Japanese Knotweed

Fallopia japonica

Origin: Eastern Asia

Route of introduction: Introduced by the Victorian plant collectors as a garden plant which soon escaped into the wild.

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Stems: 2-3m tall, hollow bamboo like, green with red/purple speckles, arching branches with a characteristic zig-zag pattern. New shoots are red/purple in colour and appear in March growing up to 40mm per day. The stems grow from a large rhizome (underground stems) which can grow out to 7m and up to 3m deep from the parent plant. The rhizome is thick and woody with an orange centre.

Leaves: green, shield-shaped, up to 12cm long, with a flat base and pointed tip, arranged alternately along the branches.

Flowers: abundant clusters of small creamy coloured flowers appear from August to October.

Seeds: fertile male flowers are very rare on British plants, seed is produced only rarely from hybridization with other exotic knotweeds, and rarely survives (although climate change could change this).

General: Leaves and stems die back in late autumn turning brown, the dead bamboo like canes can remain upright and help identify the plant in winter. Fragments of rhizome or green stem, as small as a fingernail, can produce a new infestation of Japanese knotweed. Sections of rhizome may also persist underground in a dormant state for years and is extremely resilient to a range of conditions.

Not to be confused with: Other non native knotweeds – Giant knotweed Fallopia sachalinesis and hybrid knotweed Fallopa x bohemica. Giant knotweed is even bigger than JKW as the name suggests growing up to 5m tall with leaves up to 40cm long which have a heart shaped base.

Giant knotweed leaf

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Impacts

Economic: Knotweed stems and rhizome crowns have the ability to grow through cracked tarmac and concrete causing structural damage to buildings and infrastructure such as bridges and flood defences. Large dense stands can impede water flow and dead canes can be broken off and washed away causing blockages, increasing flood risk. It is difficult and expensive to eradicate and its removal from development sites can add on considerable costs due to the expensive techniques required to control it.

Biodiversity: Rapid growth rate allows large stands to form in both open and riparian areas, the large canopy cover results in heavy shading excluding native vegetation and prohibiting native plant regeneration. The loss of native ground cover species can lead to an increase in riverbank erosion in winter when the die back leaves banks exposed.

Amenity: dense stands of knotweed can block access to watercourses and footpaths. Litter washed downstream can easily get trapped by the large block of canes and this can encourage vermin.

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Control:

Mechanical: Under no circumstances is cutting or strimming recommended as this only serves to spread the plant. This method also raises the problem of dealing with the cut material which becomes a controlled waste and must be sent to a licensed landfill facility willing to accept this waste. Leaving the cut material to decompose in situ is not acceptable as it can re-grow thereby increasing the problem.

Chemical: Spraying and stem injection methods are both acceptable and effective methods of control.  Timing is important and best results from spraying are achieved from August – October.  The herbicide is absorbed  and translocated to the roots killing the plant from the ground up.  On-going monitoring of treated stands is essential as this weed can be extremely persistent and difficult to kill. It is likely that more than one year’s treatment will be required, three years is the minimum length of control for some stands of JK.  Any person using herbicide near a water course must have a PA6AW certificate and have appropriate approval from the environment agency.  This is the minimum qualification required to carry out this task.