In part two of our series on invasive, non-native species, Frances Giaquinto describes the main mechanical methods used to contain and eradicate Japanese knotweed from residential and commercial sites

This article describes the main mechanical methods used to contain and eradicate Japanese knotweed from residential and commercial sites. Read the first part of the article here: ‘Japanese knotweed jeopardises delivery of Irish engineering projects‘.

Japanese knotweed (Fallopia japonica) is a botanical opportunist. In the same way that a ruthless businessman may spot an opportunity and aggressively out compete others in order to succeed, Japanese knotweed smothers surrounding vegetation by growing taller and faster and uses a simple, yet brilliant mechanism of dispersal and expansion.

It also takes advantage of land-management practices. The expansion of the road and rail networks provide a linear path along which it can spread and the recession left a substantial acreage of abandoned and neglected land with road frontage which provides ideal habitats for colonisation. The spread of Japanese knotweed across Ireland continues to be relentless; it has not been brought under control and the construction sector is increasingly exposed to the cost and complexities of containment and eradication before works can start.

The terms eradication, control, containment, treatment and remediation tend to be used interchangeably by invasive weed specialists and can lead to misunderstanding. When appointing a contractor, it is advisable to clarify the intended goal of removal works from the outset.

Eradication is the complete removal of Japanese knotweed, defined by the Environment Agency (UK), as no re-growth for two years. Regardless of the method adopted, eradication can be rarely if ever achieved in one go and is likely to take a minimum of three-to-five years.

Term Meaning
Containment Legislation in Ireland makes it an offence to knowingly disperse or allow to escape Japanese knotweed & other regulated non-native invasive species (Regulation 49, EC Birds and Habitats Directive, SI 477, 2011). Containment is the term used for methods that prevent dispersal. Containment methods include installation of root barriers, chemical treatment, and limiting access.
Control Control is a generic term, meaning an action or series of actions to stop the infestation getting worse. A control programme may lead to eradication but this should not be assumed.
Eradication The Environment Agency UK defines eradication as, no re-growth for two years.
Remediation Remediation is a generic term, like control.
Treatment Treatment refers to the method used, specifically the application of herbicide. Mechanical soil screening may also be described as a treatment.

Some companies offer a five- or ten-year, insurance-backed guarantee on eradication. Environet UK was one of the first in the UK to offer this assurance, yet in this case eradication refers to 98% control. Unfortunately, the remaining 2% can quickly re-grow into a major infestation. If eradication is the required goal for a site, it is essential to ensure the contractor offers a post-control monitoring and treatment programme.

Methods to contain and eradicate Japanese knotweed

There are two main approaches to eradication: mechanical and chemical. Biological control is a third approach but the method, developed by CABI in the UK, is still in field trials . Mechanical methods involve physical excavation and removal of infested soil and ground materials, either off-site to licensed landfill or to an area on site for further treatment in situ.

Eradicating Knotweed

CLICK TO ENLARGE: Eradicating Knotweed

Excavation of infested soil and its disposal off site is probably the most effective, once-off method to eradicate Japanese knotweed. Monitoring for re-growth is still required for at least 2 years but the bulk of the infestation can be removed and at least some development of the site in question may be permissible. Licenses from the National Parks and Wildlife Service (NPWS) are required for removal off-site and the Environment Protection Agency (EPA) require a detailed paper trail, all of which are time-consuming. The cost of disposal may be extremely high. Currently it is difficult to find licensed landfill sites that will accept infested materials. However, if these barriers can be overcome it is a valid method for high-value sites requiring immediate development. INVAS Biosecurity is expert in this approach.

Excavation and treatment in situ involves one of two methods: excavation and burial on site, or soil screening to remove all rhizomes from the soil.

Japanese knotweed must be buried on site at a depth of 5m or more and this is rarely feasible. Bunding, in which infesting materials are encased in concrete below ground or partially above ground, can be considered only when there is sufficient available space on site and the geology and planning requirements allow partial burial of the bund. Bunded materials are chemically treated and this may involve herbicides with a high level of toxicity. It is rarely a cost-effective or an environmentally sensitive method.

Removing rhizomes

A more interesting and innovative method is soil screening to remove rhizomes. Fragments of rhizomes act like viable propagules and are capable of growing into new plants; as long as a node is present a fragment weighing 1g or less can regenerate. This poses a significant challenge to soil screening in which the grading mesh must be large enough to allow soil particles to move through without clogging (even when wet), yet small enough to contain rhizome fragments.

eradicate Japanese knotweed screened soil

Screened soil

Knotweed Ireland Services has designed soil screening equipment which is suitable for sites with a maximum of 50 tonnes of infested soil (165 m3), ie residential sites and contained areas of infestation on larger construction sites. The innovative grading mechanism is highly effective and the company is currently scaling up its equipment for larger-scale work. The advantage of this method lies in the infested soil being dealt with on site so permissions for off-site disposal are not required and the risk of unwitting dispersal is minimised. However, the resulting volume of screened rhizomes may be significant and these must be disposed of in a manner that prevents their dispersal. The local Council may grant permission to incinerate on site and, if not, permission from NPWS to dispose to licensed landfill will be necessary.

Like excavation and disposal to land fill, soil screening rarely eradicates in one go and a programme of monitoring and on-going treatment is essential until eradication has been achieved. In some situations vertical and/or horizontal root barriers can be installed to prevent incursion of new growth. For example, a vertical barrier may be installed at a boundary to prevent re-colonisation of a site from infestations on adjacent land, or a horizontal barrier may be installed beneath a lawn footprint. The EA recommends Dendro Scot barrier against Japanese knotweed although drainage issues must be factored in when considering its use. Various permeable root barriers are available.

The next article in this series reviews chemical treatment of Japanese knotweed and illustrates how mechanical and chemical methods can be put to best effect in combination. Regardless of the approach adopted it is critical to prepare a site specific management plan based on evidence drawn from a detailed assessment of the site. Factors to be considered include:

  • The extent and nature of the infestation;
  • Other non-native invasive species and noxious weeds on site and in the locality;
  • Proximity of infestation to property, water courses, utilities, residential areas, public spaces, EU and nationally protected sites (SACs, SPAs, NHAs);
  • The extent to which the infestation has been disturbed or previously treated;
  • Composition of ground materials;
  • Client’s budget and schedule;
  • The required goal (eradication or containment).

The local authority may require a site specific management plan and a Screening for Appropriate Assessment before planning is granted or development can proceed. The management plan is most useful when it is prepared as a detailed schedule of works, including the biosecurity measures to be implemented, the methods to be adopted, the rationale for the choice of methods, the specification of herbicide formulations and their method of application, and a schedule for post-treatment monitoring. Such a document can provide valuable evidence that good practice has been adopted, compliant with EU and national legislation and guidelines.

Frances Giaquinto is a PhD botanist, applied ecologist and invasive non-native plant species specialist who lives in County Clare. To contact her, email: or telephone: 061 924 287 (mobile: 087 966 2935). See O'RiordanCivilconstruction,environment,infrastructure,legislation
This article describes the main mechanical methods used to contain and eradicate Japanese knotweed from residential and commercial sites. Read the first part of the article here: 'Japanese knotweed jeopardises delivery of Irish engineering projects'. Japanese knotweed (Fallopia japonica) is a botanical opportunist. In the same way that a ruthless...