ENVIRONMENT Pervasive, invasive: Japanese Knotweed

A very knotty problem


Andy Wilson

Top of the list of invasive plants in the West of Ireland is Japanese Knotweed. It appears to have arrived in Ireland in the late 19th century, though was relatively rare in the west until recent times. In the last decade, however, Japanese knotweed has become very firmly established along roadsides, on derelict land and along roadsides throughout the western counties.
In Westport, very large clumps can be found along the town side of the Railway Walk about 100-150 metres west of the High Street Bridge, and also behind the Elms housing estate. The last time I looked it was also all over the Council Depot on Altamont Street.
Japanese Knotweed has been spread mainly through the earth moving operations associated with development projects and also by strimming, which simply scatters parts of the stem far and wide where they then can grow into new plants. A piece of stem or root as small as 4mm (not much more than one eighth of an inch) has the potential to grow into a new plant.
Perhaps more worrying, Japanese Knotweed has the potential to burrow down into the soil several metres and send out lateral rhizomes three or four times that distance, where it will remerge. It is extremely persistent – even companies that specialise in Japanese Knotweed eradication only offer limited guarantees of success.
Knotweed is also known to burrow through asphalt and concrete. An article in the Daily Telegraph earlier this year revealed that UK mortgage providers were refusing to offer mortgages on premises infested with knotweed, or even in cases where knotweed was known to exist on adjacent properties, citing the risk to building structure and foundations. There would also be a number of related legal implications, whereby property owners with knotweed on their land could be held liable for damage – including loss of value – to adjacent properties.
Eradication is a specialist operation costing up to €200 per square foot of land infested or at risk of infestation. Although the use of glyphosate weedkillers can contain the plant in the short term, the only sure bet to eradication appears to be painstaking removal of the plant and its entire root system, carried out with the diligence of a high-profile archaeological dig.
All pieces of knotweed removed from the site have to be accounted for and destroyed by incineration. Even then, a clean bill of health can only be given after three to four years have elapsed since the last sighting, and constant vigilance is advisable.
The worst thing to do is attack the knotweed with a strimmer or with earth-moving machinery, as this will only spread the plant further.

NEXT TIME New Zealand flatworm, an aggressive predator of the native earthworm

Andy Wilson is a founding member of the Westport Sustainability Group, which is involved in many food-growing initiatives in the Westport area, including the Railway Walk Linear Orchard project. Andy also runs a range of horticultural workshops and hopes to run a workshop on Japanese-Knotweed-eradication early in the autumn. He is also happy to give free advice on the eradication of knotweed by phone or email. Andy can be contacted at 087 6714075 or by email andy@sustainability.ie.