots at intervals) grow rapidly underground producing long white shoots at the tips which send up shoots to the surface. Disturbance increases the stem density and if the tops are cut it is important to dispose of them properly.
All Japanese Knotweed plants within the UK are female and all male plants are shown to be infertile hybrids. Japanese Knotweed has an incredible capacity to spread: as little as a 10mm2, 0.7 gm section of rhizome has the capacity to produce a whole new plant. The plant spreads vegetatively from stems, crown and rhizomes and has spread throughout the entire UK.
Chemical – Chemical control is often the most cost-effective means of controlling Japanese Knotweed and refers to applying a professional herbicide over a period up to three years depending upon the infestation. The main consideration is the locality of the plant in relation to water. Consent is needed from the Environmental Agency if herbicides are to be sprayed near water. In situations near water it is advisable to use a method of application known as ‘Stem Injection’. This method of application is labour intensive but minimises the herbicide coming into contact with other plants and is particularly effective later in the year when the plant is marshalling resources into its extensive rhizomes for the next year’s growth.
Cultural – Japanese Knotweed can be excavated and then either removed from site which requires a specialist/properly licensed organisation to remove the waste from site where it must then be transported to a licensed waste facility. (The Environmental Protection Act 1990 contains a number of legal provisions that designate Japanese Knotweed contaminated soil as ‘controlled’ waste). Japanese Knotweed can be excavated and then buried on-site, but unless it can be covered with 5 metres or more of overburden, a specialist root barrier membrane should be installed to fully or partially encapsulate the Japanese Knotweed-bound soil, to prevent any regrowth.
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