Japanese Knotweed


Fallopia japonica, (Houtt.) Ronse Decraene.

Japanese Knotweed is an invasive, non-native species that was introduced into the UK by botanists at the end of the Victorian era. Japanese Knotweed and another species Giant Knotweed, Fallopia sachalinensis, and Hybrid Knotweed, Fallopia japonica x Fallopia sachalinensis are Schedule 9 species on The Wildlife and Countryside Act 1981 (Variation of Schedule 9) (England and Wales) Order 2010. Under this act it is an offence to plant or cause to grow any plants listed in Schedule 9.


The stems grow vigorously from mid to late May dominating areas and creating stands where it is the only species present. The dried stems from previous years are generally present. Fresh stems are green and red with a similar appearance to rhubarb and can grow in excess of two metres in height. The leaves have a stem, are oppositely arranged on the stem. Stems are hollow. Later in the season small creamy white flowers appear in the leaf axils.

Method Of Spread

Rhizomes (underground stems that are capable of producing the stems and roots of another plant) grow rapidly underground producing long white shoots at the tips which send up shoots to the surface. Disturbance increases the stem density and if the tops are cut it is important to dispose of them properly.

All Japanese Knotweed plants within the UK are female and all male plants are shown to be infertile hybrids. Japanese Knotweed has an incredible capacity to spread: as little as a 10mm2, 0.7 gm section of rhizome has the capacity to produce a whole new plant. The plant spreads vegetatively from stems, crown and rhizomes and has spread throughout the entire UK since its arrival here in 1854.

Methods Of Control

Cultural Methods Of Control

The volume of soil that needs to be excavated extends up to 3 metres down and 7 metres horizontally from the above ground growth. The Environmental Protection Act 1990 contains a number of legal provisions that designate Japanese Knotweed contaminated soil as "controlled" waste. Only properly licensed organisations are allowed to remove this waste from a property where it must be transported to a licensed waste facility. Japanese Knotweed can be excavated and then buried on-site, but unless it can be covered with 5 metres or more of overburden, a specialist root barrier membrane should be installed to fully or partially encapsulate the Japanese Knotweed-bound soil, to prevent any regrowth.

Vertical barriers can be used to prevent its spread and form part of an integrated programme of management.

Chemical Methods Of Control

Professional Pesticides

Japanese Knotweed is susceptible to systemic herbicides, such as glyphosate and triclopyr. Triclopyr is the active ingredient in Blaster Pro, Icade and Synero. These are selective herbicides that are particularly effective at controlling woody weeds such as Brambles, Buddleia, Rhododendron, Sycamore and Ash trees as well as difficult-to-control weeds such as Giant Hogweed and Japanese Knotweed. The benefit of spraying with a selective herbicide is that grasses that are present will not be affected and can happily recolonise the area occupied by the Japanese Knotweed once it has died.

Chemical control is often the most cost-effective means of controlling Japanese Knotweed and refers to applying a professional herbicide over a period up to three years depending upon the infestation. The main consideration is the locality of the plant in relation to water. Consent is needed from the Environmental Agency if herbicides are to be sprayed near water. The only herbicide approved for application near water is glyphosate, the active ingredient in Roundup Pro Biactive. Those applying professional herbicides require an application license. In situations near water it is advisable to use a method of application known as "stem injection". This method of application is labour intensive but minimises the herbicide coming into contact with other plants and is particularly effective later in the year when the plant is marshalling resources into its extensive rhizomes for the next year"s growth.

Glyphosate is a non selective (kills all plants that it touches), systemic herbicide that upon application is absorbed through the foliage and transported through the entire vascular system of the plant. It disrupts the processes whereby the plant can manufacture its own food and thereby effectively starves the plant to death. The ability of this chemical and its speed of operation is dependent on several factors:

  • How large the root system of the plant is, these are its existing resources that enable it to produce new growth before it starves.
  • The weather conditions dictate how quickly the plant is able to distribute the chemical around its system. The more actively the plant is growing, the more effective the herbicide will be.
  • How much of the active ingredient is absorbed into the plant. This is dictated by the formulation of the product and the particle size e.g. a fine particle size will be easily absorbed through the leaf.
  • The size and growth-stage of the target plant influence the effect of the herbicide. Spraying early in the year on established stands of Japanese Knotweed will have little effect because the volume of herbicide being applied as a ratio of the root system is relatively small. However as this ratio alters the proportion of active ingredient increases leading to an increase in the amount of glyphosate absorbed by the plant.

Roundup ProBiactive, Blaster Pro, Icade and Synero are professional herbicides that require the person applying them to be in possession of a valid Pesticide Application license. Icade and Synero are both products that require aminopyralid stewardship training. Please contact us on sales@pitchcare.com for further details.

Domestic Weed Killers

As with the professional pesticides, two active ingredients are available: glyphosate is the active ingredient in Asteroid Biocare, a full strength glyphosate packaged in 1 litre bottle with integrated measuring cap, allowing the product to be sold to the non professional user. Asteroid Biocare is a very effective herbicide that starts to degrade almost as soon as it is applied however it is not selective and will kill any plant it comes into contact with.

Triclopyr is the active ingredient in SBK a selective brushwood killer that will control broadleaved weeds without affecting grass. SBK is particularly effective at controlling woody weeds.

Biological Methods Of Control

The introduction of a non-native species into most eco-systems has been cited as the second greatest cause underlying species decline. Therefore approaching the prospect of deliberately introducing a species to combat Japanese Knotweed, itself an invasive non-native species, has been done with great deliberation and care. A shortlist of almost 200 plant-eating invertebrates and about 40 species of fungi from within the area where it naturally occurs were thoroughly investigated to discern if primarily they had no detrimental affects to any of our native species and ultimately if any of them could be used as an effective biological control.

After testing their candidates on 90 different UK plant species, including plants closely related to Japanese knotweed such as bindweeds and important crops and ornamental species, they discovered a psyllid called Aphalara itadori was the best control agent. The little insect feeds on the sap of the superweed, stunting its growth. Following peer review by the Advisory Committee on Releases to the Environment and a public consultation, the UK government has now given the go-ahead for release of Aphalara itadori, under licence. The next stage of the controlled country-wide release of the psyllid at a small number of carefully selected sites containing Japanese knotweed in England and Wales has now been completed. These sites, together with a number of control sites on which the psyllid has not been released, will be closely monitored for five years.

Experience from around the world has shown that bio-control for most species takes between 5 and 10 years from the initial releases to the time significant control is achieved. Whilst more traditional management techniques such as mechanical and chemical control seem to offer more immediate control, one of the most important advantages of natural control is the long-term management it provides with minimal disturbance to the environment and reduced use of chemicals.

Guidance for the control of invasive weeds in or near fresh water

Anyone with knotweed on their land should take advice on the best control measures with reference to the Environment Agency's Code of Practice and Welsh Assembly Government's Technical Advice and continue to implement control measures as appropriate. Any delay will only allow the plant to establish more rhizomes (root systems) making it harder to control, whichever method is applied. It is likely that, with time, people will want to integrate their control measures with the psyllid to get the best benefit for the least effort/cost.