Nature-based activities to support mental ill health (GREENGAGE): protocol for a pilot randomised controlled trial and process evaluation

Abstract Background Anxiety and depression are among the leading mental health difficulties worldwide. Nature-based interventions are increasingly being used to support mental health and wellbeing. There is growing policy interest in the use of nature as a setting for interventions. However, there remains a lack of robust evidence on their effectiveness and cost-effectiveness. Methods The GREENGAGE pilot randomised controlled trial (RCT) aims to test trial processes to inform the design and implementation of a fully powered, definitive RCT evaluating group, nature-based interventions in addition to usual care, compared with usual care, for adults with mild-to-moderate depression and/or anxiety. Eighty-four participants will be recruited via various routes, including general practices, social prescribing and link worker networks, NIHR Be Part of Research , social media, local universities, and regional media outlets. Participants randomly allocated to the intervention arm will undertake a 6-week programme comprising weekly, 3-h group sessions, delivered in natural environments by one of three nature-based intervention delivery organisations in the southwest of England. Activities will differ according to the setting, practices and resources of the delivery organisation but all will include key components identified within the authors’ Handbook for Nature on Prescription to Promote Mental Health: developing a relationship with nature, craft and creativity, physical activity, and therapeutic elements. Quantitative outcome measures relate to anxiety (GAD-7), depression (PHQ-9), wellbeing (sWEMWBS/ONS4), and loneliness. Resource use will be captured to measure intervention costs and health and social care use. Data will be collected at three time points: before randomisation (T0), at 6–8 weeks post-randomisation (T1), and 12–14 weeks post-randomisation (T2). Feasibility objectives include determining recruitment and retention rates, adherence, data completeness, and acceptability of randomisation and outcome measures. A brief participation survey will explore participants’ motivations for joining, experiences of participation, and barriers to engagement. Focus groups with participants from intervention and waitlist control groups, and delivery organisations, will explore the acceptability and implementation of trial procedures to inform a future definitive trial. Discussion Findings will inform methodological refinements, and the recruitment and retention strategies required to deliver a full-scale RCT. Trial registration This pilot trial was registered on 4 July 2025 as ISRCTN12900725.