Many cultures have long understood the harmonizing influences of flora. Henry Thoreau, the early American naturalist, wrote persuasively about the impact of nature on human well-being in his book "Walden." The pioneering landscape architect, Frederick Law Olmsted, "understood the need for fatigued urban dwellers to recover their capacity to focus in the context of nature," says Stephen Kaplan, who, along with his wife, Rachel, at the University of Michigan have helped found the field of environmental psychology. In the 1860s, Olmsted employed his insights in designing New York City's Central Park, with its acres of rambling walks and natural vistas, as well as a host of other city parks modeled after it.

"The gardens of the ancient Egyptian nobility, the walled gardens of Persian settlements in Mesopotamia, and the gardens of merchants in medieval Chinese cities indicate that early urban peoples went to considerable lengths to maintain contact with nature," according to Texas A&M's Ulrich. More recently, Harvard zoologist Edward O. Wilson has written extensively on this natural affinity, which he calls "biophilia" and defines as a partly genetic tendency by humans to respond positively to nature.

The latest research and writings are serving as the intellectual basis for the relatively new practice of horticultural therapy. Practitioners say their experience shows that gardening can have an especially beneficial mental-health impact because it provides a sense of control, a psychological counter to stress and anxiety. This is especially important for patients who are recovering from stroke or other traumas or are learning to live with a physical or mental disability, says Teresia Hazen, who oversees horticulture-therapy programs for Legacy Health System in Portland, Ore.

"For patients who find themselves restricted by a disability, even the simplest gardening experience, such as growing a potted plant from a cutting, gives them a feeling of control," says Hazen. "Gardening, more than most rehab activities, has the ability to be very distracting," she adds, noting that simply taking people's minds off their problems alleviates pain and depression.

Hazen recently helped design an award-winning garden in Legacy's Good Samaritan Hospital that has a dual purpose. Rehab patients receive therapy in it, she says, but also "many doctors and nurses just come by and sit or stroll or just stand and gaze, maybe just for a few moments. It's easy to see it draws them and is a source of relief."

Now, several city-run botanical gardens are hiring horticulture therapists to run public programs to expose city dwellers to nature's therapeutic benefits. Chicago's Botanic Garden provides a range of horticultural-therapy services — including planting, weeding, cultivating, watering and harvesting — both to private health agencies that treat the handicapped and to people who come in off the street.

Even some prisons are looking to gardens for relief. The New York Horticultural Society directs one such program, called the Greenhouse Project, at New York's Riker's Island facility. Inmates work in the garden, but some have also been allowed out to build gardens in public spaces throughout the city.

Several schools of architecture now have academics on staff who specialize in studying what kinds of gardens are most likely to attract users. "Some hospitals just throw in a few bushes and trees and hope they are accomplishing the wanted effect," says Clare Cooper Marcus, a professor at University of California, Berkeley, who has traveled the world analyzing gardens in health-care settings. A better garden, she says, "allows people to interact with the natural setting."

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Green ways to lose the blues

Here are some places to look for healing gardens and therapy sessions. The American Horticultural Therapy Association www.ahta.org and the Horticultural Therapy Institute www.htinstitute.org offer opportunities to learn more about the field.