“More evidence suggests that nature does something essential for our mental health. Specifically, we have learned that nature tends to result in reduced circulating levels of the stress hormones adrenaline and cortisol, lowered blood pressure, blunted “perceived stress” after stressful life events, and lower short-term levels of anxiety and depression. We also appear to ruminate less after we’ve spent time in nature, a phenomenon distinct enough to appear as differences in neural activity during brain scans. One recent study selected 541 vacant lots across the city of Philadelphia and randomly allocated each to either receive no intervention; receive regular trash removal and mowing; or be turned into open pocket parks, with trees and a pleasant, short wooden-perimeter fence. Survey teams blind to the intervention were sent out to question residents at random before and after the great experiment, eventually interviewing nearly 450 people about their mental health. When the study was complete, its architects found that residents of neighborhoods where lots had been greened were much healthier psychologically than those whose lots had merely been cleaned. Around greened lots, neighborhood-level rates of feeling “depressed” dropped by 42%, feeling “worthless” by 51% and having generally “poor mental health” by 63 percent.