Clinically depressed persons suffer from impaired mood and distortion of cognition. This study assessed changes in depression severity and perceived attentional capacity of clinically depressed adults (N=18) during a 12-week therapeutic horticulture program. The Beck Depression Inventory (BDI) and Attentional Function Index (AFI) were administered at baseline, twice during (4 and 8 weeks), and immediately after the intervention (12 weeks), and at a 3-month follow-up. Experiences of being away and fascination related to the intervention were measured at 4, 8, and 12 weeks. The mean BDI score declined 9.7 points from pretest (27.3) to posttest (p < .001) and were clinically relevant (deltaBDI > or =6) for 72% of the cases. The mean AFI score increased 10.2 points from pretest (68.8) to posttest (p = .06). The greatest change in BDI and AFI scores occurred in the initial weeks of the intervention. The reduction in BDI scores remained significant and clinically relevant at the 3-month follow-up (N=16). The decline in depression severity during the intervention correlated strongly with the degree to which the participants found that it captured their attention. Therapeutic horticulture may decrease depression severity and improve perceived attentional capacity by engaging effortless attention and interrupting rumination.