There is a growing body of evidence to suggest that community based horticultural projects can be effective interventions for people experiencing mental health difficulties. Such programmes have been shown to provide a wide range of benefits, including increased confidence and self-esteem, improved mood, extended social networks as well as the development of new skills and a circadian rhythm. Whilst there is now considerable evidence demonstrating the beneficial outcomes of group gardening, there is a lack of understanding as to how psychological processes that contribute towards therapeutic outcomes might be affected by these interventions. This study explored experiences of community gardening programmes in order to better understand how attendance may affect the psychological health of people experiencing mental distress. In particular, it sought to understand the impact of nature on psychological experience during group attendance. Eleven people experiencing mental health difficulties were interviewed about their experiences of attending community gardening groups. The study adopted an ethnomethodological approach to constructionist grounded theory, in order to develop a framework within which participant experiences could be understood. Findings suggested that key processes of feeling safe, letting go, (re-)connecting and finding place provided mechanisms of positive psychological change. Fundamental to each process were changes in construal of, and relationships with, others, nature and importantly, the self, within the gardening group contexts. An increased sense of identification with other people and non-human nature, and the development of empathy and compassion, appeared to be key psychological processes which may account for the positive impact on participants’ mental health. Directions for future research and implications for future clinical interventions are suggested.