So, you’ve bought a clematis, and you’re confused about how to prune it? Newly planted clematis vines should be pruned to the lowest pair of strong buds after their first winter (In about march) to promote a thicker and more vigorous plant. Subsequent pruning is based on when that particular species flowers, starting the following year. Doing this helps you to train the plant into the shape you want to take. Look at the plant’s tag; it should tell you which pruning group it is a member of.
Pruning Group ‘A’
These types of vines flower on wood from the previous year in early spring. Only prune weak, spindly growth after they bloom. Excessive pruning will delay flowering for one growing season.
examples: C. montana, C. armandii, or C. macropetala
Pruning Group ‘B’
Members of this group bloom on both current and old growth from summer to fall, either in two showings or one continuous one. Pruning this group takes a bit of finesse but can be done if you pay attention to blooming times throughout the season. Stems that bloom late should be hard pruned, about a foot from the ground. Stems that bloom early should be dead-headed after the flowers fade, but not cut back, as they will produce buds for the following season. Some gardeners will protect the old wood by laying it on the ground over the winter.
examples: Clematis ‘Nelly Moser’, ‘Henry’, “William Kennett’
Pruning Group ‘C’
Vines in group C bloom on current growth only. In early spring, cut every stem to 12 to 18 inches or so to keep these prolific vines under control.
examples: C. viticella, C. tangutica, C. virginiana, C. texensis, C. crispa
Sources: Classic Climbers, and
Woody Ornamentals for the Prairies by Hugh Knowles