Indeed, Stuart-Smith’s stirring summaries of the garden’s history, value in therapeutic settings, and place in literature and culture feel like the best kind of circumstantial evidence for the primacy of the act. But when she digs into the emerging neuroscience, the evidence is breathtaking. Take, for example, what actually lives in the soil. Bacterial actinomycetes, when activated by water, emit an aroma called geosmin that has a pleasing and soothing effect on most people, Stuart-Smith points out. Heart rate and blood pressure drop within minutes of exposure to natural surroundings like parks and gardens. Cortisol the fight-or-flight hormone that assaults our well-being when we endure sustained stress drops within 20 to 30 minutes. The scents of blooms from lavender, rosemary, and citrus summon mood-elevating chemicals. The scent of roses actually allows our body to hang onto endorphin highs longer, extending that blissful inhalation.