Introduction
Invasive plant species might not be the first thing that comes to mind when planning a construction project – but they should be. Japanese knotweed, Horsetail, and other aggressive species are a rising threat in the UK, disrupting construction schedules, increasing costs, and even leading to legal action. The problem isn’t just botanical – it’s financial, legal, and structural.
The Cost to the UK Economy
Invasive non-native species (INNS) are estimated to cost the UK economy £4 billion annually, according to a study led by CABI. Japanese knotweed alone is responsible for approximately £1.7 billion of this, with a significant portion of that cost falling on the construction and infrastructure sectors. When left unmanaged, these plants can derail timelines, create compliance issues, and cause serious reputational harm.
Legal and Financial Implications
- Property devaluation: Japanese knotweed can reduce a property’s value by up to 15%, even post-treatment, making resale difficult. Other invasive plants such as Ragwort pose a financial burden as they must be removed before construction can commence.
- Planning delays: Its presence often leads to halted development while specialist surveys and treatment plans are put in place.
- Legal risks: Under the Wildlife and Countryside Act 1981, it’s an offence to plant or cause invasive species to grow in the wild. Offenders can face fines up to £5,000 or even imprisonment.
- Neighbour disputes: The spread of knotweed from one property to another has resulted in litigation, and courts have increasingly ruled in favour of affected neighbours. A notable example from 2023 saw the Court of Appeal support claims for damages even after treatment had occurred (The Guardian).
When Weeds Halt Development: The London 2012 Case
In one of the UK’s most high-profile examples, the London 2012 Olympic site faced major disruption due to Japanese knotweed infestations. Eradicating the plant from just 10 acres of the Olympic Park cost an estimated £70 million and took over four years to complete (Wikipedia).
It’s a powerful reminder that failing to account for invasive species can escalate into a project-wide crisis.
Other Invasive Species: Destructive Horsetail
While knotweed is often front and centre, other species less talked about include Horsetail which is massive threat. This is an extremely resilient and tough plant, some would say virtually indestructible – it only takes two or three of these harmless-looking horsetails to quickly overwhelm a large planted area, forming a dense carpet. Not only does it grow prolifically in fields and grass areas (including gardens) but even on tarmacadam surfaces that have been unknowingly laid over untreated Horsetail infested ground this plant can penetrate through the surface and cause damage.
Building sites are often affected by Horsetail (as well as other invasive plants and chemical contaminants) and removal costs must be factored in to the project.
Regulatory Expectations and Site Responsibility
Most UK lenders and planning authorities now require formal invasive weed surveys during the early planning phases (RIBA Stage 1–2). Knotweed Management Plans (KMPs) have become standard practice on impacted sites, especially for developers seeking finance or planning permission.
Ignoring these risks can affect project viability and prevent a property from achieving its full market value – even years after construction is completed.
Best Practices for Construction Professionals
To minimise disruption, here’s what developers and site managers should prioritise:
- Pre-construction site surveys: Conducted by qualified specialists, ideally before acquisition or ground-breaking.
- Engagement of PCA-accredited contractors: To carry out safe, legal, and effective treatment.
- Document everything: A thorough paper trail supports due diligence and is often required by insurers and lenders.
- Plan for post-treatment monitoring: Invasive weeds can lie dormant for years before regrowth, so continued monitoring is critical.
The Bottom Line
Invasive plants are no longer just a landscaping issue – they’re a serious construction risk. Treat them like asbestos or contaminated land: get expert input early, act decisively, and always think long-term. The sooner they’re addressed, the more control you retain over costs, deadlines, and compliance.
If you’re planning a development or already managing a site affected by invasive species, don’t wait for the problem to escalate. Early action protects your investment, ensures smoother project delivery, and keeps you on the right side of the law.
Contact us today to arrange a site survey or learn more about our construction support services.