Policymakers and practitioners working in urban greenspace management want to know what kind of interventions are effective in promoting mental wellbeing. In practice, however, they rely on multiple forms of knowledge, often in unwritten form. This paper considers how such knowledge is interpreted and used by a range of stakeholders to identify greenspace interventions to support residents’ health and wellbeing in one UK city. It examines the interface between academic research, policy and practice, drawing on the findings of a three-year study in Sheffield, UK. The Improving Wellbeing through the Urban Nature project investigated the links between ‘urban nature’ and mental health. One strand of the research sought to influence policy and practice, and this article presents findings and reflects on some of the processes of this exercise. It highlights the role of tacit knowledge in practice and its influence on practitioners’ choice of greenspace interventions and the challenges in drawing on such knowledge to influence policy. The findings affirm practice-based knowledge as socially situated, interpretively fashioned and politically weighted. This paper concludes by demonstrating the importance of considering the local context when devising policy prescriptions for greenspace provision and management.