Japanese knotweed: What is it and why are people panicking?

Poor guidance from councils could result in people unwittingly spreading one of the most destructive plants in Britain, instead of eradicating it, say researchers from the University of Exeter.

The team behind the claim looked at advice on how to handle and dispose of infestations of Japanese knotweed, which can be so damaging it can block house sales and mortgage offers, says the Daily Telegraph.

They found some local councils are giving good advice – but others are not, or are giving inconsistent messages. The bad advice could cause needless worry or spread the plant – sometimes dubbed 'Britain's largest female' – further around the country.

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What is Japanese knotweed?

Fallopia japonica is a perennial plant related to buckwheat and native to Japan, as its Latin name indicates. It grows in dense clumps and its stems can reach around chest-height. It is listed in many non-Asian countries as an 'invasive species'.

In 1850, the botanist Philipp von Siebold sent samples collected on the slopes of a Japanese volcano to the Royal Botanic Gardens in Kew, says the Daily Mirror. Within four years, the plants were being sold by commercial nurseries and touted as a useful new fodder for animals.

Why are people so unhappy about it?

For three reasons. Firstly, the plant's remarkable ability to force its way through concrete or brick means it can damage buildings or roads. Secondly, it grows in dense clusters that exclude native species. Thirdly, it is almost impossible to eradicate.

Is it really so bad?

Japanese knotweed is "indisputably the UK's most aggressive, destructive and invasive plant", says the Environment Agency. As well as destroying concrete foundations, it can damage flood defences and architectural sites.

So destructive is the plant – and so hard to control – that lenders can refuse mortgages on homes with a 'knotweed problem' and house sales may even fall through. The Telegraph says one gardener was "driven mad by the worry" of the plant and killed his wife.

Knotweed can grow three to four metres in just ten weeks, reports The Guardian. Its roots can spread seven metres horizontally underground. And the plant can regrow from even a tiny fragment of its root – so getting rid of it is not easy.

How do you kill it?

The plant is resistant to cutting, as it regrows vigorously – and any cut pieces must be disposed of very carefully. Domestic gardeners are advised to call in experts for the purpose. It can be destroyed by chemical herbicides. Experiments suggest spraying sea water on the leaves. This can also be effective in situations where using herbicide is not possible.

How does the plant reproduce?

In Asia, the plant has a male and female variety. Only the female has reached the UK, however. That means the plant can't reproduce sexually using its pollen. Undaunted, the female plant brought to Britain has cloned itself over and over again.

Each separate plant of Japanese knotweed in the UK has regrown from a fragment of root from another plant. Often these have been moved unwittingly by gardeners or councils taking earth from one site to another.

Why is it sometimes called 'Britain's largest female'?

Because they have not reproduced sexually, technically, all clumps of Japanese knotweed in the UK are the same plant, even though they aren't physically joined together. So it is Britain's, or perhaps even the world's, biggest female.