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Japanese Knotweed

If the Japanese Knotweed is on a highway, please report it online via the Northamptonshire Highway’s Street Doctor.

What is Japanese Knotweed?

Japanese Knotweed is an invasive perennial plant capable of covering large areas in a dense growth up to 3m tall. It grows quickly in the spring from deep root (rhizome) systems, flowers in the autumn and dies down when frosts occur. Spring growth rates of 40mm/day have been recorded.

Japanese Knotweed is also known by its scientific name Fallopia japonica (and by older names Polygonum cuspidatum and Reynoutria japonica). It is related to Russian Vine (Mile a Minute plant). The deep rhizome systems are a key part of the problem, extending many metres beyond the visible stems and foliage. A small established stand may be the tip of an ‘iceberg’ of rhizome-infested material.

Are there legal requirements for eradication?

It is an offence under the Wildlife and Countryside Act, 1981 knowingly to introduce Japanese knotweed into the wild. Transporting infested soil to another site, or allowing its uncontrolled disposal, could constitute an offence. Material infested with Japanese Knotweed is classed as ‘controlled waste’ within legislation arising from the Environmental Protection Act 1990, and so can only be disposed of in accordance with requirements for the transport, storage and disposal of waste in Section 34 of the EPA.

The Environment Agency should be notified before any Japanese Knotweed is removed from site or buried on site. The Act places a duty of care on all waste producers to ensure that any wastes are disposed of safely and that a written description of the wastes, and any harmful properties, is provided to the site operator.

How do I get rid of it?

The plant can be killed with herbicides but the extensive rhizome network means that repeated applications over 3 years are required. Without a systematic and determined approach, chemical control over larger areas is often ineffective. Good control over the movement of soil materials is also needed, to prevent re-invasion. Developers require more rapid solutions, and so ‘dig and dump’ or on-site burial in deep, lined containment cells are sometimes employed, at considerable cost. Every fragment of rhizome must be excavated and so many cubic metres of waste are generated for each square meter of Japanese Knotweed visible at the surface.

Only a limited number of landfills are licensed to accept infested fill, you should contact the Environment Agency on 03708 506 506 to find the nearest landfill. The material is classed as ‘active’ waste and so attracts the higher rate of landfill tax on top of the tipping fee. It may also be necessary to import clean fill to replace the material removed from site, before redevelopment can proceed. Some sites have sufficient space for the infested material to be excavated and placed in one location to allow a programme of herbicide treatment to be undertaken in parallel with the development of the remainder of the site.

Expert supervision of the excavation and movement of material is essential, together with qualified specification and application of herbicides.

What can I do if Japanese Knotweed is invading from land next door?

Eradicating Japanese Knotweed from one side of the fence will not stop it spreading back, so it is essential to join forces with neighbouring landowners and to tackle the problem in a co-ordinated manner. If adjoining knotweed cannot be treated, it is possible to insert a deep barrier to prevent re-invasion.

Can I get specialist help?

Many weed control specialists offer herbicide treatment programmes, but it is important to ensure that they have BASIS- qualified personnel to select and apply the chemical, and to consult the Environment Agency about nearby watercourses and wildlife. Knotweed specialists and civil engineers should be called in to specify and supervise excavation works, to ensure that every fragment is removed and that deep excavations are carried out safely.

Any on site burial should also be supervised by a civil or geotechnical engineer, taking account of the end-use of the site. The potential for other site contamination of hazards should also be considered before ground is disturbed.

So how is it spread?

Japanese Knotweed does not produce viable seeds in the UK. It spreads as fragments of rhizome in soil and fill materials moved between sites or tipped as waste. Fly tipping is often the route into isolated areas of countryside and neglected town sites. It is found on many riverbanks where it can be spread downstream during floods.

Recent research shows that 1cm fragments of rhizome can regenerate into new plants, and fresh cut stems can also regenerate if left in water or wet conditions. Local authorities and the Environment Agency are taking increasingly strict measures to reduce the unwitting spread of Japanese Knotweed in regeneration and redevelopment work.