Japanese Knotweed: The Silent Nuisance

Mike Hedges looks at the damage caused across Wales by Japanese Knotweed.

Japanese knotweed is the cause of a great deal of anguish and despair in Swansea. Known locally as “Sally Rhubarb” due to its similarity in shape and colour to rhubarb, experts estimate that the Swansea area alone has around 62,000 tonnes of the stuff, which is the equivalent weight of 400 blue whales.


The truth however is that Japanese Knotweed is one of the most damaging non-British species of weeds we have in the UK. Fallopia Japonica, to use its scientific name, was first introduced around South Wales in the 1800s by wealthy Victorian families in the belief that it was an ornamental plant.

Japanese knotweed can grow at an astonishing speed and with the power to push through concrete and tarmac, damaging buildings, roads and pavements in the process.

As the plant has no natural domestic predators, it can both damage and out-compete our indigenous species at an alarming rate, with the tiniest piece of stem being able to re-grow into a fully-fledged nine foot giant. We’ve seen this pest take over gardens and land throughout Britain with only the Orkneys being clear of it.

The horrors associated with Japanese Knotweed are not just limited to the environment. In recent years, we’ve seen knotweed affect the housing market, with many mortgage lenders now refusing to give financial backing to buy a property where the weed is present, and in some cases, when it is present on a neighbouring property or neighbouring land. This effect has seen the value of knotweed-riddled properties and those in knotweed infected areas drop, with frustrated homeowners left having to spend thousands of pounds to clear it before they are able to sell their house.

It’s therefore unsurprising to learn that each year, Japanese Knotweed is seeing landowners and public authorities spend an estimated £150m to control and clear-up the pest, which is also costing the British economy an estimated £165m annually, whilst estimates put the total cost to clear the knotweed infestation at around £1.5billion. The problem for homeowners doesn’t stop there; the removal of knotweed is both expensive and difficult to eradicate because as all the roots need to be completely removed to prevent any future resurgence.  One small part of a root left behind and a new plant will grow and prosper.

So what weapons do we have in the armoury to combat the invader?

Physical control and chemical methods are the two main types of treatment; however, in the quest to find an answer, an innovative trial has already begun into using a “natural control” as a feasible and long-term solution to controlling the weed.   Up until now chemical treatment has been both expensive and lengthy, with treatment having to be carried out over at least three years but Swansea University is working on a new chemical means of destroying knotweed.

In July 2009, DEFRA announced plans to release a native sap-sucking psyllid bug, known as the Aphalara Itadori, into the wild to help control knotweed. The psyllid bug, which is the first time a biological method has been used in the EU as a natural control of a weed, was released on several test sites in spring 2011 under the control of the Centre for Agricultural Bioscience International.

The project is currently in its fourth year of five, and according to a recent update from the then Natural Resources Minister Alun Davies, “early signs are encouraging” and the bugs are coping well in the UK. However, as the Minister acknowledged, natural control is not a “quick fix” and successful bio-control methods usually take around five to ten years from release before the overall benefits can be realised.

Let’s hope that this natural control project gives us the “silver bullet” needed to tackle the knotweed once and for all, not least because so many hopes are resting upon the small shoulders of these bugs!

Mike Hedges is the Assembly Member for Swansea East.

6 thoughts on “Japanese Knotweed: The Silent Nuisance

  1. The White Rock site in Mike Hedges’ Swansea East constituency has a new infestation of Japanese Knotweed. So far it is confined to one location, but there are also plants along the cycle path. See http://www.whiterocktrails.org Site Clearance page for the latest news.

    The White Rock Copper Works were established in the Lower Swansea Valley in 1736, 70 years before the better known Hafod works. White Rock was one of the most important copper smelting works in the Lower Swansea Valley, the world centre of non-ferrous smelting for much of the 18th and 19th Centuries. Lead and silver were also smelted at White Rock.

    The White Rock project is run by the Swansea branch of the Historical Association. It is funded by the Heritage Lottery Fund under the Connected Communities scheme. We are currently seeking a second round of funding to return the site to its status as an Industrial Heritage Park, and a renewed asset for Swansea.

  2. Yes, Japanese knotweed is no doubt a nuisance but it can be brought under control taking on average 3 years. Successful treatment can be quicker for new infestations but sometime longer if the plants are well established or knotweed on neighbouring land is left untreated . Monitoring and vigilance post treatment is also required! Good advice is essential especially if you are looking to sell, re-mortgage or develop your land or property as lenders sometime require specific treatment methods and evidence of treatment. Damage caused by knotweed is often exaggerated – there has to be cracks or fratues on concrete for example.
    Get in touch with Knotweed Control if want a survey or require treatment.

  3. Notice there is no knotweed in open fields or hillsides is Wales? It is all on housing estates, in gardens or next to canals and railway cuttings. Why? Because it does have a natural enemy – sheep. Knotweed shoots are edible and sheep eat ’em. Just cut down the knot weed and when shoots reappear, let the sheep in. Knotweed is a strong plant that will regenerate from an inch of root but if you keep taking the top off before it can set leaf it will die eventually. That’s a tedious task for a person but sheep will do it quite happily for hours. Swansea council should hire a few flocks of sheep and bus them to where they are needed.

  4. PS But I don’t know what you do about Himalayan Balsam which has colonized the village where I live to the exclusion of everything except brambles.

  5. @ R Tredwyn
    I don’t think sheep are the answer :).
    Even if they clear the stems the roots can remain dormant for years before re-emerging. Also it can regrow from very small pieces of root so you’d have to do a foot and wool inspection of each sheep before moving them on otherwise other areas could be contaminated.

    The main reason you don’t get Japanese knotweed growing on open field or hillsides is not because of the sheep but because waste “soil” isn’t dumped on those areas.

  6. I can speak only from limited experience. I and a neighbour both removed a knotweed infestation in the garden by cutting it down and then religiously pulling out every shoot. It took time, three or four years until the stuff stopped coming back but I’ve been ten years clear. It will regenerate from a small piece of root but it can’t live without setting leaf so I’m not sure about dormancy. Roots aren’t seeds. Once dead, they’re dead. I take your point about the risk of sheep transporting knotweed root but the risk is low since the root is underground and sheep don’t dig; they just eat what’s on the surface. If you had seen what a few sheep will do to a n area of knotweed shoots, I think you’ld give them more credit for the clear nature of Welsh hillsides. They keep oak down, never mind knotweed!

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