Japanese knotweed: effect on farmland and how to prevent it


Horticulture was responsible for bringing Japanese knotweed to the UK via imports of seeds and living material in the 19th century.

The agricultural industry has since paid the price with the loss of pastures and damage to buildings caused by this invasive non-native species; it also prevented development on farmland through strict planning laws relating to its presence on the land.

Other key invasive non-native species in the UK include giant hogweed, Himalayan balsam, New Zealand pygmy, water pennywort and rhododendron.

See also: So you want to… sell your farm?

However, Japanese knotweed remains the most infamous and there are now very few corners of the UK that are unaffected by it, says Helen Bibby, conservation consultant at SAC Consulting.

She warns that it can sprout vigorously from a small fragment, quickly spreading into adjacent soil through the growth of rhizomes.

“It spreads to a new area when small fragments break off from the parent plant and are washed away by water, soil removal or in the treads of machine tires to regrow at a new site,” explains Mrs Bibby.

Underground, it develops a vast network of rhizomes that can extend up to seven meters from the initial plant; these can remain dormant for many years after treatment to regrow.

Its disposal comes at a cost, as it must be done by a regulated and licensed contractor, which has resulted in its introduction to some farms through dump trucks.

The presence of Japanese knotweed can affect farm values ​​and sales.

© Tomas Vynikal/Adobe Stock

Property values

Tom David, senior managing partner of Strutt & Parker, was involved in two recent farm sales where the weed was discovered.

On one, a small amount of knotweed was found around the farm buildings and in the garden of the house.

Mr David says this had no effect on the progress of the sale or the values ​​- in fact the property sold for considerably more than its guide price, with two people competing for it.

But the seller in this case was able to demonstrate that the knotweed had been treated and had a management plan.

“It probably also helped that there were cash buyers involved, so there was no bank involvement,” says David.

“If lenders are involved, they can get quite nervous about it.”

In the second case, the last-minute discovery of knotweed caused the deal to fail on the eve of the exchange of contracts.

“Ultimately, it affected the value by about 10%,” says David. “But it was more of a lifestyle farm and the knotweed was close to the house and the buildings, so that was more of a concern.”

He says having a management plan in place “is probably a must” if a farm is sold and there is knotweed, especially near a house or buildings.

“You can then show a potential buyer that they’re addressed,” he says. “Some buyers will hear the words Japanese knotweed and panic, while others may be more pragmatic about it.”

The Law Society’s Property Information Form TA6, which is completed by a seller of property to give a potential buyer detailed information about the property being sold, has a section indicating whether the property being sold has been affected by Japanese knotweed .

Litigation risk

Ruairadh Adams-Cairns of Savills, who helped produce the Royal Institution of Chartered Surveyors’ guidance note on Japanese knotweed, says careful consideration should be given to the response as it can lead to major compensation claims later. line.

“It would be foolish to check the ‘no’ box if you know the property was affected and if you really don’t know, definitely check the ‘don’t know’ box,” he said.

“But the mistake some people make is that they think they don’t have it and tick the ‘no’ box and that can get them in serious trouble – the buyer may be entitled to hire a contractor specialist at great expense to the after-sales seller and there could be a claim for compensation on the final price paid.

Mr Adams-Cairns cites the example of the sale of a small farm where the weed was discovered by the new owner growing in the flower beds next to the house.

The seller had completed the TA6 stating that the property had not been affected by Japanese knotweed.

“She assumed the question was asking if the property had been adversely affected, which it had not been, so she checked the ‘no’ box.

“Litigation ensued with the buyer suing her for a huge sum of money.”

If allowed to establish, knotweed can invade pasture areas and that pasture would be lost, but Adams-Cairns says some animals will eat it.

“It’s full of carbs and animals love it,” he says.

There is no requirement for property owners to remove knotweed from their property, but they could be prosecuted and face criminal penalties if they allow it to spread to another property.

The Wildlife and Countryside Act 1981 and the Invasive Alien Species (Enforcement and Permits) Order 2019 apply to Japanese knotweed.

Some of the penalties following a conviction include unlimited fines and imprisonment for up to 24 months.

The 2019 order also provides for a civil penalties regime overseen by Natural England and Natural Resources for Wales and includes financial penalties, compliance notices, restoration notices and shutdown notices.

The Environmental Protection Act 1990 also applies, classifying the disposal of soil containing Japanese knotweed as “controlled waste”.

The penalties under this Act are similar to offenses committed under the 1981 Act and the 2019 Ordinance.

Local authorities also have powers in this area, as they can serve notices under section 215 of the Planning Act 1990 and the Anti-Social Behaviour, Crime and Dishonesty Act 2014. police.

Landowners can also face potentially significant liabilities from their neighbors if knotweed spreads onto their land, Adams-Cairns says.

Liability may be incurred under the common law for nuisance with a private civil suit if weeds spreading from one parcel of land to another caused substantial and unreasonable interference with the plaintiff’s land.

“It’s not something landowners should ignore,” says Adams-Cairns.

How to Prevent Japanese Knotweed

Farmers can take steps to prevent Japanese knotweed from entering their land.

Good land management practices are key to preventing contamination, says Helen Bibby of SAC Consulting.

She advises farmers to keep farm machinery clean and avoid importing or exporting contaminated soil.

“Make sure machinery entering the farm is clean to avoid accidental cross-contamination,” Ms Bibby adds.

It is good practice to regularly monitor the soil for the presence of the weed.


The plant begins to grow in spring, with red or purple shoots appearing in March and April and clusters of creamy white flowers in late summer.

It grows large, heart-shaped green leaves with hollow, bamboo-like stems up to 2 m tall.

The plant may produce dense clumps of growth that turn red or orange before dying back in winter.


The control method normally involves spraying a herbicide such as glyphosate.

Ms. Bibby says spraying in late summer, after flowering, will ensure the chemical is translocated to the roots.

“This method of control can take three or four years to eradicate the plant,” she advises.

Stem injection applications deliver the herbicide directly into the hollow stem.

Burning doesn’t kill knotweed – in fact, it can make it grow more vigorously, Ms Bibby says, although it can remove some of the top growth.

“Avoid mechanical cutting because in many cases it can increase the spread of the plant and not kill it,” she says.


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