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Japanese Knotweed

Detail of the stalk so you can self identify.

A team of scientists has been trying for FIVE years to kill the invasive Japanese knotweed plant - but it has not managed it.

But homeowners are being told not to panic if the plant has made its home on their land.

The bioscientists looked at 19 methods of controlling the plant, renowned as one of the world's worst invasive species, and concluded that "unscrupulous" companies were encouraging householders to pay for expensive treatments that simply didn't work.

Professor Dan Eastwood, part of the team at Swansea University, said: "We began focusing on knotweed at a time when there was a great deal of hysteria surrounding it.

"At the time, most information for people affected by the plant was largely based on anecdote. This led to the prospect of unscrupulous companies offering expensive and ineffective treatment solutions."

The team acknowledged, for most people, Japanese knotweed represents a nightmare, particularly if they are trying to sell their home. And it says it has seen mortgages declined because of botched treatment undertaken by home owners.

Professor Eastwood said: "It was incredible to us that there was no long-term, large-scale field trial analysis of the treatment methods used to control Japanese knotweed.

"Our research tested more methods of control than any other invasive species trial ever conducted, allowing us to replace out-dated guidance based on short-term experiments and anecdotal information."

His colleague Dr Dan Jones added: "Claims made by companies, stating that they could eradicate Japanese knotweed using herbicides in short spaces of time, have now been proven to be false, based on our experiments.

"Furthermore, we have shown that applying the wrong herbicides at the wrong time of the year leads to greater herbicide use and environmental impacts."

The team believes it has now come up with an effective method to control Japanese knotweed using what it calls the 4-Stage Model that uses lower doses of herbicide and targets treatments over the seasonal cycle of the plant.


It also has some advice for homeowners who are worried about the spread of knotweed:

  • Ignore the hysteria - your house will not fall down. The team says knotweed can be controlled using the chemical glyphosate at the right time of year, though this can usually take three to five years. However, the team does not recommend you do this yourself. A spokesman said: "We have seen mortgages declined because of botched treatment undertaken by homeowners."
  • Knotweed is a resilient weed that cannot be controlled by one herbicide treatment in a single year, a claim frequently made by unscrupulous companies, says the team. Any treatment strategy should be long-term and target both the above-ground and extensive below-ground parts of the plant.
  • Effective treatment centres on working with the biology of the plant and targeting the correct herbicide when the plant is vulnerable to its effects, from summer into late autumn (depending on the weather).
  • Calling out a weed control company to control knotweed is not the same as calling out a plumber. You would expect the plumber to get the work done soon, if not immediately. If you call out a company in spring to control knotweed, quite rightly, you will need to wait until later in the year to get the best results – this will save you time, money and hassle in the long-term.
  • Once knotweed has been effectively controlled using herbicide, don't disturb this land by digging, for example, as it is likely to come back. Even if the above-ground parts of the plant are dead, the below-ground rhizome system probably isn’t. Again, however, don’t panic, if it comes back, call the contractors to regain control.
  • Don't try digging out the knotweed yourself, it is easy to miss parts of the root and spread it and you cannot dispose of this plant material along with your garden waste – in fact, it is illegal to do this.

Japanese knotweed starts growing from early spring, and can reach 1.5m by May and 3m by June.

It then dies back between September and November.

You should look out for:

  • Fleshy red tinged shoots when it first breaks through the ground
  • Large, heart or spade-shaped green leaves
  • Leaves arranged in a zig-zag pattern along the stem
  • A hollow stem, like bamboo
  • Dense clumps that can be several metres deep
  • Clusters of cream flowers towards the end of July that attract bees
  • Dies back between September and November, leaving brown stems

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15 August 2020
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