Japanese Knotweed2019-07-30T08:48:54+00:00

Japanese Knotweed

Fallopia japonica

About Japanese Knotweed

WHAT IS JAPANESE KNOTWEED?2018-10-01T17:15:43+00:00

Japanese knotweed (Fallopia japonicais a large perennial plant that grows throughout much of the UK. It was imported to Britain from Japan during the mid 19thcentury as popular ornamental feature to many gardens. Today, Japanese knotweed is recognised as one of the most invasive species present in the UK and is known for rapidly spreading and causing substantial damage to areas where it is present.


Japanese knotweed emerges as small asparagus-like shoots green/purple in colour. As the plant develops it produces small red/green shield-shaped leaves growing from the stem’s many distinct raised nodes or ‘knots’. Once mature, the leaves become a vibrant green colour reaching lengths of up to 120mm.  The red/green stems of adult Japanese knotweed have a hardy bamboo- like appearance which grows in thick clumps or ‘stands’. During the months of September and October, creamy white flowers are produced, growing in clusters at the end of the stems. As the plant sheds its leaves and dies off, the stems become hollow brown skeletal remains that are easily broken. The dead stems often remain upright amongst new growth during the following season.

WHY IS JAPANESE KNOTWEED A PROBLEM?2018-10-01T17:16:41+00:00

Being a non – native species to the United Kingdom, Japanese knotweed is able to grow rapidly and in many cases dominate its surrounding environment. Unaffected by the natural controls such as predators and diseases that target the native flora, Japanese knotweed infestations can quickly spread. Japanese knotweed is classed as an invasive weed in the UK and is included in the Wildlife and Countryside Act 1981, making it an offence to ‘plant or otherwise cause Japanese knotweed to grow in the wild’. Significant costs can result from the presence of Japanese knotweed growing in urban environments due to the damage it can cause to property.


Japanese knotweed is able to colonise a variety of urban and rural environments. Due to its ability to exploit weaknesses in construction material, such as concrete and tarmac, new knotweed growth can cause damage to structures and roads thus affecting the value of property as it rapidly develops. As Japanese knotweed grows along riverbanks it is able to colonise large areas as stems and rhizomes are carried downstream to ultimately spread the infestation causing severe damage to flood defence structures. Ecosystems can be disrupted by the presence of Japanese knotweed which can starve the surrounding native plants of resources, seriously impacting on the delicate balance of local food chains.

HOW CAN YOU KILL JAPANESE KNOTWEED?2018-10-01T17:17:40+00:00

Japanese knotweed infestations can be controlled and eliminated through a variety of methods. Depending on factors such as timescale, location of infestation and its surrounding environment the method can vary but through correct application and management Japanese knotweed growth can be stopped and prevented from returning. The main methods of control are:

Chemical – Qualified and competent personnel are permitted to use herbicides to control and eliminate Japanese knotweed. The application method and specific chemical used will depend on factors such as the infestations proximity to watercourses and local wildlife.

Burial – If time is a factor and it is unfeasible to wait for the numerous growing seasons that herbicide treatment may take to eradicate an infestation Japanese knotweed can be excavated and buried on site. All material contaminated with Japanese knotweed is encased in a root barrier membrane that prevents further growth and buried at a depth greater than 5m below the surface to prevent the material being disturbed and re-infesting the area.

Bund – If burial on site is not an option a bund can be created by designating an area of the site to place all material and soil contaminated with Japanese knotweed. The bund is then treated with herbicide allowing the original area of infestation to be developed or managed without the presence of Japanese knotweed being an issue. This method can be applied if there is sufficient time and space to relocate contaminated material and treat it on site, preventing the high costs of disposing Japanese knotweed at a licensed waste management facility off site.

Cutting – Japanese knotweed stems can be cut and dried out however this will only address the growth above ground and will take many years to effectively control the infestation without the use of chemicals or digging out the Japanese knotweed rhizome.

Burning – As with cutting this method can be ineffective without excavating Japanese knotweed rhizome from the soil and burning it with all plant material.

Biological Control – After careful research and development, tiny plant-eating insects called psyllids have been identified as a possible means to control Japanese knotweed, with tests being done to establish whether to release them nationwide. The ‘knotweed bugs’ have been released in some parts of England as part of a trial. Although they may control the Japanese knotweed they will not eradicate it fully otherwise the bugs would have nothing left to eat and they would die.

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Examples of Japanese Knotweed Treatment

Japanese Knotweed

Stem injection of Japanese Knotweed was undertaken as part of the EU Interreg Invasive Non-Native Species project in order to control this species in order to improve biodiversity and allow the public better access to the rivers.

Stem injection of herbicide was the most suitable method of controlling the Japanese knotweed on the banks of the River Urr & the River Dee with minimal impact on the surrounding environment.

River Urr & River Dee, Stem Injection of Japanese Knotweed, Galloway Fisheries Trust

Japanese Knotweed

Stem injection was used to eradicate a stand of Japanese knotweed in the front garden of a house in St Andrews. The infestation was surrounded by ornamental garden plants so it was not feasible to spray herbicide or dig out the Japanese knotweed. The direct application of herbicide into the Japanese knotweed’s stems was very effective at eliminating this invasive non-native weed.

Front garden, residential property, St Andrews, Private homeowner

Japanese Knotweed

A small stand of Japanese knotweed was growing close to an electrical substation within the grounds of the former nuclear powerstation at Chapelcross. Stem injection of herbicide was used to eradicate the Japanese knotweed quickly and at a low cost.

Stem Injection of Japanese Knotweed at Chapelcross Powerstation, Annan, Magnox North