Japanese Knotweed Frequently Asked Questions

What is Japanese knotweed?

Japanese knotweed (Fallopia japonica) is a large perennial plant that grows throughout much of the UK. It was imported to Britain from Japan during the mid 19^th century 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.

What does Japanese knotweed look like?

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?

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.

What damage can Japanese knotweed cause?

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?

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.

How do I prevent Japanese knotweed spreading?

Japanese knotweed spreads through fragments of rhizome and cut stems, capable of producing new shoots and roots when buried in soil. It only takes a piece of rhizome the size of a finger nail to generate into a new plant. This material can be carried far and wide, completely un-noticed on the sole of a shoe or the tracks of construction vehicles. If Japanese knotweed growth has been discovered it is important not to attempt to move or break the stems or uproot the plant. Take measures to prevent access to the infestation.

Japanese knotweed is spreading into my land from my neighbour’s land. What is the law on this?

Although it is not illegal to have Japanese knotweed growing on your property, your neighbour should be made aware that allowing Japanese knotweed to grow from their property onto other people’s property may be regarded as a private nuisance and this would be a civil matter under common law. If you think that you are at risk from Japanese knotweed on neighbouring property contact Invasive Weeds Agency for impartial advice.

Can I kill Japanese knotweed myself?

It is highly recommended that any attempt to control Japanese knotweed should only be carried out by trained and qualified persons. Due to the highly invasive nature of the plant, its complete eradication may require the use of potentially harmful chemicals that could put the user and the surrounding environment at risk if incorrectly applied.

Can I eat Japanese knotweed?

Young Japanese knotweed shoots are edible and provide an excellent source of vitamin A. It can be used in soups, sauces and dessert dishes, providing a taste similar to rhubarb. It is not advisable to harvest Japanese knotweed in the UK due to the possibility that it is growing in heavily polluted soil. Also, there is a possibility of spreading the weed if incorrectly handled or disposed of, as well as the possibility of prosecution for the incorrect transportation of this weed.

Can I plant Japanese knotweed?

Under section 14 of the Wildlife and Countryside Act 1981, it is an offence to plant or otherwise cause to grow Japanese knotweed in the wild. It is not an offence to plant it in your garden, however, doing so may seriously affect the value of your property.

Is Japanese knotweed poisonous?

Japanese knotweed is not poisonous and is not harmful to touch, however, always use caution when in close proximity to avoid inadvertently allowing the infestation to spread.