Do you know of any correspondence courses concerning landscape design criticism and critique?
There is a book called North American Horticulture–A Reference Guide, compiled by the American Horticultural Society (second edition, 1992, edited by Thomas M. Barrett). The book includes a chapter on education, including correspondence courses.
Three community colleges in the Puget Sound area offer horticulture degrees, and they might know something about correspondence programs. They are Edmonds Community College Horticulture and South Seattle College Horticulture, and Lake Washington Institute of Technology.
More colleges and universities are offering online courses now. Here are two examples :