Japanese knotweed: economic and ecological scourge


Japanese knotweed Fallopia japonica, is a non-native invasive herbaceous perennial plant. It was introduced as an ornamental plant in the 19th Century from Japan, but has spread across Ireland, particularly along watercourses, roads, railways and waste grounds where its movement is unrestricted.

Japanese knotweed causes a number of major problems, both economic and ecological. It can seriously damage houses, buildings, and infrastructure by growing around, “heaving” from below hard features and sometimes even growing through them. Once established underneath or around the built environment, Japanese knotweed can be particularly hard to control. Japanese knotweed also causes a number of significant ecological impacts. It grows vigorously and out-competes native plants and it can also threaten native plants and animals by forming dense growths and blocking wildlife commuting corridors. Japanese knotweed can also damage flood defence structures such as embankments, and can reduce the flood conveyance capacity of river channels. It dies back in winter leaving river banks vulnerable to erosion.

Japanese knotweed is listed on the ‘most unwanted’ list in Ireland. Regulations 49 and 50 of the European Communities (Birds and Natural Habitats) Regulations 2011 (S.I. No. 477/2011) make it an offence to:

  • plant, disperse, allow dispersal or cause the spread of Japanese knotweed.
  • keep the plant in possession for purpose of sale, breeding, reproduction, propagation, distribution, introduction or release;
  • keep anything from which the plant can be reproduced or propagated from without a granted licence;
  • keep any vector material, in this case soil or spoil taken from Japanese knotweed, for the purposes of breeding, distribution, introduction or release.
Japanese knotweed Fallopia japonica.
Japanese knotweed on a river channel after being dispersed by river dredging works.
Japanese knotweed on a river channel after being dispersed by river dredging works.
There is some work being undertaken to tackle the Japanese knotweed problem on Ireland’s roads network but we are just at the start of controlling this major problem.

Problem for development sites

The presence of Japanese Knotweed (and indeed many other non-native species) can result in limitations to overall site management objectives during the construction phase of projects, due to delays in scheduling of works as a result of the requirement to treat infestations of this plant on a site, or as a result of damage caused by this noxious plant. There is also potential for spread of Japanese knotweed from within and outside the site boundary as it has extensive roots systems.

Japanese knotweed roots can remain viable for up to 20 years if they are disturbed – adding large, often unexpected, costs to a development site.  Japanese knotweed treatment needs to start as soon as possible. In the dormant (winter) season, the plant may not be visible particularly if the site has been cleared of vegetation, so all development sites should be surveyed at the earliest opportunity.

Interest is resuming in building houses and other developments in Ireland, but we are finding that invasive species such as Japanese knotweed are a major issue on many these derelict sites. Many stalled development sites, NAMA lands and ghost estates have been left derelict for over 5 years now and this has unfortunately allowed many non-native invasive species, such as Japanese knotweed, take hold and grow unchecked. These sites are a risk to other areas potentially allowing these plants to spread.

Invasive species such as Japanese knotweed can be a major issue for development for development sites as these plants will have to be eradicated first. It is illegal to disturb or disperse these plants, and even if it wasn’t you could not build houses over a field covered with Japanese knotweed as the shoots can grow up through buildings and roads, both damaging and devaluing property. It can take up to 3 years of herbicide treatment with monitoring and follow up control for up to 5 years to eradicate this plant from  site.

Japanese knotweed
Winter canes of Japanese knotweed on a proposed construction site. Often non-native species can be missed during the winter months.
Japanese knotweed
Japanese knotweed on a commercial premises beside a Natura 2000 river in winter. Use of machinery and other disturbances result in a high risk of further dispersing this plant with broken fragments on machinery and via the river.


Once Japanese knotweed stands become established, they are extremely persistent and difficult to remove. This plant has the ability to grow through tarmac and concrete, and in some cases within dwellings. It can also “heave” from below buildings and roads – which can cause significant damage. Spraying is normally the cheapest method of control but usually takes at least three years of continued repeat sprays. The current most widely recommended chemical for Japanese knotweed control is glyphosate which breaks down in the soil relatively quickly. Glyphosate does, however, because of its broad spectrum nature, have the disadvantage of being potentially damaging to non-target plants.

The Environment Agency* have advised that combining digging and spraying can be effective in reducing the time needed for chemical control. By digging and breaking up the rhizome, the aim is to stimulate leaf production leaving the plant more vulnerable to treatment with a plant protection product. Knotweed spraying is undertaken in the growing season of May to September. The site then has to be monitored for a minimum of five years after any viable plant material is found. Japanese knotweed control is done under licence.

Other removal options also include stem injection – a new technology offers the quickest and most cost-effective method of control, but can still take up to 3 years. Whole area Japanese knotweed removal within the site or to controlled waste site is the quickest option on a development site, but also the most expensive. This option removes the whole area of contamination to a waste disposal site or a more suitable area of the site. Root barrier membranes can also be used. They are expensive and can provide a quick solution in some cases. They can be particularly useful for use as a barrier on a site boundary to prevent horizontal spread of Japanese knotweed from an joining infected site. Root barrier membranes can also be used under a building or structure to significantly reduce the risk of future problems. To prevent damage from ‘heave’ a layer of sand is installed under the membrane.

It is noted that all information on the Japanese knotweed infestation of a development site, and the work to eliminate it, should be passed onto to future owners. Where a site has been affected it is therefore necessary that a guarantee is provided by the contractor used to undertake the non-native invasive species control and management work.

Whatever treatment method approach is taken it is important that monitoring of treatment, excavation and/or burial sites will be undertaken by a suitably qualified ecologist.

*Note that this document has now been withdrawn.

Further reading

For further reading see:-

Who to contact?

ECOFACT can advise on the management and control of Japanese knotweed and other non-native invasive species in Ireland. If you need a non-native invasive species survey or any ecological advice about developing site please do not hesitate to contact us at info@ecofact.ie or at +353 61 419477. Also visit our main website at www.ecofact.ie.

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