CPD: Managing invasive species on construction sites

Invasive non-native species (INNS) such as Japanese knotweed, Himalayan balsam and giant hogweed present a growing challenge for the construction and remediation industry.

While their ecological impact is well documented, their significance in construction project delivery is often underestimated. These plants can delay programmes, damage infrastructure, trigger legal liabilities and significantly inflate costs if not managed correctly from the outset.

According to the UK government, invasive species already cost the UK economy almost £2bn annually, with construction and development projects bearing a significant share of that burden. As developments become more constrained to brownfield sites and urban infill, the risk of encountering invasive plants increases.

Early identification and robust management planning can safeguard project timelines, protect budgets and reduce environmental risks. With increasing regulatory scrutiny and sustainability targets to meet, it is essential that contractors, consultants, and developers bring invasive species management into project planning at the earliest stage.

One of the most insidious aspects of invasive species is that what appears benign above ground may belie a complex subterranean threat.

Knotweed stems might be sparse, but underground the rhizome network can spread widely, often infiltrating foundations, retaining walls or utility corridors. Without timely intervention, this hidden proliferation can force work stoppages, trigger redesigns or require costly remediation.

From a cost standpoint, late detection often proves significantly more expensive than proactive action. Remediation can recur over several years, or demand full excavation, both of which carry financial and programme risks.

Additionally, reputational risk is real: failure to address invasive species can erode client trust, unsettle insurers and even provoke legal claims if contaminated material spreads beyond site boundaries.

Projects with ambitious sustainability or biodiversity net gain (BNG) targets are particularly vulnerable. If invasive species aren’t controlled, they may undermine ecological gains, derailing ESG strategies and potentially triggering noncompliance against net gain commitments.

In practice, two strategies dominate invasive species control: herbicide application and excavation with soil disposal.

Herbicide treatment offers a cost-efficient solution, particularly in areas where future disruption is unlikely. A typical treatment plan begins with a mapping survey, after which a suitable herbicide such as glyphosate, triclopyr or aminopyralid is selected depending on species, site sensitivity and environmental constraints.

Treatment often involves knapsack spraying, stem injection or cutting-and-spraying, repeated over multiple seasons, with monitoring and follow-up for two to four years.

Because of the risk of off-target impact, especially near watercourses, registered practitioners must hold recognised pesticide qualifications (PA1, PA6, and in some cases, PA6AW), and applications must comply with HSE and code of practice requirements.

When development timelines do not accommodate a prolonged herbicide regime, or when plant infestation is dense, excavation is often the preferred route.

In such cases, contaminated soils and root material are carefully excavated under a tightly controlled biosecurity regime. Conventional earthmoving plant, 360-degree excavators, tracked dumpers and dozers are typically used, provided their operations follow strict cleaning, transit and containment protocols.

Excavated material is either contained onsite in bunds or cells, or removed to licensed landfill facilities as controlled waste. The key is not the type of machinery, but the stringent management of soil, haul routes, stockpiles and waste handling to prevent spread.

The legal framework governing invasive species in the UK requires construction professionals to treat them with the seriousness they deserve.

Under the Wildlife and Countryside Act 1981 (Schedule 9), species such as Japanese knotweed, giant hogweed and Himalayan balsam are controlled and their management is regulated.

The Infrastructure Act 2015 strengthens these obligations, empowering authorities to enforce species control orders where necessary. Alongside this, the Environment Agency’s RPS (regulatory position statement) 178 guidance provides clear protocols for correct treatment, including handling and disposal.

On a practical level, contractors must also heed their duty of care obligations to ensure that contaminated soil, plant fragments or machinery do not spread invasive species beyond site boundaries. Increasingly, insurers, lenders and warranty providers demand documented evidence of invasive-species risk assessment, mitigation and ongoing management before they will provide cover.

Managing invasive species effectively demands that all key stakeholders, clients, consultants and contractors integrate species risk into every stage of the project.

This begins at feasibility and design: ecological or invasive-species surveys should be commissioned early, ideally during the planning phase. These surveys typically include a desk study, a walk-over by an ecologist or specialist, species mapping and a risk assessment that informs methods, cost and planning.

Note that invasive species can be seasonal, site-specific, and easily overlooked without trained eyes, so specialist expertise is important for detection.

Once species presence is confirmed, its implications should be integrated into project design. This may mean reconfiguring works to avoid deep excavation in contaminated areas, designing burial cells, capping, or using phased sequencing to minimise disturbance. In some cases, combining long-term herbicide treatment with containment may prove more sustainable than full excavation.

Biosecurity measures must also be strictly enforced. Construction teams should receive toolbox talks that explain species appearance, contaminated zones, approved routes for plant movements and hands-on protocols such as how to clean plant and machinery.

Soil movement becomes high risk when contaminated material is mistakenly carried into clean zones, either on wheels, tracks or in poorly segregated heaps. This risk is mitigated through controlled haul routes, machine segregation and mechanical cleaning, scraping, pressure washing and drying, rather than reliance on detergents.

Maintaining a golden thread of information is critical: all survey data, treatment plans, logs (herbicide, excavation), method statements, disposal records and photographic evidence should be stored in a common data environment (CDE). This ensures transparency, traceability and accountability across the project lifecycle and supports future maintenance or redevelopment needs. RPS 178 requires records to be kept for two years and these records made available to the Environment Agency on request.

A construction workforce capable of identifying, managing and mitigating invasive species risk requires tailored training at multiple levels. For general operatives and supervisors, short induction sessions and toolbox talks (typically 15-30 minutes) are sufficient to raise awareness. These should be updated when new species are found or when working zones change.

Herbicide applicators require formal certification: PA1 for theory, PA6 for handheld application and PA6AW if spraying near water. Courses are typically one to three days in duration, followed by an assessment.

Specialists overseeing high-risk remediation works are often ecologists or environmental professionals with Invasive Weed Group or membership of the Invasive Non-Native Specialists Association; they design strategies, supervise applications or excavations, and ensure compliance with regulatory obligations.

On this Churngold Construction project, although initial above-ground evidence suggested only limited growth, during excavation we uncovered extensive Japanese knotweed rhizomes, some alarmingly close to the building foundations.

To manage the risk, the Churngold site manager supervised the works throughout. Using a 360 excavator, soil was scraped back in 100mm layers and inspected after each pass, with excavations carefully enlarged wherever rhizomes were detected. Around the foundations, the project team removed loose material by hand where required to achieve full clearance.

In total, Churngold safely removed 10.62 tonnes of contaminated soil via a licensed waste carrier to a permitted landfill, before backfilling the excavation with clean site-won material.

This project highlighted how easily invasive species can be underestimated and reinforced the importance of early surveys, meticulous supervision, and robust remediation planning in protecting both programme and structures.

As the demands on construction increasingly align with sustainability goals, biodiversity, ESG and regulatory compliance, managing invasive species must become part of mainstream project risk management – not an afterthought. The industry needs to equip its teams with the awareness, tools and systems to deal with INNS as routine construction risk.

This means embedding invasive-species modules into CPD programmes, regular toolbox refresher training and leveraging digital platforms for tracking, reporting and transparency. By prioritising ecological diligence, construction firms not only protect their programmes and budgets but also deliver projects that stand the test of time, environmentally and structurally.

Steve Dyke MCIOB is environmental & remediation director at Churngold Construction.