Japanese Knotweed is classified as a high-impact invasive species. Native to Japan, Taiwan, Korea and northern China.
In Ireland, without this evolved food chain, it is now one of the most problematic invasive alien species. It is an increasingly common sight on waste ground, the fringes of waterways and roads. It is classified as one of the top 100 worst invasive species worldwide because of its serious impact on biological diversity, impact on human activity and its capacity to invade new environments.
A number of other knotweed species of concern are established throughout the county – details of which are listed below.
|Japanese Knotweed (Fallopia japonica)||Classified as a high impact invasive species by the Irish National Biodiversity Centre. Third Schedule listed species under Regulations 49 & 50 in the European Communities (Birds and Natural Habitats) Regulations 2011.||
|Classified as a high impact invasive species by the Irish National Biodiversity Centre. Third Schedule listed species under Regulations 49 & 50 in the European Communities (Birds and Natural Habitats) Regulations 2011.||
|Classified as a medium impact invasive species by the Irish National Biodiversity Centre. Third Schedule listed species under Regulations 49 & 50 in the European Communities (Birds and Natural Habitats) Regulations 2011.||
|Classified as a high impact invasive species by the Irish National Biodiversity Centre. Not a Third Schedule listed species under Regulations 49 & 50 in the European Communities (Birds and Natural Habitats) Regulations 2011.||
Japanese knotweed (Fallopia japonica)
- Mature stands grow to approx 1m – 2.5m tall
- Heart shaped leaf with flat base near the stem. Up to 5 inches long
- Smooth profile edge on leaf
- Zig zag structure to the canes/stems
- Green canes/stems with red/purple speckles. Brown and woody in winter.
- Bamboo rings at nodes. Stems are hollow.
Giant Knotweed (Fallopia sachalinensis)
- Much taller than Fallopia Japonica 4-5m
- Leaves 2–3 times bigger. Can be much bigger than an adults hand
- Have a crinkle around the edge
- Don’t have the distinctive flat base on the leaf
Bohemian knotweed (Fallopia x bohemica)
- A hybrid between Japonica and Sachalinesis
- 2- 3m
- Leaves an intermediate between the two. No flat base and crinkled edge
- Spreading quicker and could be potentially harder to eradicate
- Can produce male plants and often produce viable pollen, which could potentially back pollinate with either a Giant or Japanese knotweed plant to produce viable seeds
Himalayan knotweed (Persicaria wallichii)
- Often mistaken for Himalayan balsam as it bears similar colour flowers and also has long lance-like leaves.
- Leaf edge is not as serrated as Himalayan balsam and its stems are bamboo-like, similar to other knotweeds
- Flowers can range in colour from white to pink and are loosely clustered.
- Like other knotweeds it can spread vegetatively from cuttings and fragments.
- Not as common in Ireland as the other knotweed species, although it is considered to be in expansive mode at present.
- Damage houses, buildings, services, 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 defense structures and reduces the capacity of channels to carry flood water.
Chemical: Treatment can be carried out between May and October.
Foliar treatment that is conducted towards the end of the growth season is far more effective. This is due to the reversal of the vascular system of the plant towards the rhizome, making it more receptive to herbicide intake. It is recommended that a bi-seasonal treatment for knotweed species is undertaken for the first year, with foliar spraying with glyphosate herbicide in August and the second in September (approx.). Treatment can be reduced to once per year after initial years treatment is completed.
Treatment at the end of the growth season will occur before senescence occurs. Treatments and monitoring of sites should continue over a five-year programme (or until three consecutive years of no regrowth has been recorded)
Physical: Complete excavation and removal of Japanese Knotweed is another method of treatment for Japanese Knotweed. This completely removes all knotweed and knotweed contaminated material from an area. This involves excavating knotweed following a biosecure procedure that is supervised by experienced personnel. Japanese Knotweed is excavated and a 7m area across and 3m down is also excavated to ensure no viable knotweed material is left behind. This contaminated Japanese knotweed material can then be buried in a deep cell on site, bunded and treated in situ on site, or transported using a licensed haulier to a suitable licensed waste facility.
Excavation and removal is a popular method of treatment for those in construction and planning as it leaves the site knotweed free for development in a shorter space of time than is possible for herbicidal treatment.
Biological: Some studies have been conducted on a few species including using the plant louse, Aphalara itadori, Black Vine Weevil, Otiorhynchus sulcatus with some evidence of success.
(National Biodiversity Data Centre, Ireland, Japanese Knotweed (Fallopia japonica), image, accessed 18 May 2022,)