When Dr. Robert Zarr wants to help kids with obesity and diabetes in Washington D.C., he doesn’t just order in another set of pills. He looks up a database of green spaces and asks his patients if they’ve been outside recently. Then he writes a prescription–to a park.

Over the last three years, Zarr has been chief evangelist for Park Rx, a system that makes it easier for doctors to recommend outdoor activity, offering an alternative or supplement to drug treatments.

We wanted to know whether actually prescribing a park during a doctor’s visit would change behavior.

With the help of the National Park Service, the D.C. Department of Parks and Recreation and other groups, Zarr mapped and rated 380 parks for their activities, cleanliness, safety, and accessibility to transit. Now when DC’s doctors look up a patient’s electronic record, they can see a database of greenery alongside other options. They can ask patients about their physical activity and what they like doing, then print out a page with details of a nearby park with a map and picture.

Since launching Park Rx last July, 30 doctors at Unity Health Care have signed up to the program, issuing about 550 prescriptions. Follow-up interviews show an average increase of 22 minutes of activity per week across 400 kids. “We wanted to know whether actually prescribing a park during a doctor’s visit would change behavior,” Zarr says. “And the answer was ‘absolutely yes’.”

The National Park Service came up with the original Park Rx concept. But it’s taken Zarr’s efforts to build the database and see it being used by colleagues. In an interview, he says he was inspired by a book by Richard Louv called Last Child in the Woods, which explores “nature-deficit disorder” and its effect on youth health.

Zarr doesn’t think prescribing parks is a radical step, though it may require a little getting used to. “Once you get over the conceptual hurdle of prescribing park, and you believe the scientific literature that clearly says being outside is good for health, then all it takes is to push a button on a computer. They have to do that anyway,” he says.

Zarr now hopes to develop a mobile app, and perhaps get the “have you been outside recently?” question included in patients’ pre-interviews alongside other vital signs queries.