Development of the Nature Impact Mental Health Intervention for People Experiencing Mild to Moderate Anxiety, Depression, and/or Stress—Co-Producing a Programme Theory and Logic Model

Background: Nature-based health interventions (NBHIs) show promising potential for supporting people experiencing mild to moderate anxiety, depression, and stress. However, their underlying programme theories are rarely made explicit, limiting transparency, implementation, and transferability within healthcare contexts. The Nature Impact Mental Health Intervention is a context-adapted, nature-based programme designed to support mental health and well-being. This article aims to describe its development through a structured co-production process and presents its programme theory and logic model. Methods: The co-production-based development process followed a three-stage framework. Stage 1 established a scientific foundation through a systematic review, stakeholder analysis, dialogue meetings, and a Delphi study to synthesise evidence and identify knowledge gaps. Stage 2 involved a co-production workshop with practice partners and researchers to translate evidence and refine intervention components. Stage 3 consolidated outputs and site visits into an operational intervention catalogue for prototyping the resulting programme theory and logic model. Results: The co-production process yielded a coherent programme theory comprising clearly defined mechanisms of change and aligned intervention activities. These were iteratively refined through workshops and prototyping, resulting in a consolidated logic model that articulates hypothesised causal pathways linking activities to outcomes. The model also provides a practical framework for guiding subsequent feasibility testing, implementation, and evaluation across contexts. Conclusions: This study demonstrates a transparent development process for co-producing a programme theory and logic model for NBHIs. The resulting model provides a theoretically grounded and implementation-sensitive foundation for subsequent feasibility testing and contributes methodological guidance for integrating NBHIs within healthcare systems.