The RHS is the International Registration Authority for the genus Clematis.
Registration
Download registration forms here:
Application form to register a Clematis cultivar name
Application form to register a Clematis cultivar name in German
Publications
The RHS publishes The International Clematis Register and Checklist 2002 and subsequent supplements, some of which are available as downloads from the link below.
Clematis registers
Further information
Evison, R. 1985. May Large-flowered Clematis cultivars. The Garden 110(5): 207-213.
Fretwell, B. 1989. Clematis. Deer Park, Wisconsin: Capability's Books.
Lloyd, C. 1989. Clematis. Deer Park, Wisconsin: Capability's Books.
Rogerson, B. 1985. Clematis viticella and its progeny. Pacific Horticulture 46(1): 40-44.