Japanese knotweed

Japanese knotweed is a tall vigorously growing perennial plant which originates from Japan and was brought to Europe in the mid 1880s. It is a highly aggressive, invasive non-native plant and can colonise most habitats including:

• stream banks
• woodlands
• grasslands
• coastal areas
• road verges
  
• gardens
  
• waste ground

Report Japanese knotweed sightings

​You can help us gather data on where Japanese knotweed is found in Jersey by reporting and sending photos of plants you find.

The best way to do this is to use either the iRecord or the Jersey Biodiversity Centre websites, or you can email Natural Environment, include photos and location details, or phone us.

If we confirm it is knotweed, your sighting will be plotted on a map showing the invasive plant's progress, informing future control activities.

What it looks like

It is easily recognisable at all stages of its growth. Early season shoots are often reddish in colour. These grow into characteristic tall, hollow, bamboo-like stems which are usually pale green and purple in its mature state. Mature Japanese knotweed stems can be 2 metres high, growing up to 10 centimetres per day.

The plant produces small white flowers in late summer, and its leaves are arranged in a zigzag pattern up the stem.

Japanese knotweed and the Law

Natural Environment is raising awareness with landowners, encouraging and supporting them to control their knotweed, in order to help minimise future spread and ideally eradicate it.

However, it is important to note, that since June 2021, Japanese knotweed (Fallopia japonica) has been listed in the Wildlife (Jersey) Law 2021 as an invasive, non-native species (as defined in Article 25(1) of the Law). The Law makes it an offence to possess Japanese knotweed, or to deliberately plant or knowingly cause or permit another person to plant it in the wild, without a licence under the Law to do so.

The Law additionally provides for authorised officers to enter into species control agreements with landowners for the control of Japanese knotweed. In certain circumstances, including if a landowner refuses to enter into or take reasonable steps to comply with a species control agreement, the Minister for the Environment may make a Species Control Order requiring control measures to be undertaken. 

Damage caused by Japanese knotweed

It is strongly recommended that this plant is at least controlled, and preferably removed or destroyed, wherever it is found, to minimize its spread across the island.

The foliage forms a dense canopy which restricts the growth of native ground flora, and other plant species.

Shoots are able to push through asphalt, damaging:

• pavements
• car parks
• patios
• roads
• houses
  

• penetrating foundations of walls
• penetrating land drainage works
• exploiting and opening up cracks in concrete and walls
  

Controlling Japanese knotweed

There are a variety of options available to control the plant. The most appropriate method will depend on several factors. Whatever method is used, killing the extensive rhizome system is essential if lasting control of Japanese knotweed is to be achieved. This may mean that the management programme will take a number of years to be effective.

It is important to realise that there is no quick solution for the control of this plant.

Natural Environment maintain a list of contractors that have carried out training, including the targeted stem-injection method that can help control Japanese knotweed. However, there are also measures that individual householders can take to cost-effectively manage this invasive weed:

• chemical treatment using herbicides - currently the use of a Glyphosate based herbicide is the most effective method of controlling and eradicating Japanese knotweed. It is important that the treatment is applied at the right time of year. Always read the label carefully, follow the instructions, and wear appropriate PPE. Only approved herbicides can be used near water
  
• cutting - a sharp blade should be used to cut shoots on a regular basis. It is imperative that stems and rhizome material are disposed of appropriately
  
• hand pulling – pulling up stems from the base - this is only suitable for small infestations. It is imperative that stems and rhizome material are disposed of appropriately
  
• grazing – constant grazing can reduce the spread of Japanese knotweed but will not eradicate it
  
• excavation - on larger sites where it is not possible to treat the infestation over a number of years, excavating the knotweed, followed by screening, is an option. Contaminated soil should be retained on site using a bunding / burial system, utilising an appropriate root barrier membrane. An appropriately qualified person should develop a Knotweed management plan in line with best practice, to ensure the success of the treatment and prevent it from spreading
  

Disposing of Japanese knotweed

As knotweed can regenerate from small fragments, it is important that any plant material is disposed of carefully and not allowed to spread.

It is particularly important that fragments should not be allowed to enter water courses and ditches.

All machinery used to cut knotweed, or cultivate knotweed contaminated ground, must be thoroughly cleaned before leaving the site.

You must separate Japanese knotweed from other green waste.

Disposal options are:

• burning cut knotweed stems on site
• piling it up and allowing it to dry out and eventually die
• carefully and securely bagging it up and taking it to the Energy Recovery Facility at La Collette for incineration. It is important that you phone ahead and book in any deliveries by telephoning 448383. If you need to dispose of large quantities, it may have to be delivered over an agreed period of time. 
  

Further information

UK Environment Agency - The Knotweed Code of Practice

RAPID - Reducing and Preventing Alien Invasive Species Dispersal - Good Practice Managment Guide for Japanese Knotweed

INNSA (Invasive Non-Native Specialists Asssociation - Japanese Knotweed Code of Practice