Japanese knotweed
Japanese Knotweed
Japanese knotweed

Meet the plant

Japanese knotweed (Fallopia japonica) has a bamboo like stem, with purple speckles and roughly triangular green leaves between 10-15cm long, on a zig-zag twig.  It has an extensive root system of rhizomes, making it difficult to remove. Roots can grow up to 3m deep, and travel up to 7m laterally in all directions. 

It was introduced from the far East to Europe in 1849, first being recorded in the wild in GB in South Wales in 1886.  Japanese knotweed was considered an ornamental "architectural" plant and widely grown in larger gardens and parks. It was also thought to have been planted by local governements and businesses to shore up embankments and pathways, as it was believed to protect the ground from mudslides and movement.

It tends to grow on disturbed ground, on riverbanks, railways, road sides, waste land and in urban areas. Occasionally it may be found in woodland.

Download the Japanese knotweed ID guide

Read more about Japanese knotweed

 

Japanese Knotweed

Impacts

Japanese knotweed can form dense stands which stops native flowers and shrubs from growing. However, the actual impact of this is poorly researched as it usually grows on sites that are already degraded and bare of other plants.

It does alter the habitat structure and wildlife of a river bank, which is known to directly impact on salmonid fisheries, both due to restricting access for angling and hindering conservation efforts of rivers.

The rhizomes of the plant are extremely tough and re-grow readily. It has caused problems by growing through foundations of houses and pavements and this has led to the plant having a high economic impact.  It is a legal requirement to eradicate Japanese knotweed from construction sites, to do so is very costly, in the actual removal and the disposal of what is classed as contaminated waste.

Stem injection

Management of Japanese knotweed

Herbicide Application

Given how hard it is to fully eradicate Japanese knotweed, research suggests the only effective way of controlling it is to use herbicide (Glyphosate).  We are using one of two techniques of application – stem injection and spraying using knapsack sprayers.  Stem injection is more targeted and has proved to be more effective, but it is much slower and labour intensive, so on large infestations the more practical method used is spraying.  We visit each treatment site twice; with an initial treatment between Aug - Oct, with a follow up treatment if necessary.

Treatment is needed over subsequent years, while initial success can be achieved in the first year, at least some of the root will remain and will grow to form a new stem if the treatment isn't followed up in years 2-3. The average time for a chemical treatment programme is 3-5 years.

Remember – you may need permissions from SEPA to use pesticides near water, and you may need permission to remove or dispose of invasive species and contaminated soils.

 

CAUTION - Do not attempt to cut or strim Japanese knotweed. Repeatedly cutting with a mower will not kill it, the plant will simply re-grow from the root.  Attempts at digging the plant out should also be avoided, the surrounding area will be contaminated with the roots and it only takes a discarded 2cm piece of root to start a new plant. 

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Get involved!

We want to create a network or local people who have their pesticide application qualifications who can treat Japanese knotweed in their local area, these might be individuals, students, estate workers, ghillies, rangers, wildlife groups, community groups.

Find out more about volunteering and getting qualified

 

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Where is it?

Japanese knotweed is widespread throughout our project area and is in all our partner fishery trust catchments.

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