Westerplatte Clematis                          (Photo by Carlotta Lucas)

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