Japanese knotweed in the UK: how it got here and what the problem is

The very words ‘Japanese knotweed’ strike fear into the hearts of home owners, because it’s Britain’s most invasive non-native plant. It’s a world-wide problem and this triffid doesn’t only invade gardens and wild places – it breaks through hard surfaces as well, and spreads along watercourses and railway lines.

Japanese knotweed damage

The rapacious growth of Japanese knotweed, some 10cm per day, can damage buildings, paths and tarmac and it has popped up through flooring and cavity walls.

Those on the western side of Britain, in places where the climate tends to be warmer and wetter, as well as those in inner city areas with warmer microclimates, are most affected. It’s also a problem in the north-west. Bolton is said to be the Japanese knotweed hotspot of the UK, according to invasive plant specialists Environet, although no one quite knows why. Environet offers a free photo identification service.

Why is Japanese knotweed Invasive?

Japanese knotweed (Reynoutria japonica), also known by its old name Fallopia japonica, is native to Japan, Taiwan and northern China. It’s a well-behaved plant in its native habitat, kept in check by the climate and thinner soil. In Japan, for instance, it’s found naturally on sloping sites that consist of well-drained volcanic lava. However, when it’s grown in richer conditions in a mild climate it becomes a thug. The book Alien Plants (Clive A. Stace and Michael J. Crawley) records that it shades out native species and also alters the soil’s chemistry by releasing allelochemicals that inhibit the root growth of other plants.

How did Japanese knotweed get into the UK?

Japanese knotweed came to the UK in the mid-19th century after a Bavarian doctor named Philipp Franz Balthasar von Siebold (1796-1866) sent back Japanese specimens to leading botanic gardens and societies, including Kew and the RHS. By 1850 Kew Gardens were selling this new ornamental to private buyers. The clone was female and it’s thought that all the Japanese knotweed in the UK (and possibly the world) is genetically identical to the original collection made by Siebold.

By 1886, it had escaped from gardens and was colonising the cinder tips of Maesteg near Cardiff. Some estate owners planted it to stabilise sloping banks and this included some railways cuttings and rivers as well.

In 1870, William Robinson actively encouraged gardeners to plant it when he published The Wild Garden, one of the most influential gardening books of the Victorian era. These days we are far more careful about introducing alien plants than we were, because we understand that they can (and do) behave quite differently when exposed to different climates and soils.

What’s the law about Japanese knotweed?

In 1981 The Wildlife and Country Act made it an offence to introduce Japanese Knotweed into the wild. It is not illegal for you to have Japanese knotweed on your property, but it is against UK law to cause or allow the plant to spread in the wild. If you do spot it in your garden, it’s best to eradicate it before it travels into a neighbouring garden. If you see it in the wild near you, report it to your local council. Network rail can be contacted on Network Rail their helpline (03457 11 41 41). It’s a much more complex affair to report it on waterways, because different areas of the country are managed by different authorities. However, it is known to escalate flooding because it blocks up drainage channels. See gov.uk for more on disposing of Japanese knotweed.

Winning the battle

The Japanese Knotweed Agency predict “that the UK will have reduced the amount of Japanese Knotweed currently in the UK to 50% of its current amount in five years’ time, and that the UK will lead the way for other European Countries in the fight against these dreaded invasive weeds.”

Got Japanese knotweed? Call in a specialist

Standard herbicide treatments available to householders and gardeners don’t work. If you have Japanese knotweed, you will probably need a specialist company’s expert help to eradicate it. This will take an average of three to five years and it could cost several thousands of pounds.

Buying a house?

If you’re planning to buy a house with Japanese knotweed, and it’s within 7m of the building, you will need to get specialist help to eradicate it in order to get a mortgage. Its presence puts buyers off and therefore devalues the price of the property by 15% on average. That will be a problem if you intend to sell quickly. However, if you’re in for the long term, a property price reduction may be a bonus. It has been successfully removed from the Wembley Stadium site and from the Olympic Village, although it did take four to five years.

Some reassuring new research about soil and spread

New research, carried out by Mark Fennell, Max Wade and Karen L. Bacon in 2018, is more reassuring however. It investigated the 7m rule, the distance it must be away from the building, before it becomes a problem. They concluded that problems could be caused by plants growing on shrinkable clay soil. However, this type of soil isn’t common in the UK.

The research team looked at the spread of the rhizomes and found that they were no longer than two metres and not beyond four. 75% of larger stands had rhizomes extending no further than 2.5m. They received only one report of rhizomes over 4m in length. This shows that the fear of Japanese knotweed commonly having seven-metre rhizomes is unfounded and the use of the 7m rule is not robust for determining the likely lateral extent of these underground shoots.

The worst-case scenario study

The researchers (Fennel, Wade and Bacon) assessed 68 abandoned properties on three streets in northern England with a significant Japanese knotweed infestation, to determine whether these houses had damage caused by the presence of the plant. “Many properties had Japanese knotweed present or nearby but the team found no evidence to suggest that the dilapidation of the properties was caused by the plant. In fact other plants, particularly woody trees, were visually associated with more damage. This area represented a near worst-case scenario for houses that had been left to the mercy of Japanese knotweed and still we found no evidence for the plant causing damage.”

A survey of professionals

The researchers also conducted a survey of members of the Royal Institute of Chartered Surveyors and the Property Care Association. Their members have been involved in property surveys and they have supervised the treatment or removal of Japanese knotweed in the UK. They were asked to report whether they had observed damage to buildings occurring with Japanese knotweed. The conclusion was that where Japanese knotweed was associated with damage, it was likely that the plants had exacerbated existing damage, rather than being the initial cause of the damage. Overall, their study “found no support for the commonly suggested ideas that Japanese knotweed routinely damages buildings and that its influence extends seven metres from plants above ground. Nor did we find evidence that it poses a major risk to built structures.” The best advice is to maintain your property.

Attempts to control Japanese knotweed with aphids

The standard treatment for eradicating Japanese knotweed is to inject the stems with a strong herbicide. This is not only mega-expensive, it’s also damaging to the environment, because many of these chemicals could potentially leach into water courses, or linger in the soil. Although there was a scheme to introduce a Japanese knotweed munching aphid, a trial near Swansea in 2011 proved disappointing. There were also concerns about introducing an alien species of aphid into the UK. Scottish legislation regulates against the deliberate introduction of alien species, so the idea has been shelved.