Prune your clematis based on whether it flowers on last year’s wood, this year’s wood, or both. Experts divide the genus into three groups: Group I are all the early spring-flowering evergreen clematis and early- and mid-flowering species. This group includes Clematis alpina, C. armandii, C. macropetala, and C. montana. These clematis flower on last year’s wood and should be pruned after the flowers fade but no later than July. The only pruning really needed is to remove weak or dead stems and whatever is needed to confine the plant to its allotted space.
Group II consists of clematis that also flower on last year’s growth, but will produce a second flush of bloom on new growth. Here is included mid-season large-flowered cultivars such as ‘Bees Jubilee,’ C. ‘Henryi’, ‘Nelly Moser’, and ‘Vyvyan Pennell’. Remove all dead and weak stems in late winter or early spring, and cut the remaining stems back to a pair of strong buds which will produce the first blooms. Occasional pinching after flowering will stimulate branching.
Group III consists of late-flowering cultivars and species that flower on this season’s growth such as C. flammula, C. integrifolia, C. x jackmanii, C. viticella, C. tangutica, C. texensis, and the herbaceous species. These can also be pruned in late winter or early spring. For the first two or three years they may be cut back to a foot from the ground. Later cut them back to two feet. If not cut regularly this group can become very leggy and overgrown.
The boundaries between these three groups are not absolute. Certain Group III clematis, for example, can be treated as Group II to produce early blooms on the previous year’s wood, but they serve as a rough guide to safely keeping your clematis vines within bounds.