To quote the Canadian Horticultural Therapy Association: "Horticultural Therapy (HT) is a formal practice that uses plants, horticultural activities, and the garden landscape to promote well-being for its participants. HT is goal oriented with defined outcomes and assessment procedures. HT sessions are administered by professionally trained Horticultural Therapists. Research indicates that HT is proven to be beneficial in a wide variety of healthcare, residential, school, and rehabilitative settings". (https://www.chta.ca/about_ht.htm).

Horticultural Therapy has  a long history in the United States. As the American Horticultural Therapy Association points out -  "In the 19th century, Dr. Benjamin Rush, a signer of the Declaration of Independence and recognized as the "Father of American Psychiatry," was first to document the positive effect working in the garden had on individuals with mental illness." (http://www.ahta.org/what-is-horticultural-therapy).

Both these websites provide lots of information on Horticultural Therapy. If you want to get formal training in HT, there are numerous programs in American universities and botanical garden's that you can take. In the New York area, the New York Botanical Garden has an outstanding certificate program in Therapeutic Horticulture. . In New Jersey, Rutgers University has a similar program which is accredited by the American Horticultural Therapy Association.

Courtesy of NYBG Plant Information