Untangling the issue

Long delays and landfill costs from dealing with Japanese knotweed could be a thing of the past thanks to new methods available to self- builders, explains Nic Seal of Environet.

Discovering the presence of Japanese knotweed on a site where you’re hoping to build your dream home can understandably send self-builders into a cold sweat. But treatment methods have changed dramatically over recent years, and it doesn’t need to spell disaster with long and costly delays on site.

How big is the problem?

Number one on the Environment Agency’s list of the UK’s most invasive plant species, Japanese knotweed is described as “indisputably the UK’s most aggressive, destructive and invasive plant”, costing an estimated £166m each year to treat.

It was brought to the UK in the mid- 19th century by Bavarian-born botanist Phillip von Siebold and delivered to Kew Gardens in a box of 40 Chinese and Japanese plant varieties. It spread rapidly across the UK as keen gardeners obliviously shared cuttings and disposed of garden waste and can now be found all over the UK.

An extremely resilient plant thanks to large energy stores in its root system, Japanese knotweed is very difficult to kill when fully grown. The source of its resilience lies in its native habitat. It was dug up from volcanic ash near Nagasaki, where it thrived amid lava and poisonous gases thanks to its extensive network of underground stems, or rhizomes, that were able to suck up the limited nutrients.

Interestingly, Japanese knotweed is not a problem in Japan where it has natural enemies in the form of bugs and fungi, but in the UK it is predator free.

Dealing with the invasive intruder

If knotweed is discovered on a site, it needs to be dealt with immediately. DIY attempts to dig it out, tackle it with weed killer or burn it won’t solve the problem and could cause further knotweed spread, so it’s a false economy to cut corners.

Herbicides can be applied over a period of two to three years, which will achieve an element of control, but without digging up all the infested soil, there is no way of knowing for certain that all the rhizome is dead. Also, disturbing ground containing knotweed rhizome is the catalyst for new growth. Therefore, the need to deal with the problem quickly, and the fact that on construction sites the ground will be disturbed, means herbicide treatment is seldom the answer for self-builders.

Alternatively, infested soil can be physically excavated, with all materials consigned offsite to an authorised landfill site. Named the “dig and dump” method, it’s not particularly sophisticated and is referred to by the Environment Agency as the “method of last resort”. It also results in significant haulage and landfill costs, which isn’t ideal when you’re on a tight budget.

A better, eco-innovative solution, which is used extensively on development sites, is to separate the knotweed rhizome from the soil onsite, thereby avoiding the huge environmental and financial costs of consigning vast quantities of otherwise good soil to landfill and importing clean fill. It costs about half that of the dig and dump method, produces zero waste and is environmentally friendly. But most importantly, it takes just a matter of days to complete, meaning builders and contractors aren’t hanging around for weeks unable to move on with the project, and draining the finances.

Guarantees & responsibilities

For any brownfield residential development project it is essential that the knotweed specialist is able to provide insurance backed guarantees acceptable to the banks and building societies. Prior to 2012, guarantees were simply offered by individual companies, but during the credit crunch two notable knotweed specialists ceased trading, rendering their guarantees useless overnight. The banks and building societies responded by making it a condition of their lending that guarantees were insurance backed to protect the lender and the homeowner.

Self-builders should also be aware of their legal responsibilities around preventing knotweed spreading onto neighbouring land. In February 2017 judgement was passed against Network Rail for allowing knotweed to grow on their land, thereby causing actionable nuisance to adjoining property owners. Network Rail was ordered to pay not only for the treatment but, more interestingly, for diminution, the reduction in property value resulting from the knotweed. Hailed as a landmark case, it has very significant ramifications for owners of land affected by knotweed. It reinforces the legal precedent that it is the duty of the landowner with knotweed to prevent it from spreading and failure to do so can very easily lead to court.

Nic Seal is founder and MD of Japanese knotweed removal specialist Environet