The main purpose in pruning is to help plants produce the maximum number of flowers. Annual pruning is recommended. Sometimes older, neglected plants can be cut back into older wood and new buds may break. Growth from old wood will likely be weak and slow, however. If no pruning were done at all, plants would still grow and flower profusely, though not where you may want them to. Some flowering would occur high in the plant and out of sight. Not all clematis can be pruned in the same way. There are three methods that can be applied to major groups depending on the time of year the plant flowers. No new growth must occur to enable the earliest flowering clematis to bloom, but the later flowering types must make new growth in order for flower buds to form. A few plants are not strictly bound to the following groups but may cross lines. Because vines will likely be entangled, make cuts carefully among the intertwining vines and spread and train them in various directions in order to cover the maximum possible area. This enables the plant to display its blooms rather than be bunched up.
Plants in this group bloom in early spring, generally in April and May, from buds produced the previous season. Prune these back as soon as possible after bloom but no later than the end of July. This allows time for new growth to produce flower buds for the next season. Remove shoots that have bloomed. You can prune out more vines to reduce the size or to form a good framework of branches. Do not cut into woody trunks. Plants in this group include: C. alpina, C. macropetala, C. armandii, C. montana and C. chrysocoma.
Large-flowered hybrids bloom in mid-June on short stems from the previous season's growth and often again in late summer on new growth (these blooms are smaller). Prune in February or March by removing dead and weak stems, then cut back remaining stems to the topmost pair of large, plump green buds. This cut could be a few inches to a foot or two from the stem tips. Plants in this group have the tendency to become bare at the base as they mature. Underplant to help conceal the stems. You may be able to force a flush of new growth from the base by cutting the vine back to 18 inches immediately after the flush of bloom in June. Plants in this group include: 'Nelly Moser,' 'Miss Bateman,' 'Lasurstern,' 'Duchess of Edinburgh,' 'Mrs. Cholmondeley' and others.
Plants in this group flower on the last two to three feet of the current season's growth. Some types begin blooming in mid-June and continue into the fall. This is the easiest group to prune since no old wood needs to be maintained. In February or March cut each stem to a height of about two to three feet. This will include removal of some good stems and buds. Eventually the length of the bare stem at the base will increase as the vine matures. Plants in this group include: C. viticella, C. flammula, C. tangutica, C. x jackmanii, C. maximowicziana, 'Perle d'Azur,' 'Royal Velours,' 'Duchess of Albany' and others.
Credit:Information from Ohio Stae Online Search at http://ohioline.osu.edu/cgibin/htsearch