The effect of horticultural therapy (HT) on immune and endocrine biomarkers remains largely unknown. We designed a waitlist-control randomized controlled trial to investigate the effectiveness of HT in improving mental well-being and modulating biomarker levels. A total of 59 older adults was recruited, with 29 randomly assigned to the HT intervention and 30 to the waitlist control group. The participants attended weekly intervention sessions for the first 3 months and monthly sessions for the subsequent 3 months. Biological and psychosocial data were collected. Biomarkers included IL-1β, IL-6, sgp-130, CXCL12/SDF-1α, CCL-5/RANTES, BDNF (brain-derived neurotrophic factor), hs-CRP, cortisol and DHEA (dehydroepiandrosterone). Psychosocial measures examined cognitive functions, depression, anxiety, psychological well-being, social connectedness and satisfaction with life. A significant reduction in plasma IL-6 level (p = 0.02) was observed in the HT intervention group. For the waitlist control group, significant reductions in plasma CXCL12 (SDF-1α) (p = 0.003), CXCL5 (RANTES) (p = 0.05) and BDNF (p = 0.003) were observed. A significant improvement in social connectedness was also observed in the HT group (p = 0.01). Conclusion: HT, in reducing plasma IL-6, may prevent inflammatory disorders and through maintaining plasma CXCL12 (SDF-1α), may maintain hematopoietic support to the brain. HT may be applied in communal gardening to enhance the well-being of older adults.