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Japanese Knotweed — A Focus for Action

What is Japanese Knotweed?

Japanese knotweed (Fallopia japonica) was introduced into Europe during the mid-nineteenth century as an ornamental plant. It has since become one of the most problematic invasive weeds in Europe.
Japanese knotweed is a perennial weed, producing tall canes up to 3m in height during the summer. The canes have characteristic purple flecks, and produce branches from nodes along its length. The branches support shovel shaped leaves and clusters of white flowers in autumn. The canes die off in winter,turning brown and shedding their leaves. This produces a dense mulch that precludes the growth of native species


How Does Knotweed Spread?

Japanese knotweed canes grow from dense crowns that also produce extensive underground stems called rhizomes. These can produce new plants and research has shown that rhizomes can penetrate the ground up to 3m and spread 7m laterally. Roots are very destructive to tarmacadam, brickwork, drains and foundations.
Japanese knotweed is often spread from top soil imported on to site (fly tipping is reportedly a big issue in the UK), and from shard or slivers of stem created by trimming or flailing of knotweed in attempts to control it. A 5mm sliver of knotweed can generate a new plant.
Road verge flailing must be avoided as it can create linear infestations along the road treated as we have in Glounthaune.

Our Knotweed Survey Results

In September, Glounthaune Tidy Towns undertook an extensive survey of Knotweed between Bury’s Bridge and Killacloyne to ascertain the extent of the problem in Glounthaune. The results were astounding.
We catalogued 0.6 Hectare in 76 individual stands with a distinct linear infestation along the railway and road. We estimated over 176000 stems need treating. We calculate 98% of knotweed is on land in public ownership managed by Cork County Council and Iarnrod Eireann.
This is not a new problem for Glounthaune. We have documentation showing we highlighted knotweed infestations to Iarnrod Eireann as early as 1991, In addition we have been fighting an infestation around the Ashbourne pond for six or seven years.

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How Can Knotweed be Controlled?

Eradicating knotweed is difficult, time consuming and expensive. The London Olympic site spent over £80M controlling knotweed in a scheme where tall rhizomes were excavated, sorted from the spoil and then buried and covered with a membrane!
Using a herbicide with appropriate controls is effective. Foliar (foliage) spraying requires several applications to be effective and can take 3 years of repeated spraying. A more effective treatment is stem injection – here one injects each cane like stem with a small amount of herbicide at the end of the summer. We invested in a stem injector and experimented in three locations at the end of last summer The results will be seen at the end of this spring. This is just a very good solution, especially for waterside locations and prevents collateral damage. We will use foliar spraying where accessibility is an issue on health and safety grounds and stem injection in all other cases.

Our Proposal to Cork County Council

We have proposed that Tidy Towns will manage a knotweed eradication plan in partnership with Cork County Council. We have proposed a budget and we await a positive response. Once this is in place we will approach Irish Rail and the National Roads Authority for action on their Knotweed infestations.
The complete eradication of knotweed will require a multiyear effort and take several years commitment by all to eradicate it from our community.

Can I help?

There are many ways in which you can help:

  • Look around your property for knotweed and commence a treatment plan. We have a stem injector for hire.
  • Do not cut, strim or flail knotweed – as it can rapidly spread. Cork County Council have been approached to alter their roadside maintenance approaches
  • Call Garry on 086 231 3963 to add any new infestation to our survey map
  • Do not remove soil from any knotweed infested site and be aware of the source of any imported top soil
  • Only dispose of knotweed to a licensed site – this is a legislative requirement! We have not found one yet…..
  • Call your TD and County Councillors and make them aware of YOUR concern over this scourge
  • Volunteer to help with stem injection when this commences in the summer


Knotweed effects us. all Please join with us to eradicate it from our community