See: Howells, J. The Rose and The Clematis as Good Companions. P.168
THE 'SEVERE' PRUNING CLASS
Six clematis groups are in this class. They all flower after June. Firstly there is the Late Large Flowered Group containing plants such as 'Jackmanii', 'Comtesse de Bouchaud', 'Victoria', 'Hagley Hybrid', 'Gipsy Queen', etc. The second group is the trouble-free Viticella Group containing such plants as 'Madame Julia Correvon', 'Ville de Lyon', 'Etoile Violette', 'Abundance', 'Kermesina', etc. The third group, the Herbaceous Group, contains all the integrifolias and heraclefolias. The fourth group, the Texensis Group, contains tulip like flowers such as 'Princess Diana', 'Gravetye Beauty', 'Sir Trevor Lawrence'. The fifth group is the Orientalis Group with its yellow flowers such as 'Bill Mackenzie', the tanguticas, serratifolia, rehederiana. The sixth group are the Late Mixed Group containing such plants as C. flammula, C. vitalba, C. terniflora and C. fargesi.
This used to be a troublesome group to prune. I have now simplified it. It's so very simple. You just cut all stems at ground level. I was very slow to appreciate this was the way to do it. Indeed the plants told me because left to themselves they withered each year to the ground.
When do you cut to the ground? In February-March time. Gently scratch the ground and you will find the new shoots just waiting to spring up. There is no virtue in pruning too early as you do not want to subject these tender new shoots to hard late frosts. Also they are all strong growing plants making a lot of quick growth and, of course, they won't be flowering until late in the season.
There is one modification you may care to use. Many plants in this class are climbing up shrubs. The clematis can be unsightly with their black leaves. So in the autumn prune to 1m (3ft), bring the stems together with a tie and now you can tuck them away out of sight. The stems will still give some protection to the crown of your plant. Complete the pruning in late winter.
Yet another modification can be useful if your plant has a long way to climb. Instead of pruning to the ground, prune 1m (3ft) from the ground every year. The plant will develop strongly woody stems in its bottom 1m (3ft). You have now given the plant a lift to help it on its way upwards. It's cleverly done on the tall pergola in the Rose Society's Garden of the Rose at St. Albans, U.K.
See: Howells, J. 1999. Garden News. Jan.13.