Doctors are prescribing a walk in the park.

In January, a 13-year-old patient named Kelssi came to Dr. Robert Zarr’s office at Unity Health Care, a community health center in Washington, D.C. Kelssi had struggled with her weight for as long as Zarr had known her and was now obese. But during this visit, she looked at Zarr and told him she was finally ready to do something about it.

Zarr told me that exhorting patients to “get more exercise” was too vague. Last year, he decided to start trying something different. He stopped asking his patients, “Do you move?” and began asking “Where do you move?” He discovered that many spent very little time outdoors, and he began prescribing time outside for conditions as wide-ranging as ADHD, high blood pressure, asthma, obesity, anxiety, diabetes, and depression.