Clematis are perennial vines that add a colorful vertical element to gardens. Clematis in the Jackman group are prized for their large blooms. They are the largest and most widespread type of clematis in America, according to Ron Smith of North Dakota State University. There are three groups of clematis, divided based on the pruning method or time of year that pruning should be done. Clematis x jackmanii is considered a late-flowering cultivar and should be pruned accordingly.
Late-flowering clematis, like Clematis x jackmanii, should be pruned heavily in late winter or early spring. This group of clematis produces flowers on new wood from the current year’s growth. There is one main flush of growth, typically in July and August. To prune, the vines can be cut within 6 to 12 inches of the ground in late winter or early spring. Alternatively, wait until bud break later in the spring and prune back to the highest new growth.
The other two groups of clematis include clematis that blooms on the previous year’s growth or old wood and clematis that produces flowers on both old and new growth. Clematis in the first group bloom between April and June and should be pruned back to live wood only after bud break. Clematis that produce flowers on both old and new growth have two waves of blooming — one wave in May to July on growth from the previous season and another in late summer on the new growth. These clematis should be pruned in early spring to remove deadwood.
Although clematis do not have any serious insect pests, clematis wilt can be a problem on occasion. Affected clematis vines experience a sudden wilting and die. The dead vine should be pruned to ground level, removed and destroyed. The clematis will typically send up new, healthy shoots.
Clematis can be pruned back after planting to encourage growth. After planting, trim the small vine to about 12 inches above ground level or to a set of low buds that will force multiple branches and additional shoots.