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What do you know about Japanese knotweed?

It has halted developments across the country, caused untold damage to existing housing stock and rendered some properties completely unsellable. How much do you know about Japanese knotweed? As the sun coaxes this nuisance from the ground, Joe Bradbury of buildingspecifier.com investigates:

What is it?

Japanese knotweed is a non-native outdoor plant which grows at an alarming rate of up to 10cm per day. It proliferates in any type of soil and spreads incredibly easily, often leaving extensive damage in its wake.

By aggressively spreading its roots underground (up to 10 feet deep and 23 feet horizontally), it creates a serious threat to foundations of buildings and waste water management solutions.

If left untouched for a long period of time, the species can become very expensive to remove. It is estimated that total annual costs of Japanese knotweed damage, control and removal to the British economy £166 million. Defra’s Review of Non-native Species Policy states that a national eradication programme would be prohibitively expensive at £1.56 billion.

Needless to say, it is listed by the World Conservation Union as one of the world’s worst invasive species and also classed as “controlled waste” in Britain under part 2 of the Environmental Protection Act 1990. This means that all traces of knotweed need to be disposed of at licensed landfill sites only.

What does it look like?

Could you identify Japanese knotweed if you found it on your property? Because a study undertaken by online garden shop GardeningExpress.co.uk found that only 44% of customers were actually able to identify the invasive plant, and 56% identified the weed as a plant they would actually welcome in their gardens.
Here are some of the easiest ways of identifying knotweed by sight:

  • The plant is a lush green colour
  • Its leaves are shovel shaped
  • The stem looks similar to that of the bamboo plant
  • Between September and October it produces white flowers
  • It grows at an accelerated pace

What to do if you find Japanese knotweed

If you think that you have Japanese knotweed on your land you need to do something about it as soon as possible, to prevent further risk to your property and those close by.

The first thing you need to do is to alert a professional. DO NOT attempt to remove it from the ground yourself; this will merely serve to disperse its stem fragments and cause it to spread even further.

There are three main methods of getting rid of the weed. These can be split into three areas:

Non-chemical control

It is possible (but not always feasible) to dig out Japanese knotweed, but due to the depth of the roots, regrowth often reoccurs regardless. This method also creates problems with disposal, due to the waste needing to be disposed of at a licensed landfill site only. Alternatively, it can be destroyed on site by burning the waste, but only after it has competently dried out.

Biological control

A plant sucker (psyllid) is being released in the UK as a biological control for Japanese knotweed. It is currently only being released at a handful of trial sites and is not available to gardeners. However, if successful it will be released more widely and will become widespread in Britain over the next five to ten years by natural spread.

Chemical control

Perhaps the most effective treatment method is to use special weed killers to keep the plant at bay. Often it takes a lot longer this way (up to three seasons),but can produce great results in the long run. Injecting glyphosate herbicide into the stems of the plant can kill the plant completely with no impact on the surrounding vegetation, landscape or wildlife.

In summary

Japanese knotweed is a major issue for British builders. It is in everybody’s interest to be vigilant towards this invasive plant and prioritise the control and removal of it from our properties altogether – particularly those with surrounding parkland and riverbanks from where the infestations usually originate and spread from in the first place.

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Japanese Knotweed deters nearly 8 in 10 from buying a property

New YouGov research suggests a high level of anxiety around Japanese knotweed and alarming levels of myth and misinformation.

New research released today reveals that 78% of those aware of the infamous Japanese knotweed would be put off buying a property if they discovered the weed was present in the garden. Reasons for this included the concern that it cannot always be removed (69%) or that it would be too costly (56%) or time consuming to do so (57%).

The survey, carried out by YouGov and Japanese knotweed removal specialist Environet UK, suggests whilst many are aware of the weed, there is a high level of myth and misinformation around the threat posed by Japanese knotweed and the options available to homeowners who discover it on their land.

Japanese knotweed was first introduced into the UK from Japan in the 1850s as an ornamental plant, but it is now number one on the Environment Agency’s list of the UK’s most invasive plant species, described as “indisputably the UK’s most aggressive, destructive and invasive plant”. Growing up to 3 metres in height, it spreads rapidly and can push up through asphalt, cracks in concrete, driveways, cavity walls and drains in its quest for light and water.

Despite only 4% of those aware of the weed having had Japanese knotweed growing on their property, awareness of the threat is high, with 75% of Brits knowing about it. This awareness is particularly high in areas where the spread of the weed has been most prolific according to Environet’s own records of treatment, such as Wales, where 95% of respondents are aware of it, and in the south of England (80%).

Japanese knotweed

Those aware of the plant are also largely oblivious to their legal obligations to deal with Japanese knotweed if it is discovered on their land. Only around half (49%) know that a homeowner is legally responsible for preventing it from spreading from their property, and just around one in five (21%) are aware that they could receive an ASBO if knotweed on their land is allowed to spread to their neighbour’s garden.

In fact, knotweed can now be completely removed within a matter of days, at any time of the year, using a digging out method that sifts the earth to remove all viable rhizome roots from the infected soil. Once the problem has been swiftly tackled and an insurance backed guarantee has been secured, there are no difficulties in obtaining mortgage finance and property sales can proceed unhindered. For worried homebuyers, a professional indemnity insurance policy is now available, enabling them to protect themselves from the risk of Japanese knotweed from as little as £67. Despite this fact, only 3% of those aware of the weed said they would not be at all deterred from buying an affected property.

Nic Seal, MD and Founder of Environet comments “Homeowners are right to be concerned about the threat posed by Japanese knotweed. Attempting to deal with it by cutting it down repeatedly, burning it, burying it or using common weed killers simply won’t work as the plant can lie dormant beneath the ground, only to strike again when people least expect it.

“Yet for those wishing to buy or sell a property, it doesn’t have to be a deal breaker. Japanese knotweed can be dealt with once and for all, within a matter of days from discovery, so there is hope for buyers who may have otherwise walked away from their dream home.”

Chartered Surveyor Philip Santo FRICS, Director at Philip Santo & Co, added “RICS shares concerns that many people believe Japanese Knotweed poses a much greater risk than it really does. Since RICS issued guidance in 2012 the situation for buyers and sellers has greatly improved. For most affected properties there is now access to mortgage finance once an approved Japanese Knotweed Management Plan is in place. DIY remedies can make matters worse and should not be attempted.”

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