Not all Clematis are the same!
When is the best time to prune Clematis? This climbing vine can be separated into three pruning groups.
Group 1: Early flowering cultivars
These cultivars flower in the spring, only on growth produced the previous season. They generally do not need to be pruned at all, but if you feel it is necessary to keep them in bounds, trim them back within one month after they are finished blooming. Old, woody plants can be pruned very hard at this same time, cutting them back to 1-2 feet. This will remove the old, non-flowering growth. It may take a season or two for it to start flowering again.
Group 2: Early to mid-season flowering cultivars and rebloomers
These varieties begin to bloom in early summer, with the first flush of flowers appearing on the previous year's growth. This is followed by a second flush of flowers appearing on the new growth. These flowers will likely be smaller than the first ones and flowers that were double the first time may be single the second time. The time to prune this group is in late winter or early spring. Dead and weak shoots should be removed and the remaining shoots cut back to 6-9 inches above a couple of well-developed buds.
Group 3: Late flowering cultivars
These varieties bloom first on new growth beginning in midsummer and then again on new shoots in early fall. The second round will likely produces smaller flowers than the first and flowers that were double the first time may be single the second time. Though their growth habit makes them difficult to prune, it can be done in late winter or early spring. All shoots can be cut back to the previous year's wood, just above the base of the plant. This pruning is necessary to avoid getting a bare stem with a thick tangle above it. The spring flowers will be eliminated for that year, but the late summer flowers should still be produced. Alternatively, if you don't want to cut the entire plant back all at once, pruning can be done in stages over a period of 3 years. Each year, prune back 1/3 of the stems to 6-9 inches above a couple of well-developed buds.
Small-flowered clematis do not need to be pruned regularly. Only when they grow too large for their space should they be cut back.