It was brought to Europe in mid-19th century and was much-admired for its ornamental foliage, after being uprooted from its natural habitat on the slopes of Japanese volcanoes, but it has become our most challenging invasive plant.
In 1847, it was awarded the gold medal by the Society of Agriculture and Horticulture, in Utrecht, Holland, as the most interesting new ornamental plant of the year. It was seen as a thing of beauty and was in great demand by gardeners. Once used in bouquets, people clearly thought it looked well among flowers.
Now, the very mention of Japanese Knotweed is enough to strike fear into some — it has even led to a couple’s murder-suicide in England — and its eradication is one of the most demanding tasks facing authorities in Ireland and Britain.
Fallopia japonica, to give it its scientific name, grows prolifically and is found in all 32 counties, along rivers, roads, and waste ground where it spreads at will.
The weed is listed by the World Conservation Union as one of the worst invasive species on Earth because its root system and strong growth can damage concrete foundations, buildings, flood defences, roads, paving, retaining walls, and archaeological sites.
It was brought to Europe by a Dutch explorer in 1847, and was marketed as cattle forage, a sand-dune stabiliser, and a flower for bouquets. It was added to the Kew Gardens collection, London, in 1850.
Knotweed can grow through concrete, tarmac and other hard surfaces and also undermines some native plants and animals by forming dense thickets which stifle their growth. It blocks routes used by wildlife, damages flood-defence structures and reduces the capacity of channels to carry flood water.
In Cork last year, the Douglas and Glanmire/Sallybrook flood relief schemes were hit by delays after the knotweed was found in the areas intended for construction works.
The weed is notoriously difficult to remove because cutting will only spread it further and it is resistant to some weedkillers. It can take years of regular chemical treatment to successfully remove it from areas where it is widespread. Its roots can be up to 3m deep.
Deep excavation and burial is another option that is used in situations where there is a pressing development need for the site and time which would not allow for in situ herbicide control over a longer period of time.
Some local authorities have set up special teams to remove the scourge and create awareness of it.
Red/purple shoots appear early in spring and, in some cases, have an asparagus-like appearance. But, as the canes grow, the leaves unfurl and the plant takes its more characteristic appearance. The plant can grow to more than 3m high.
Flowering occurs in late summer/autumn (end July — typically August) and consists of small, creamy white flowers. During the winter, the leaves die back and reveal orange/brown woody erect stems, leaving river banks prone to erosion.
Only female Japanese knotweed plants have been recorded to date in Ireland and Britain, so it is spread entirely from site to site through the deliberate or accidental movement of parts of the plant or cut stems. Some reports suggest a fragment of knotweed as little as 0.6g can result in new growth.
Knotweed grows intensely and takes over from native plants. Native plants can rarely compete with it and local plant biodiversity is reduced. Rivers, hedgerows, roadsides, and railways can form important wildlife corridors for native plants and animals and large infestations of knotweed can block these routes for wildlife.
How to fight knotweed is a tantalising difficulty.
However scientists at the Centre for Agricultural Bioscience, in the UK, have been looking at almost 200 insect species which feed on knotweed in Japan.
They have come across one, a psyllid, which only eats knotweed. Next year is the final year of a field trial to properly test the superbug.
It is illegal to dump knotweed waste in the countryside and to plant or otherwise cause knotweed to grow. It is also illegal to dispose of Japanese knotweed at a landfill site without informing the landfill site what the waste material is.
Knotweed is one of Ireland’s most unwanted species, posing a huge environmental and economic threat.
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