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Scientific name:
Fallopia japonica
Japanese Bamboo, Polygonum cuspidatum, Reynoutria japonica
Native to:
Japan, Taiwan, northern China
Common in urban areas, particularly on waste land, railways, road sides and river banks

What is Japanese Knotweed

japanese knotweed

Japanese knotweed is a non-native alien invasive plant species that is rated among the 100 worst invasive alien species in the world by the Global Invasive Species Programme (GISP). The weed is a robust, herbaceous perennial with deeply penetrating woody rhizomes and bamboo-like stems that can grow to 3 metres tall. The weed was introduced to England as an ornamental in the mid-19th century and has spread throughout Great Britain and Ireland since.

Only female plants are present in Ireland and most seeds produced are sterile. Propagation is via fragmentation (of stems and rhizomes) and underground rhizome growth. Japanese knotweed is a very fast growing plant that can rapidly outcompete native vegetation and can cause structural damage to foundations and hard standings.

Japanese knotweed is recognised as being very difficult to control, reflecting the ability the weed has to regenerate from small fragments and to spread rapidly via its very large and intricate rhizome network.

Identification of Japanese Knotweed

It is a rhizomatous (produces underground stems) perennial plant with distinctive, branching, hollow, bamboo-like stems, covered in purple speckles and often reaching 3 m high. The leaves of the mature plant are up to 170 mm in length with a flat (truncate) base and pointed tip and are arranged on arching stems in a zig-zag pattern.

The plant flowers late in the season, August 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 can penetrate to 3 m deep and 7 m beyond the last emerged stem. During winter, the leaves die back to reveal orange/brown coloured woody stems which may stay erect for several years. 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 grow rapidly due to stored nutrients in the extensive rhizome system. Growth rates of up to 40 mm a day have been recorded.

Source: Inland Fisheries Ireland


japanese knotweed throughout the year


japanese knotweed key features identification

Problems caused by Japanese Knotweed

Japanese Knotweed was first introduced into Ireland by the late Victorians as an ornamental plant for their gardens. However, outside its native habitat of Japan and Northern China the plant has proven to be an aggressive coloniser without the checks and balances of the predators and pathogens present in its native habitat. Japanese Knotweed poses a serious threat to:

  • Biodiversity through light exclusion
  • Erosion to riverbanks after winter dieback
  • Access to river banks for anglers, boating, swimming or inspection
  • Local tourist economies through habitat destruction.
  • Foundations, tarmac, hard standings, roads, railway infrastructure, etc.

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Long-term Solution for Japanese Knotweed

Japanese knotweed is acknowledged to be a very difficult weed to effectively control and an even more difficult weed to fully eradicate. However, long-term control and even eradication is possible with conscientious and science-based treatment measures, assiduous and timely application procedures, rigorous monitoring and appropriate after-care.

Once Japanese knotweed has been identified at a site, it is important to ensure that the weed is not disturbed or interfered with until a definite biosecurity and control plan is instituted. INVAS Biosecurity has considerable experience in this regard, having effectively controlled this highly invasive weed in both domestic and large-scale construction situations.

Japanese knotweed can be effectively treated using chemical methods or excavation. Cutting or strimming is not recommended and indeed should be actively discouraged as it will merely spread the infestation.


Chemical treatment is the most environmentally friendly method to control Japanese knotweed. Using this method all treatment work is confined to the site. This eliminates the requirement to disturb, excavate or remove the infested soil, thus significantly reducing the risk of spreading the weed. Treatment using herbicides can effectively control Japanese knotweed. However, it is highly unlikely that a single application of any approved herbicide will effectively kill this pervasive weed and complete eradication may take three years or longer.

It is important to only use herbicides that are cleared for use by the Pesticide Control Service of the Department of Agriculture, Fisheries and Food. Where the Japanese knotweed infestation is near water, only glyphosate-based herbicides can be used. Herbicide treatment must only be conducted by competent and qualified personnel.

When planning any herbicide treatment for Japanese knotweed control, INVAS Biosecurity staff will take into account the following issues:

  • need for licenses or permits to conduct work
  • area and extent of infestation(s)
  • age and maturity of the infestation(s)
  • timing of the treatment
  • proximity of water courses
  • adjacent native flora
  • proximity of wildlife habitats
  • most appropriate herbicide and adjuvant.

Herbicides can be applied from May to October but recent research indicates that later applications (July to October) are often most effective. Appropriate herbicides can be applied to the foliage or injected directly into the stem.

Foliar applications can be made directly onto the leaves from June to October, although the maximum uptake of herbicide by the plant is recorded when applied between July and September.

Stem injection is the preferred method where the extent of the infestation is not too large, as it is necessary to inject each individual stem to derive the desired kill. It is also the preferred method where the infestation borders running water or is in a protected conservation area. Using this method a concentrated solution of the herbicide and marker is injected into the flowering stem towards the base of the plant. This operation is best conducted in September or early October.

Excavation is very expensive and highly disruptive. Hence, it should be the method of last resort. However, where it is deemed necessary to excavate, effective control of Japanese knotweed can be achieved.

Because of the capacity of the rhizomes to penetrate to 3 metres deep and 7 metres beyond the leading edge of the foliar infestation, sites for excavation must be clearly marked and take into account the underground extent of the weed. Barriers should be erected at least 10 metres distant from the last obvious stems and leaves, and all soil excavated to at least 7 metres from this leading edge.

Two options for excavation are available: excavation and containment or excavation and removal.

Excavation and containment

Soil containing Japanese knotweed material can be buried on site, once a suitable and contained location is available. This is far less disruptive and less costly than transporting the spoil to landfill. It is advisable to apply a non-persistent herbicide to the spoil before burial to further inhibit any knotweed growth. Soil that is contaminated with Japanese knotweed must be buried to a depth of at least 5 metres and then covered with a reputable root barrier membrane layer. The area must then be filled with inert fill or topsoil.

It may be possible to stockpile the Japanese knotweed-infested soil, if a suitable area on the site is available. This site must be free from disturbance and should be secured using barriers and suitable signage. The soil should be treated with herbicide as it is being stockpiled and regularly monitored for any Japanese knotweed growth. Regular treatment of regrowth will probably be required.

Excavation and removal

Where spoil removal is the only method of safe disposal possible, contaminated material must be removed from the site in biosecure trucks and transported to licensed landfill. It is important to give prior notice to the operator of the landfill site as not all sites will accept Japanese knotweed waste.


Japanese knotweed control programmes that are operated by INVAS Biosecurity are individually tailored to suit the requirements of each site and all our staff are both qualified and experienced in the application of herbicide. It is important to use professionals for this work, whether the job is big or small. In the end it will cost you less.