Background <p>Nature-based interventions are increasingly recognised for their potential to support psychological well-being and social inclusion among vulnerable populations. However, few studies have examined the short-term effects of therapeutic gardening among individuals experiencing homelessness, a group at high risk of poor mental and physical health.</p> Methods <p>This mixed-methods study evaluated a participatory gardening programme implemented at a homeless shelter in Córdoba, Spain. Participants play an important role in the co-creation and co-management of therapeutic green spaces through a structured “co-co-co-co” methodology (co-design, co-deployment, co-management, and co-assessment). Quantitative data were collected via adapted scales from validated instruments (Well-Being Index, Psychological Distress Scale, Self-Rated Health) adapted to assess immediate pre- and post-session changes. An additional environmental perception scale conceptually grounded in an integrated restorative environment framework, drawing from the biophilia hypothesis, Attention Restoration Theory, and Stress Reduction Theory, was administered. In-depth semi-structured interviews explored participants’ experiences and perceived outcomes.</p> Results <p>Quantitative analyses revealed statistically significant improvements in well-being, perceived physical health, and psychological distress among gardening participants, with large effect sizes. In contrast, the comparison group, which engaged in unstructured shelter-based activities, showed no meaningful change. The environmental perception data indicated strong appreciation for features such as water and sensory comfort, with less emphasis on biodiversity. In-depth semi-structured interviews corroborated these findings, highlighting the importance of structure, emotional support, and social connection. The participants described the intervention as providing a sense of purpose, calmness, and motivation. In brief, structured exposure to therapeutic gardening was associated with measurable improvements in mental health-related outcomes among people experiencing homelessness.</p> Conclusions <p>The participatory design facilitated both individual and collective benefits, suggesting that such interventions may offer low-cost, scalable pathways for emotional restoration and social inclusion. The findings suggest that nature-based approaches may offer valuable complementary support within homelessness services, with attention to cultural, psychological, and environmental dimensions.</p>