Timing for green prescription: comparing the immediate and cumulative effects of different duration horticultural therapy programs on college students' mental health

BackgroundAlthough the mental health benefits of horticultural therapy (HT) are well-established, the influence of intervention duration and the temporal dynamics of effects remain insufficiently explored, lacking evidence for optimal protocol design.MethodsThis study employed a comparative experimental design with a short-duration HT program (4 sessions, n = 12) and a long-duration program (8 sessions, n=14), both conducted weekly. Immediate effects (mood states, affects, facial expressions, salivary cortisol) were measured before and after each session. Cumulative effects (meaning in life, depression, anxiety) were assessed at baseline, post-intervention, and at a 4-week follow-up. Non-parametric tests were used to analyze within-group changes and between-group differences in effect sizes.ResultsFor immediate effects, all psychological and physiological indicators showed significant improvement after each single session (|r| > 0.5, p < 0.05), with no significant difference in effect sizes between groups. For cumulative effects, the long-duration group demonstrated a significant and sustained increase in the presence of meaning in life (PML), which was maintained at the follow-up. While the short-duration group also showed a statistically insignificant trend of improvement in PML. Both programs significantly reduced depression levels immediately post-intervention, but these effects rebounded at follow-up. Anxiety decreased significantly only in the short-duration group post-intervention, with both groups showing a rebound at follow-up.ConclusionHT activities provide stable and significant immediate mental restoration, unaffected by the overall program duration. While shorter programs are effective for transiently alleviating anxiety and depression, longer durations are crucial for fostering a stable and enduring sense of meaning in life. This study reveals a differentiated “dose-response” relationship for HT, providing refined evidence for designing time-sensitive intervention strategies tailored to specific mental health objectives.