Two or more years of this treatment may be necessary for the plant to develop a satisfactory framework, but the rewards in terms of future flowering and general appearance are well worth it. During this time flowering is not sacrificed, rather delayed until later in the season. Late season pruning of clematis is not recommended in areas which experience cold winters. Any unexpected warm spell will encourage new growth which will certainly be killed by the cold spell that follows. H.F. Young, Mrs. Cholmondeley, etc.
Light pruning to remove any dead bits and neaten it up is all that's needed.
If after a hard winter some stems appear dead, wait until the leaves are emerging on the rest of the plant and then prune slowly in sections, one stem at a time, from the top so as to avoid the premature removal of what is actually live wood. If you have a Montana which has outgrown its allotted space and is in a terrible tangle, you can control it by pruning hard immediately after flowering. This will allow plenty of time for it to put out new growth which will in turn flower next spring. Do wear gloves when pruning clematis as the brittle wood splinters easily under attack. Upon concluding a pruning session it is a good idea to feed with a good all-purpose fertilizer and lime as needed.