Whether rambling through rose bushes or billowing over mailboxes, clematis can be a show-stopper when in bloom. You may struggle with the proper time to prune clematis, as seemingly similar vines need pruning at different times. Clematis cultivars have been divided into three classes for proper pruning season. The class designation is usually indicated on the plant label.
Group 1: Blooms On Old Wood
Clematis vines that bloom only on old wood belong to Group 1, which is sometimes referred to as Group A. These earliest-blooming clematis vines bloom in the spring and should be pruned after flowering and no later than July, according to Ohio State University. This class needs minimal pruning, to shape the plant or control size.
Varieties in Group 1 include White Swan, with 2-inch, white blooms, and Pamela Jackman, an 8-foot vine with azure-blue flowers, from the alpina species. Other cultivars in this group include Lagoon, of the macropetala species, with deep blue, doubled flowers, and Elizabeth, a vigorous vine with fragrant, light pink flowers from the Montana species.
Group 2: Blooms On Both Old And New Wood
Group 2, or B, clematis bloom on both old and new wood. These vines bloom twice, first in late spring or early summer, then again in late summer or fall. They require only minimal pruning to remove dead stems in early spring. If the vines have become leggy, though, cut them back to 18 inches from the ground in early spring to rejuvenate the plants; you will lose the first flush of flowers but the plants will bloom in the fall.
Cultivars belonging to this group include Mrs. Cholmondeley, a reliable bloomer with light lavender flowers, and Nelly Moser, with 8-inch wide, pink flowers with a darker midrib. The President has 5- to 7-inch, purple-blue flowers and grows to 8 feet tall.
Group 3: Blooms On New Wood
Group 3, or C, clematis are the easiest to prune, according to the University of Nebraska. Since all flowers are produced on new growth, prune all vines back to 6 to 12 inches from the base of the plant in late winter or early spring.
Sweet Autumn clematis belongs to this group of plants; these rampant vines are smothered in sweetly scented white flowers in fall. C. jackmanii, popular with gardeners since its introduction in 1862, has bluish-purple flowers. Other reliable clematis in this group include Madame Julia Correvon, with red flowers, and the golden clematis, C. tangutica, a 10- to 15-foot vine covered in yellow flowers in late summer to fall; the seed heads last until winter.