Philadelphia Vacant-Lot Greening — A Randomised Trial of Low-Cost Urban Greening and Neighbourhood Safety (USA)

Philadelphia holds thousands of vacant lots. In a cluster randomised controlled trial led by the University of Pennsylvania, 541 lots were randomly assigned to 'cleaning and greening' (debris removal, grading, grass and a few trees, low perimeter fence, ongoing maintenance), a trash-only clean-up, or no intervention.
Published in PNAS (2018), the study found significant reductions in gun violence, burglary and nuisance offences near greened lots, with up to a 29% reduction in gun violence in neighbourhoods below the poverty line, and large falls in residents' perceived crime and safety concerns. A companion JAMA Network Open trial (2018) reported 41.5% less self-reported depression among residents living near greened lots (more than 68% in neighbourhoods below the poverty line); the overall mental-health measure was significant only for specific items.
Scientific American reports a typical cost of about US$9,300 per 1,000 sq ft lot and roughly US$50 per year upkeep.
Caveats: the evidence concerns social and health co-benefits. The trial did not measure carbon storage, biodiversity or stormwater, and effects depend on continued maintenance. Reported effect sizes differ between summaries (e.g. 9.1% overall vs. 29% in sub-analyses), so treat headline figures as indicative.

Read the full analysis: https://www.publichealth.columbia.edu/news/how-reduce-crime-gun-violence-stabilize-neighborhoods-randomized-controlled-study