The U.S. city of Louisville, Kentucky isn’t known as a hotbed of environmental action and innovation, but that could change as it has recently become home to a first-of-its-kind collaboration between environmentalists, city leaders and public health professionals. The Green Heart Project, funded in part by the United States National Institutes of Health, will plant trees in neighborhoods throughout the city and monitor how they affect residents’ health. It’s a boundary-pushing medical trial—a controlled study of nature as a medical intervention.