‘Strimming is the worst you could do’

Warning about the dangers of Japanese Knotweed

Cavan County Council is once again warning of the dangers of Japanese Knotweed. In the summer, the invasive species can reach three meters in height. It has bright green, shield-shaped leaves and spikes of small, creamy white flowers in September. Japanese knotweed can take over an area, wiping out all other plants - including precious native species - in a single season and can cause havoc on building sites.

It is important not to cut the plant, once it is identified as Japanese Knotweed. “The only way it spreads is through its rhizomes or fragments of its own vegetation breaking off and re-growing", explains a council spokesperson.

This means, when people cut it to try to get rid of it, they actually help it to form new clumps. “Strimming it is the worst thing you can do as it creates millions of tiny pieces, each of which can sprout into a new plant. Any cut or broken material should be burnt.”

Lack of knowledge about how it spreads has meant that hedge cutting contractors have been cutting and spreading it unknowingly. Very often, when there is a roadside area of knotweed, there are several clumps further down the road where sections from the flail cutter have landed in previous years.

“Even a small plant can grow to an out-of-control infestation quickly, so it shouldn’t be ignored,” advises the local authority spokesperson.

Not only is it an invasive species, but it is a menace to native biodiversity and to property. “The plant has the ability to grow through concrete and tarmac, and its roots grow extremely deep. So, unless completely eradicated through careful, repeat treatments, it can cause havoc on building sites and near homes,” warns the spokesperson.

Cavan County Council treats instances of Japanese Knotweed on council land and, when resources allow, on national and regional roadsides. However, the onus is on private landowners to responsibly treat any Japanese Knotweed found on their land. For small infestations in gardens, a glyphosate herbicide can be used but the council advise to seek professional advice.

To get rid of the pest, it needs to be sprayed in August or September after flowering but before the vegetation dies back. Spraying at this time means the weed killer will be transported to the roots of the plant increasing its effectiveness. “To protect pollinating insects, try not to spray plants with herbicide while they are still in flower. Under no circumstances should you ever strim, flail, or chip it.”