Karl Foord, University of Minnesota Extension Educator
Clematis is a genus that is best known for its vining members that produce large, colorful, showy flowers. This is at best only half of the truth. In fact many of the cultivars do produce spectacular flowers with colors from almost all 360 degrees in the gardener’s color wheel. In addition there are varieties with smaller nodding flowers that add a certain delicacy to the garden as well as some herbaceous types that are more shrub-like and die back to the ground each year. Clematis require a certain effort to make them thrive but it is well worth the effort.
The dizzying array of cultivars can be intimidating, but this can be simplified by categorizing plant types by when they set their flower buds. This will then determine when they will flower and how they should be pruned. As such, the categories are often labeled as pruning groups.
In this group flower buds are initiated on this year’s vine in July and then produce flowers in the late spring of the following year. If you prune off old wood you also prune off flower buds. So if you have a clematis vine and do not know the variety, observe its time of flowering. Those that flower before early summer are likely in this group. Pruning of this type should only serve to maintain the framework. Do so only after flowering and before July. Common species in this group include Clematis alpina and C. macropetala and are characterized by smaller 1” to 3” nodding flowers. Clematis alpina ‘Pamela Jackman’ and C. macropetala ‘Markhams Pink’ are cultivars of these groups that flower in May and thereafter produce attractive seed heads. Simple Rule: “If a clematis flowers before early summer, do not prune it.”(1)
Groups B1 & B2
The B1 group flowers in early summer (May-June) from buds initiated the previous summer and in late summer (September) from buds initiated on the current year’s growth. The late summer flush is typically smaller than the early summer flush. Cultivars showing this pattern of flowering are ‘Haku Ookan’, ‘Miss Bateman’, ‘Lincoln Star’, and ‘Nelly Moser’. Interestingly, the cultivars ‘Belle of Woking’ and ‘Daniel Deronda’ produce double and semi-double flowers, respectively from old wood and single flowers from new wood.
Remove dead and weak stems in late spring prune after first flush of growth. Simple Rule: “Do not indulge in large-scale pruning of old wood made during the previous season(s) or there will be a loss of early flowers.”(2)
The B2 group shows a continuous flowering pattern lasting from June to September. Flower buds were initiated the previous year (old wood) and in the current year (new wood) in the same manner as the B1 group. However, this group does not demonstrate a rest period between two growth flushes. Cultivars showing this flowering pattern are ‘The President’ and ‘General Sikorski’ (photo).
Group C plants only initiate flower buds on the current year’s growth. This pushes the flowering time to later in the season- July through September. The goal in pruning this group is to eliminate all of last year’s growth and leave the lowest pair of live buds on the plant to begin the current season’s growth. This encourages plants to produce strong new shoots from the base and flower well. Typical of this flowering pattern are the Tangutica, Texensis and Viticella groups and cultivars ‘Madame Julia Correvon’ (photos). Clematis integrifolia is a group C plant with a bush habit that only reaches two to three feet in height (photo).
I recommend that you consider clematis for your garden or if you have some already, consider adding more as part of you next gardening adventure. Always provide a climbing surface for the climbing clematis forms and secure stems to the surface. If this is not done winds can buffet and break the vines leading to a very unsatisfactory outcome.
*The A, B1 & B2, and C groups are often named 1, 2, and 3, respectively by some authors.
- Mary Toomey & Everett Leeds, An Illustrated Encyclopedia of Clematis, Timber Press, Portland, Oregon, 2001.
- Fred Wein et. al., The Concise Guide to Clematis in North America, Clearview Horticultural Products Inc., Home of ClematisTM