What’s in a fence? More than you’d think. In neighborhoods where as little as about $1,000 was spent transforming a vacant lot with some grass, a few trees, and a short wooden fence, people felt less depressed and less worthless.

“The beauty of the intervention is that it’s pretty simple,” says Dr. Eugenia C. South, one of the authors on a new study that tracked hundreds of vacant lots across Philadelphia. “Which is good for replication in other cities and being able to scale it up. Also, the cost is relatively inexpensive compared to other types of interventions you might do for health.”

The study, published in JAMA Network Open by a group of five doctors at the University of Pennsylvania including South, is the first to observe a cause and effect between access to “greened” vacant lots and improved mental health through a randomized controlled trial. Their research paints a vivid picture of how our neighborhoods impact our well-being and provides new evidence for why cities should be investing in low-cost but high-impact design interventions like lot greening in blighted neighborhoods.

South is an emergency medicine physician who now spends half her time on research into the underlying causes of violence prevention. “In the healthcare setting, we never really address or think about what’s happening in peoples’ neighborhoods to cause these problems in the first place,” she explains. “So that got me interested in looking outside the walls of the hospital to figure out what we could do to improve people’s health.”

Over the past few years, she and her colleagues have studied the association between green spaces and community health, including decreased violence and stress. In their latest study, they created a randomized trial of 350 people and more than 500 vacant lots, which were broken into three groups: lots that received no intervention, lots where trash was removed, and lots that were remediated–by adding grass, trees, and a fence–by the Philadelphia Horticultural Society, whose LandCare program has greened 12,000 parcels since 2004. Participants took a Kessler-6 Psychological Distress Scale survey before and after the greenings, revealing a 40% reduction in feeling depressed and a 50% reduction in feeling worthless. The impact was even more pronounced in participants living below the poverty line.

What’s so fascinating about their research–beyond observing a direct link between mental health and green space–is that the “greening” itself was extremely limited.

The Horticultural Society spends between $1,000 and $3,000 on each project and greens about 400 lots every year. The group clears away trash, razes the ground, plants some grass and maybe a couple trees, and then adds a low wooden fence, which seems to visually symbolize that the space as cared-for and invested-in. “Fences were originally installed to discourage short dumping of construction debris, a common occurrence on many blighted lots,” the group explains. “The wooden post-and-rail fence is not intended to keep people out of the lots, but rather to define the perimeter and signal that the lot is a well-maintained property and part of a citywide program. The fence has become the ‘brand’ of the Philadelphia LandCare program.”