Nature-Based Interventions for Individuals with Psychiatric Disorders: A Mixed Methods Systematic Review with Random-Effects Meta-Analysis of Mental Health and Functional Outcomes.

Nature-based interventions (NBIs) are increasingly used in mental health services, but their effectiveness in people with psychiatric disorders, and how these individuals experience them, remains unclear. This review synthesised quantitative and qualitative evidence on NBIs in psychiatric populations. Eligible studies evaluated outdoor NBIs against controlled comparators, excluding neurodevelopmental/degenerative conditions and indoor or virtual interventions. Quantitative outcomes were synthesised using random-effects meta-analysis; qualitative data were analysed using thematic synthesis. Twenty-eight studies were included, mostly involving people with diagnoses of schizophrenia or depression. NBIs were associated with greater improvements in clinical symptoms than controlled comparators (pooled effect size 0.71 [95% CI 0.29-1.12]; p = 0.0009), with moderate heterogeneity (I2 = 48.6%). The qualitative synthesis identified five themes: Being in Nature, Personal Growth, Psychological Wellbeing, Social Relationships, and Physical Benefits. Participants reported reduced stress, improved mood and coping, strengthened identity, enhanced social connection, and increased energy. NBIs, particularly horticultural programmes and guided outdoor activities, may offer promising recovery-oriented adjuncts to psychiatric care. The next step is to build a translational evidence base by harmonising recovery-relevant outcomes and developing pragmatic, scalable models of delivery that can be embedded within routine mental health services, informed by mixed methods evaluation.