The researchers suggest that the novelty of gardening may have been enough to jolt some of the participants out of their doldrums, but some experts have a much more radical explanation for how gardening might ease depression.

Christopher Lowry, Ph.D., an assistant professor of integrative physiology at the University of Colorado at Boulder, has been injecting mice with Mycobacterium vaccae, a harmless bacteria commonly found in soil, and has found that they increase the release and metabolism of serotonin in parts of the brain that control cognitive function and mood -- much like serotonin-boosting antidepressant drugs do.

Digging in the dirt isn't the same as taking Prozac, of course, but Lowry argues that because humans evolved along with M. vaccae and a host of other friendly bugs, the relative lack of these "old friends" in our current environment has thrown our immune systems out of whack.

This can lead to inflammation, which is implicated in a host of modern ills, from heart disease to diabetes to depression.
"By reintroducing these bacteria in the environment, that may help to alleviate some of these problems," Lowry says.