To prune clematis plants, first determine what type of plant it is, prune it at the right time and prune it regularly. A clematis that isn't managed regularly results in masses of old stems and few flowers.
The when and how of pruning clematis depends on the cultivar. Prune spring bloomers after flowering in the spring, summer and fall bloomers in the early spring or after they flower in the fall, and repeat bloomers after their biggest blooming period of the growing season.
When pruning a spring bloomer, determine if it is vigorous or slow-growing and if it blossoms on growth from the previous year. If so, trim only stems on new wood, avoiding old wood that doesn't produce new growth very quickly or at all; however, you can cut C. armandii or other vigorous varieties to the ground every year.
Prune repeat bloomers -- those plants that bloom heavily once a year and then sporadically throughout the rest of the growing season -- by pruning them lightly and thinning them in both the early spring and early summer. Because summer or fall bloomers produce flowers on new growth, prune them before they bloom. A light pruning promotes longer, fuller blooming, but the plant may need a heavy pruning every few years to control its size.