Renovating vacant lots into green spaces can lower feelings of depression for those living nearby, especially in poor neighborhoods, a new Philadelphia study indicates.

Researchers found that planting grass and trees in formerly empty and blighted lots led residents within a quarter-mile radius to feel 40 percent less depressed. In neighborhoods that fell below the poverty line, the effect was even more pronounced, with feelings of depression dropping 68 percent.

Researchers say the study provides support for a low-cost public health measure to address one of the most costly health conditions in the United States. Individuals and insurance programs spend more than $70 billion annually on depression treatment. In comparison, renovating a vacant lot cost about $1,600 per 1,000 square feet and $180 per year to maintain, according to the study, published Friday in JAMA Network.

"This really is a relatively low-cost intervention compared to the amount of money that is spent on other health problems," said Eugenia South, coauthor of the study and assistant professor of emergency medicine at the University of Pennsylvania. Researchers at Rutgers University and the U.S. Department of Agriculture also contributed to the study.

This is not to say a green space can replace traditional therapies or medications, South explained. But it could work in conjunction with those treatments, and provide the added benefit of affecting an entire neighborhood. It could be particularly useful in low-income communities, where there is a prevalence of mental health concerns and people often find it difficult to access traditional health care.

>>READ MORE: Gun violence can be reduced by clearing vacant lots, study finds

For the study, more than 500 vacant lots throughout Philadelphia were grouped into 110 clusters. Each cluster was randomly assigned to either a greening intervention, a trash cleanup, or no intervention. The greening involved removing trash, leveling the land, planting new grass and trees, and installing a low wooden fence. Trash cleanup involved removing trash and mowing any pre-existing grass. The Pennsylvania Horticultural Society performed the renovations.

Researchers surveyed more than 300 residents around the lots in the 18 months before and after the renovation, asking them how nervous, hopeless, or depressed they felt. While residents near greened lots reported significant improvements in feelings of depression, those near lots that only received trash cleanup did not.

The findings indicate that "green space in and of itself is important for mental health benefits," South said.

It adds to a growing body of evidence showing that a lack of parks and other infrastructure is associated with depression, while spending more time in and around green spaces can lower depression rates and reduce physiological symptoms of stress.

Tracy Jamison, 54, says she's noticed that difference after several lots were renovated in the area of North Philadelphia where she's block captain. (The lots were not necessarily included in the study, but were renovated in the same way.)

"The lots used to have old cars and trash piling up. Now kids bring coloring books and use the space for camp," she said. "It makes the community happier."

>>READ MORE: Let's promote health and green space one empty lot at a time

Researchers are only just beginning to understand how and why green spaces affect mental health.

Some studies have shown that green spaces can decrease mental fatigue by providing a reprieve from the rush of stimuli in urban environments. In previous research, South found greening lots can also reduce gun violence, which could make people feel safer, she said. They're also used as a gathering space for residents, building a sense of community.

That's been an added benefit for 72-year-old Rafael Martinez, who likes to spend his free time in a greened lot near his North Philadelphia home chatting with friends. It's even inspired him to clean up another lot on his own. He planted a garden of kale, collard greens, and tomatoes there, and he shares the produce with his community. "It beautifies the neighborhood and it makes you feel good," he said.

Andrew Lee, an associate professor of public health at the University of Sheffield in England, says finding the reason green spaces have positive effects is key to informing public policy. Is it the social aspect of a park that improves mental health? If so, cities should not just build green spaces, but host barbecues and other gatherings, he said. If it's the physical activity component, then every green space should be built with exercise equipment.

Lee coauthored a review of studies on green spaces and health in 2011 and found few investigated that aspect.

"Sometimes you can spend a lot of money on an intervention and realize not much is coming out of it," Lee said. "But if you understand how an intervention works, you can maximize the benefits and ensure it's likely to work."

>>READ MORE: Why my South Philly urban garden made me want to get up in the morning

Kimberlee Douglas, director of the landscape architecture program at Thomas Jefferson University, said the new study provides concrete evidence for something people intuitively know: "We have an innate need for nature."

From her experience building parks in Philadelphia's low-income neighborhoods, she's seen them provide benefits beyond mental health. They're great places for kids to explore, use creativity, and build social skills, she said. They encourage people to exercise more.

"It gives communities a different way to think about themselves," Douglas said. "They can be proud of where they live."

Green spaces in urban areas have also been linked to cooler temperatures and fewer heat-related deaths. When properly designed, they can reduce noise levels and air pollution and improve stormwater management.

>>READ MORE: These Philadelphia neighborhoods get the hottest in a heat wave

One concern that arises in some communities is that green spaces will increase real estate prices and force residents out of the neighborhood. Douglas and South said renovations need to be done with the community's input.

"Our intention is not to push people out," South said. "We want it to be something that is helping the neighborhood be healthier for the people who live there."

Philadelphia Media Network is one of 19 news organizations producing Broke in Philly, a collaborative reporting project on solutions to poverty and the city's push toward economic justice. Follow us at @BrokeInPhilly