While a garden can blunt that harm, it won’t eliminate it, she said. But it does appear to help.

A 2008 study by Alison Laichter, a graduate student at Columbia University, showed that participants in the GreenHouse program had a 40 percent lower rate of reconviction than inmates in the general prison population. Similar results are reported for the garden at San Quentin.

But according to Sander van der Linden, a professor of social psychology at the University of Cambridge, it is difficult to say whether green prison programs are the cause of this difference.

“Serious or violent offenders are unlikely to qualify for such programs,” he said. “So perhaps those who are selected into them were less likely to reoffend to begin with.”

While the precise impact may be hard to quantify, Taariq King credits the garden with his transformation from a frightened young inmate without job skills to someone who has found a sense of direction in his life.

Mr. King was sent to Rikers at the age of 18, accused of second-degree murder and possession of a weapon. Early in his jail stint he tried working in the garden, which was way outside of his comfort zone, he said. But the garden grew on him quickly.

After two years of waiting for his trial, he wa s eventually acquitted. He entered the Hort as an intern, and now is a full-time employee.