Clauses relating to Japanese knotweed still feature in construction contracts despite the arrival of killer bugs

Developers will have been excited by recent news that a not-for-profit agricultural research organisation has discovered a tiny sap-sucking insect from Japan that feeds exclusively on Japanese knotweed (see

Defra will now seek independent review of this research before it is submitted for public consultation. But why is this weed such a problem and how can developers and contractors protect themselves?

Japanese knotweed (or fallopia Japonica) and all its variants and hybrids are part of the genus “fallopia” and members of the plant family “polygonaceae”.

It is a non-native invasive species of plant introduced to the UK in the mid-19th century, and has been the scourge of developers for years. Its rampant shoots push through most obstacles that stand in its way (including buildings and hard surfaces), and its presence on site can lead to significant project delays and costs. Once thought of as a collectable plant, Japanese knotweed is no longer so prized – it is difficult to eradicate and there is a raft of legislation concerning its control. As such, it presents a particular challenge for removal from construction sites.

Contractors looking for protection where Japanese knotweed is suspected tend to seek clauses placing the responsibility for it on the developer. Such clauses commonly allow a contractor to seek a variation to the contract and claim an increase in the contract sum (and extra time, if appropriate) if Japanese knotweed is discovered and needs to be treated.

A common catch-all is that the contractor has no continuing liability to the developer following initial treatment, even where it deals with it initially (the clause is: “notwithstanding anything contained elsewhere in this contract the contractor shall have no liability to the employer in the event that Japanese knotweed returns to the site or the works whether prior to or after practical completion”).

A common catch-all is that the contractor has no continuing liability following initial treatment

So, how can a developer protect itself? The good news is that in the past few years, awareness of Japanese knotweed has grown considerably, and there are specialist companies that can assist in identifying and developing a programme of treatment for the weed, which can involve control and/or eradication.

The Environment Agency has also published a code of practice: “The Knotweed Code of Practice: Managing Japanese Knotweed on Development Sites”, which contains a great deal of practical advice.

Hence, the following steps should be considered where Japanese knotweed is suspected:

  • Undertake a professional investigation of the site to identify its extent and location – is the weed in a contained location that can be kept undisturbed from construction work?
  • Ensure that an appropriately qualified consultant is appointed, and that the benefit of its report and advice can be extended to other interested parties, for example the development’s financier, future purchaser and possibly the contractor
  • Ensure that any specialist programme of eradication and treatment is followed, and that those engaged are adequately insured
  • Check that the construction contract places the risk sensibly between the developer and contractor for treatment and continuing control of the Japanese knotweed – consider who is best placed to deal with the initial and longer-term treatment.

Despite news of the sap-sucking insect, developers and contractors would be wise to continue current best practice for now. It might be a while before this treatment gets approval for use in the UK, and even if it does, it is likely to take a long time to bring the weed under control, which means that Japanese knotweed is likely to remain a problem for the foreseeable future.

Regardless of the arrival of killer bugs, killer weed clauses are likely to remain a feature of construction contracts for the time being, and the risks and remedies are still going to be issues for negotiation, particularly in the light of the impact that Japanese knotweed can have on construction costs and site values.