Cutting at Planting
Clematis needs well-drained soil and the top of the plant requires full sun. In a tricky twist of fate, the roots need to be in a shady to partially shady location. You can plant in spring but early September is better to allow the plant to establish and hopefully get some blooms the first year. Cut the plant back to 12 inches at planting. This prevents injury during the installation process, but more importantly, it encourages the plant to branch out as it grows.
The time the vine flowers determines when it is pruned. Early bloomers flower in spring and should be cut immediately after they have finished. They flower off the growth that is produced in that year, so early pruning gives new growth time to form and mature. Pruning too late could cause new growth that will be injured in the cold fall and winter temperatures. Very fast-growing vigorous plants may be cut all the way to the ground.
Some clematises flower in late spring to early summer and produce flowers all season until cold temperatures arrive. These types of vines should be thinned and untangled in late fall or late winter. After the first flush of flowers, prune back the branches that produced those flowers. You could also prune the entire plant back by half in late winter every other year, which produces a well-maintained clematis but will sacrifice the first blooms in alternate years.
Late-blooming clematises may flower in summer or even fall. They bloom off growth produced that season, which means you can cut the plant back in early spring before growth begins. Cut the stems all the way to within a foot of the ground. Make the cuts 1/4 inch before a strong growth node and angle the cut so water is deflected from the bud. If you don't mind how high your clematis is climbing, you may also do nothing. It will not affect the number of blooms, they will just be higher on the vine.