"Horticultural therapy is a clinical process with measurable objectives and outcomes that are closely monitored by a medical team of therapists or rehabilitation professionals," he said.
Because of his interest and expertise in the field, Airhart is often called on to lead workshops such as one he led recently at the Memphis Botanic Garden.
Clara Ray, a technical horticultural specialist who's working on a project in Putnam County's Dry Valley Alternative School, attended the session.
"The workshop really helped me put in perspective the specific issues of assessing children with behavioral problems, as opposed to children and even adults with mental or physical disabilities, who are the clients for many of the other workshop participants," she said.
"I know plants have had a major impact on my own life, so I will feel good if I can use plants to help just one other person turn their life around," Ray said.
In addition to addressing a wide range of client issues, horticultural therapy also draws attention to a wide range of environmental aspects, Airhart said, such as the relaxing response people have to the color green.
"What I like best about it is that at its core horticultural therapy has the ability to increase self-esteem and self-confidence on both individual and social levels," he said.
"It can be as simple as taking a cutting from a plant, rooting it in soil, giving it to someone as a gift and seeing them smile, but horticultural therapy teaches people skills they can use to help make themselves and others happy," Airhart continued.
At its greatest expression, horticultural therapy provides a wide range of social and vocational benefits. For instance, a former patient may respond so well to the therapy that horticulture becomes a new source of income for him or her.
The groups who may benefit most from horticultural therapy include:
People with physical, mental, psychological or developmental disabilities.
Someone who's suffered a traumatic brain injury and needs to work on motor skills, for example, may be encouraged as a traditional mode of physical therapy to try squeezing a soft rubber ball a certain number of times per minute.
"If a patient loses interest in that activity, a therapist who's skilled in the practices of horticultural therapy might then hand her a pruning tool and discover that the patient is happy to clip dead leaves from plants until her hands are too tired to prune any longer," Airhart said.
People whose age ranges from the youngest of children to the oldest of adults.
"A child with a chronic illness who doesn't want to take his medicine can be given a plant to care for and instructed that the water and fertilizer are the plant's medicines," he said. "That way, the child sees first-hand how the plant can thrive with proper care or wilt from neglect."
Victims of abuse or the abusers.
People recovering from illnesses or hospice patients seeking enhanced quality of life in their last days.
And accomplished or novice gardeners and even people who've never gardened before.
"One of the first horticultural therapy activities I learned was to make a bird feeder from a pine cone smeared with peanut butter and rolled in bird seed," Airhart said. "That's an easy, inexpensive activity that can be applicable to patients of all ages who are being treated for a wide variety of conditions."
Although the positive interaction of people with plants has been noted as far back as ancient Egypt, Dr. Benjamin Rush a medical professor in Philadelphia who is often considered the first psychologist noted in the late 18th century that field labor in a farm setting had a curative effect on mentally ill patients.
In the two world wars of the early 20th century, horticulture provided a diversion for wounded soldiers with long hospital stays.
But it wasn't until Alice Burlingame, a trained psychiatric social worker, started a geriatric horticulture program in Michigan State Hospital in 1951 that she began to realize horticultural therapy warranted study as a distinct profession, Airhart said.
With professional interest in the new field steadily increasing, the National Council for Therapy and Rehabilitation Through Horticulture formed in 1973.
"I first developed an interest in horticultural therapy as a graduate student at the University of Georgia in 1978, and I began serving on the board of directors of the NCTRH in 1980," Airhart said.
When the NCTRH changed its name in 1988 to the American Horticultural Therapy Association, he served as the first president of that new organization.
At TTU, Airhart teaches the agriculture department's horticultural therapy course every other spring semester.
"During my introductory lab session, I take students to the greenhouse and tell them to pick out a plant that looks how they feel," he said. "It's late in the afternoon, and they're tired from a long day, so most pick out plants that are just a little bit wilted."
Once, however, one of the students picked out a plant that was "green, perky and looked like it was just absolutely beaming," Airhart said. The student who'd selected it said it was the best day of her week because she'd been asked out on a date that morning.
"Students in most classes don't have the opportunity to interact with each other or their instructor on such a personal basis but that just goes to show you the power of plants!" he said.