Japanese knotweed: Effect on farmland and how to prevent it
© Pixaterra/Adobe Stock Horticulture was responsible for Japanese knotweed’s arrival to the UK via imports of seed and live material in the 19th century.
The farming industry has since been paying the price with the loss of grazing and damage to buildings wrought by this invasive non-native species; it has prevented development on farmland too through strict planning laws relating to its presence on land.
Other key invasive non-native species in the UK include giant hogweed, Himalayan balsam, New Zealand pygmyweed, floating 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 left unaffected by it, says Helen Bibby, conservation consultant at SAC Consulting.
She warns that it can grow vigorously from a small fragment, quickly spreading into adjoining ground through growth of the rhizomes.
“It spreads to a new area when small fragments break from the parent plant and are carried away by water, soil removal or in treads of machine tyres to regrow in a new site,” says Ms Bibby.
Underground, it develops an extensive network of rhizomes that can extend to seven metres from the initial plant; these can lie dormant for many years after treatment only to regrow.
There is a cost to disposing of it, as it must be done by a regulated, licensed contractor and this has resulted in it being introduced onto some farms by fly-tippers.
The presence of Japanese knotweed can affect farm values and sales.
© Tomas Vynikal/Adobe Stock
Property values
Tom David, senior associate director for Strutt & Parker, has been 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 values – in fact, the property sold for considerably more than its guide price, with two people competing for it.
But the vendor in this case was able to show that the knotweed had been treated and was under a management plan.
“It probably also helped that there were cash buyers involved, so there was no bank involvement,’’ says Mr David.
“If lenders are involved, they can get quite twitchy about it.”
In the second case, the last-minute discovery of knotweed resulted in the deal falling apart on the eve of the exchange of contracts.
“In the end it affected the value by about 10%,” says Mr David. “But this was on more of a lifestyle farm and the knotweed was close to the house and buildings, so was more of a concern.”
He advises that having a management plan in place “is probably a must” if a farm is being sold and there is knotweed, particularly near a house or buildings.
“You can then show a potential buyer that it is being 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 TA6 property information form, which is completed by a property seller to give a potential buyer detailed information about the property being sold, has a section on whether the property being sold has been affected by Japanese knotweed.
Litigation risk
Ruairadh Adams-Cairns, of Savills, who has helped to produce the Royal Institution of Chartered Surveyors’ guidance note on Japanese knotweed, says careful consideration must be given to the response, as it can result in major claims for compensation further down the line.
“It would be foolish to tick the ‘no’ box if you know the property has been affected and if you genuinely don’t know then definitely tick the ‘don’t know’ box,” he said.
“But the mistake some people make is that they think they haven’t got it and tick the ‘no’ box and that can get them into serious trouble – the purchaser may have the right to hire in a specialist contractor at great expense to the vendor post-sale 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 borders next to the house.
The seller had completed the TA6 by stating that the property had not been affected by Japanese knotweed.
“She made the assumption that the question was asking whether the property had been adversely affected, which it hadn’t been, so she ticked the ‘no’ box.
“Litigation followed with the purchaser suing her for a huge sum of money.”
If it is allowed to establish, knotweed can take over grazing areas and that grazing would be lost, but Mr Adams-Cairns says some livestock will eat it.
“It is full of carbohydrates and animals love it,” he says.
There is no obligation for landowners to remove knotweed from their land, but they could be prosecuted and face criminal sanctions if they allow it to spread onto another property.
Both the Wildlife and Countryside Act 1981 and the Invasive Alien Species (Enforcement and Permitting) Order 2019 apply to Japanese knotweed.
Some of the sanctions following conviction include unlimited fines and a jail sentence of up to 24 months.
The 2019 order also has a civil sanctions regime overseen by Natural England and Natural Resources for Wales and includes financial penalties, compliance notices, restoration notices and stop notices.
The Environmental Protection Act 1990 applies too, classifying the disposal of soil containing Japanese knotweed as “controlled waste”.
Penalties under this act are similar to offences committed under the 1981 act and 2019 order.
Local authorities have powers in this area too as they can serve notices under section 215 of the Town and Country Planning Act 1990 and the Anti-social Behaviour, Crime and Policing Act 2014.
Landowners can also face potentially large liabilities from neighbours if knotweed spreads onto their land, says Mr Adams-Cairns.
Liability can arise under the common law of nuisance with a private civil court claim if the weeds spreading from one parcel of land to another caused a substantial and unreasonable interference with the claimant’s land.
“It is not something that landowners should ignore,” says Mr Adams-Cairns.
How to prevent Japanese knotweed
There are steps farmers can take to stop Japanese knotweed getting onto their land.
Good land management practices are key to preventing contamination, says SAC Consulting’s Helen Bibby.
She advises farmers to keep farm machinery clean and to avoid the import or export of contaminated soil.
“Ensure machinery entering the farm is clean to avoid accidental cross-contamination,” Ms Bibby adds.
It is good practice to regularly monitor land for the presence of the weed.
Identification
The plant starts to grow in the 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 that can be up to 2m tall.
The plant can produce dense clumps of growth that turn red or orange before it dies back in the winter.
Control
The method of control is normally by spraying a herbicide such as glyphosate.
Ms Bibby says that spraying in late summer, after flowering, will ensure that the chemical translocates to the roots.
“This method of control may take three or four years to eradicate the plant,” she advises.
Stem injection applications deliver herbicide directly into the hollow stem.
Burning does not kill knotweed – in fact, it can cause it to grow more vigorously, says Ms Bibby, although it may remove some of the top growth.
“Avoid mechanical cutting as in many cases this can increase the spread of the plant and does not kill it,” she says.