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 branch. 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 Great Britain in South Wales in 1886. Japanese knotweed was considered an ornamental “architectural” plant and widely grown in larger gardens and parks. Historically, it has also been planted 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

Impacts

Japanese knotweed forms dense stands which can stop or limit the growth of native flora. However, the actual impact of this is poorly researched as it often grows on sites already degraded and bare of other plants. It does alter the habitat structure and wildlife of a river bank and can restrict river access for angling or other recreation activities.

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 so the plant can have a high economic impact. It is a legal requirement to eradicate Japanese knotweed from construction sites, but to do so is very costly, as the plant material and soil are classed as contaminated waste and must be removed and disposal of appropriately at licensed sites.

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Japanese knotweed management

Japanese knotweed grows in dense stands along many rivers in our project area. The rhizomal root system can be left exposed when erosion and flooding wears away soil from riverbanks, allowing fragments to be washed downstream and establish elsewhere. Similarly, Japanese knotweed frequently spreads along rivers and waterways when the plant is cut, strimmed or mowed as this creates loose stems and fragments which are carried downstream and establish in new locations. Most plants in Great Britain are sterile and so do not produce viable seed after flowering so it is not a particularly mobile species – if not being cut or strimmed! We are mostly dealing with long-established Japanese knotweed stands in our control programme.

In most of our control we apply approved herbicide to the plants via knapsack sprayer or stem injection. This work is only possible thanks to the support and work of numerous land managers, carrying out their own control, and volunteers who come along to help and work with us.

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Herbicide application

Due to its persistent rhizomes and ability to regenerate, the most effective way of controlling knotweed at scale in the wider environment is by herbicide (Glyphosate) application. Treatment is needed over multiple years. While initial success and progress can be achieved and made in the first year, at least some of the root will remain and will grow to form a new stem if treatment is not followed up in years 2-3. In general, a chemical treatment programme for an infested site is 3-5 years. After that time sites should be monitored as rhizomes can remain dormant for a number of years before becoming active again – for example if the ground is disturbed.

We treat plants from late summer to autumn, usually between August and October. It is possible to treat the plant earlier in the season but late summer application is more effective as, at this time, the plant is diverting energy resources to the rhizome to support growth the following year. Therefore, herbicide applied then more effectively reaches the root systems and better reduces the growth of the plant. This work is undertaken by qualified staff and volunteers.

Japanese knotweed control

Spraying

We commonly use backpack sprayers to apply herbicide by foliar spray. This is an effective means of treating Japanese knotweed to reduce the extent of growth. We use specific nozzles to target and apply herbicide directly to the target plants and foliage.

Spraying is only carried out in appropriate weather conditions (dry, little wind) to maximise effectiveness and minimise the risk of off-site application or drift.
Stem injection

We also use stem injectors to apply herbicide. These deliver a measured dose of Glyphosate directly into the stem of the plant, making this method very targeted, plant specific and effective. However, it can be slow work and labour intensive – particularly in larger stands.

This method is preferred on sites where there is particular concern for off-target application – for example, on a nature reserve or Site of Special Scientific Interest (SSSI) – or where we are dealing with small stands where treating plants directly is preferred.
Digging

Excavation requires the removal of all the rhizome and all the surrounding soil – which is classed as contaminated. Such material is then classed as controlled waste and a licence is required for disposal. As such excavation is normally only carried out at a commercial development sites, often with the deployment of membrane barriers to prevent regrowth from any remaining rhizome fragments in the soil. This method is not used within our control programme.
Winter works

We have found it useful on some sites to visit in the winter and strim or cut back dead knotweed stems. This allows us easier access the next season carry out spraying or stem injection.

*IMPORTANT – This is only carried out on dead stems in winter. Never cut or strim living Japanese knotweed!*