Knot weed may result in refusal of permission to build.

Japanese Knot weed is wreaking havoc with our indigenous plant life and potential developments around the country. This invasive plant could prevent the development of houses on sites as planning can be refused if the plant is present.

The key features of the plant are summarised below:

  • Produces fleshy red tinged asparagus like shoots when it first breaks through the ground in an established stand.
  • Has large, heart or spade-shaped green leaves which are approximately the size of your hand.
  • Has leaves arranged in a zig-zag pattern along the stem.
  • Grows up to 3 metres in height.
  • Yellow / cream flowers in late summer (Typically the start forming from late July onwards).
  • Hollow bamboo like stems which have distinctive ring like nodules at regular intervals along it.
  • Brown stem in winter once it has died back.
  • Extensive rhizome system (roots) (7m x 3m approximately)
  • Orange centred root.
  • Spread entirely via the movement of plant and root fragments.

knotweed stem                             knotweed leaf

Just recently the presence of the plant resulted in the halting of an €8.7 m housing development in Clonakilty,  Co.Cork which would have delivered 56 houses according to Senator Tim Lombard.

Japanese knotweed can:

  • Seriously damage houses, buildings, hard surfaces and infrastructure growing through concrete, tarmac and other hard surfaces in some cases.
  • Threaten native plants and animals by forming dense thickets.
  • Block routes used by wildlife to disperse.
  • Riverside Japanese knotweed damages flood defence structures and reduces the capacity of channels to carry flood water.

As a result of the plants ability to grow through concrete and tarmac and its extremely wide root system, which can travel 7m from the parent, Japanese knotweed poses a serious long term threat to houses and buildings.

There is also a significant cost involved to have the plant eradicated from an area. In a recent interview Senator Lombard indicated that between £70-£80 million was spent in London to have the plant removed from the Olympic village. Following that colossal spend, the British government have allocated £5 bn to tackle the problem across the country which is expected to save money in  the long run.

In his recent radio interview, Senator Lombard expressed the opinion that there needs to be a national strategy in place to tackle the problem effectively.

“Local authorities in Ireland have struggled to deal with the issue of Japanese Knotweed because the current strategy of all state agencies working individually is not extensive enough to eradicate such invasive plants,” he said.
“The responsibility of dealing with invasive plants comes under the Department of Regional Development, Rural Affairs, Arts and the Gaeltacht, and this issue needs to be dealt with by the Department of Environment, which would have the ability to coordinate the local authorities, the NRA, the Inland Fisheries, and Iarnród Éireann in a national task force.
“To gain an understanding of the magnitude of this issue, all local authorities should carry out a survey on all their lands to record the prevalence of invasive plants such as Japanese Knotweed.
“Once the size of the undertaking is known, a national strategy can be put in place to eradicate this issue so that crucial infrastructure projects, such as the Clonakilty housing project, are not delayed in future.”

This could be an unexpected issue for any body in rural areas who might have been hoping to build on a site. The presence of the knotweed could result in the halting of any building project on that site.

If you identify Japanese Knotweed on your land,site or in your garden, take note of the following:

  1. Remember that it is illegal to dump Japanese knotweed waste in the countryside.
  2. It is illegal to plant or otherwise cause Japanese knotweed to grow. Hence you should be careful to ensure that you do not cause further spread.
  3. It is illegal to dispose of Japanese knotweed at a landfill site without informing the landfill site that the waste material is Japanese knotweed.
  4. Japanese knotweed can regenerate from very small fragments of root (as little as 0.7 grams).- Do not pull, strim, shred or pile knotweed as fragments of the plant can grow into new plants.
  5. Plant material should not be composted as it is ineffective and may result in further spread.
  6. Plants should be treated in the same season as they are identified. Try not to let stands of Japanese knotweed become established as this species is very difficult to control. If it is a recent introduction it is best to tackle it quickly to prevent the rhizome system from fully establishing.
  7. Japanese knotweed is not an easy plant to control due to its extensive underground root system. Therefore treatment often needs to be repeated until no regrowth is observed over several years for eradication to be achieved.

For more information about Japanese Knotweed click on the following links:



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