Aims: Nature-assisted therapy for mental health problems receives increased attention. However, quantitative evaluations are rare. This study evaluates the effects of an all-outdoors vocational rehabilitation program for individuals on long-term sick leave due to sustained stress-related symptoms. Methods: In a comparative pre-post intervention design the intervention group contained 48 participants from Mariendal Gardens (MG), while 45 participants at Stress- & Jobmanagement (SJ) formed the comparison group. At MG all activities took place outdoors, while activities at SJ were mainly indoors. Questionnaires were completed at baseline, 3- and 6-month follow-up. Outcomes included Perceived Stress Scale (PSS-10) and measures of sleep, mindfulness, self-efficacy, daily functioning, and work ability. Data were analyzed using mixed model repeated measures analysis of variance. Results: At baseline the MG-group PSS-10 mean score was 25.15 points (SD=7.20), while the SJ-group mean score was 23.91 (SD=7.48). At 3-months the MG within-group score dropped 4.61 [2.71; 6.52] points (p<0.01), corresponding to at standardized mean difference (Cohen's d) of d=0.64 [0.38; 0.91], while the SJ within-group score dropped 4.16 [1.73; 6.59] points (p<0.01), corresponding to d=0.56 [0.23; 0.88]. The between-group mean difference was not significant (p=0.77). Similarly, results for sleep, mindfulness, self-efficacy, daily functioning, and work ability demonstrated significant within-group effects and minimal between-group differences.

CONCLUSIONS BOTH INTERVENTIONS DEMONSTRATED SMALL TO LARGE PRE: -post effect sizes. Negligible differences were observed between the effects of the two interventions, indicating no added effect of the all-outdoors setting. Results should be interpreted with caution as unequal lost to follow-up rates threatens the comparability of changes in the two groups.