Do any colleges or universities in the Seattle area have programs on horticultural therapy?
At one time, Edmonds Community College offered a certificate in therapeutic horticulture but they do not any longer. You can find other programs available around the U.S. on the website of the American Horticultural Therapy Association.
The University of Washington doesn’t offer a horticultural therapy program, though there may be individual courses through the department of Landscape Architecture. Professor Daniel Winterbottom specializes in “ecological urban design and the role of restorative/healing landscapes in the built environment.” The Miller Library has a good selection of books and articles on the horticultural therapy, including information about making raised beds, appropriate tools, gardening for the differently abled, and the like. This booklist will give you an idea of the resources available. For information about library hours and directions, go here.