The California Garden and Landscape History Society is "dedicated to celebrating the beauty and diversity of California’s gardens and cultural landscapes."
The New England Garden History Society published a journal from 1991 to 2003, indexed here. Alas, it now appear to be defunct.
Do please let me know of other organizations I've missed and I'll be glad to pass them on.
[Above is the Bellwood Plantation, Upson County, Georgia, drawn in pastel by Loula Kendall ca. 1850. Source is the Southern Garden History-Cherokee Garden Library, housed at the Atlanta History Center. Unfortunately, their wonderful collection is not available online. Somebody give them a grant!]