Japanese Knotweed: the Invasive Plant that Knows no Boundaries

Probably our most invasive and ruthless invasive plant species is Japanese knotweed (Fallopia japonica). A German botanist called Philipp von Siebold first saw the plant in 1850 growing on a volcano in Japan. Thinking it would look pretty in Victorian gardens, he decided to send it back to the Royal Botanic Gardens at Kew, England. Alas, this began its relentless invasion throughout the UK and Ireland including County Monaghan. It is now classed as one of the top 100 most invasive plants in the world. Jennifer Mc Aree of Transition Monaghan elaborates below.                     Knotweed

A clump of Japanese Knotweed being observed by Liam Murtagh, Transition Monaghan.

Japanese knotweed grows along riverbanks, canals and roadsides, in derelict sites and gardens. It is a plant with green, shield -shaped leaves and a bamboo like stem. It produces cream-coloured flowers in late summer and the female variety that we have can grow rapidly to 2-3 metres (up to 1metre in three weeks), eradicating every other plant species in its path through pure force and blocking sunlight.  This kills off native plant species and thus reduces biodiversity very quickly. The plant can force its way through concrete, brick and tarmac, causing considerable damage to buildings and roads. House sales have fallen through and lenders have refused mortgages in the UK in cases where properties are affected by the deadly weed. Construction projects must be delayed where the species is found until it is properly treated, sealed and removed from the site by licensed experts. The British Government spent over £88m eradicating knotweed from the Olympic village site before even beginning to build.

The plant can take hold from fragments as little as 0.6cm, so when cut it will surely spread. Its roots can grow down as far as 3m and create an extensive underground rhizome system. Only the female of the species has spread in the UK (and in Ireland) since being introduced – it has basically cloned itself from the very start.  It is illegal to dump any cuttings. Glyphosate or ‘round-up’ can be used to treat knotweed, but this takes several applications at various stages of the plant’s life cycle. In addition glyphosate is hazardous to human health and the environment, so it needs to be handled with extreme care.

The knotweed issue has come to the fore in Ireland over the past few years. In January 2016, the then Minister for Transport Paschal Donohoe announced a €298m investment to tackle Japanese knotweed along 2,000km of national roadways. While it has rampaged its way across all parts of the country the plant is particularly prevalent in the south-west where the mild, rainy climate has encouraged further proliferation. Kerry County Council has set aside €100,000 for a targeted spraying programme. The weed has taken over large swathes of the Ring of Kerry and Killarney National Park.

A Kerry county councillor recently suggested that knotweed root should be used for making jams and desserts (it apparently tastes like rhubarb). However the idea is unlikely to become popular. To cook with it you need a special licence and it must be highly controlled. Japanese knotweed is used for medicinal and cooking purposes back in its native Eastern Asia, but here it is a very risky business.

A tiny bug called Aphalara itadori is being trialled in the UK where Japanese knotweed persists to determine if it can help eradicate the vicious plant. Until the programme is deemed successful this treatment is unlikely to be adopted elsewhere. For now, it is advised not to cut the plant, but to seek advice and treat it carefully at various stages with glyphosate-based weed-killer, or to dig out the weed as far down the roots as possible and burn it in situ. Never add the cuttings to normal household waste or remove them off site. If the knotweed has grown out of control, you must contact the local council or a registered removal expert.

The Invasive Species Ireland website is a mine of information on Japanese knotweed and how to treat it. Another gem is www.knotweedsurvey.ie, which encourages people to report knotweed locations. Through a nationwide group effort by local authorities, landowners and householders, we might have some chance of destroying this bad boy (apologies, bad girl!) once and for all.

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