When to prune a clematis.
Pruning group A (or 1)
“A” is for After bloom
When and how to prune: Don’t prune until after the flowers are finished. Flower buds were formed the previous year, so pruning before they flower means no flowers that year.
Includes: Species that bloom in early spring, such as C. montana, C. armandii, or C. macropetala
Pruning Group B (or 2)
“B” is for Before bloom
When and how to prune: In spring, cut back to a set of live buds, about a third down from the top. Hard-prune (to about 12 inches) for the first two years after planting to develop a strong root system.
Includes: Species that bloom in late spring/early summer, including most large-flowering types.
Pruning Group C (or 3)
“C” is for Cut back hard
When and how to prune: In early spring, cut every stem to 12 to 18 inches or so.
Includes: Species that bloom in summer/early fall: C. viticella, C. tangutica, C. virginiana, C. texensis, C. crispa
Source:
Organic Gardening
Article By Therese Ciesinski
http://www.organicgardening.com/learn-and-grow/pruning-clematis