American Horticultural Therapy Association (AHTA) - http://www.ahta.org/
Advancing the practice of horticulture as therapy to improve human well-being. The sole US organization that offers professional registration to horticultural therapists.
Horticultural Therapy Institute - http://www.htinstitute.org/
A small non-profit organization that focuses its efforts and resources on education, reaching students from across the U.S. and internationally.