Identifying and managing Japanese knotweed

There is too much information from our guide to include here, please download full information on the link above.

What is Japanese knotweed?

Scientific name: Reynoutria japonica var. japonica.

Perennial plant (herb) in the Knotweed family (Polygonaceae).

Japanese knotweed, a long-lived (perennial) and polycarpic (flowers many times) competitive dominant, thrives through rapid early-season growth from an underground rhizome (root). Its dense canopy restricts light access for native plant species, while the thick mulch formed by its decomposing leaves and stems limits their germination.

How does Japanese knotweed grow and spread?

In the UK, invasive knotweeds can reach up to 4 metres in height at full growth, spreading both by direct rhizome (root) expansion, at a rate of up to 0.5 metres per year, and vegetative (asexual, clonal) dispersal through fragments of plant material. This spread is facilitated by human activities, such as road and railway construction, as well as natural disturbances like flooding. Additionally, the rate of invasive knotweed proliferation in the landscape is exacerbated by ineffective control methods, such as cutting and flailing of roadside vegetation, and the illegal disposal of soil contaminated with knotweed material.

Where does Japanese knotweed grow?

Common habitats where Japanese knotweed is found include:

• Roadsides
• Railway embankments and cuttings
• Waste ground and spoil-tips
• Rivers and other watercourses (riparian habitats)
• Amenity areas

What impact does Japanese knotweed have ?

Competitive growth of invasive knotweeds causes negative ecological and socioeconomic impacts, including:

• Reduced habitat availability and quality – reduction of native plant, invertebrate and vertebrate species diversity, altered soil conditions and reduced ecosystem services.
• High management costs – estimated at around than £165 million per annum UK-wide.
• Property valuation – the presence of invasive knotweeds can affect property value and obtaining a mortgage on affected properties.
• Litigation – recent court cases have established Japanese knotweed and other invasive knotweeds can give rise to both criminal and civil liability.

Control and management – Invasive knotweeds

When we talk about effective control and management of invasive knotweeds with herbicides, what we are describing is ‘knocking out’ points where stems emerge from the below ground rhizome bud bank, year after year.

As time goes on, fewer and fewer of these growth points can produce new stems in the following spring. In many ways, this process is similar to spraying seedling weeds as they emerge each year.

However, whereas controlling seed banks is more effective early in the growing season, effective management of the rhizome bud bank involves herbicide (glyphosate) application late in the growing season.

Note that while the full downloadable guide does not provide advice on remediation methods for invasive knotweeds, many cultural (preventative) and physical (mechanical) control methods are ineffective and are likely to result in further spread.