Pruning clematis always causes confusion and it seems that it always will. It is probably the most-asked pruning question but I think that is partly because clematis are so popular. In theory, clematis don’t need to be pruned and they will carry on flowering year after year with minimal attention. But we prune them to shape them, to get rid of dead and unproductive stems and to keep the flowers where we can see them, rather than leaving the flowers for the birds and the bees. So, assuming that we want to control them, we need to prune.
Part of the confusion comes from the way clematis are ‘grouped’ into 1, 2 and 3 or A, B and C or other classifications. These roughly translate to flowering time; 1 or A being those that bloom first, usually in April or May, 2 or B being the large-flowered, May-July flowering clematis and 3 or C being the late-flowering clematis that bloom from August-September-ish.
Those that flower in spring have produced their flower buds on all the growth that occurred last summer. These buds break and the flowers open and then they make more growth at the ends of these shoots over summer. They include the C. macropetala and C. alpina kinds as well as the popular C. montana. There are also the evergreen New Zealand hybrids such as ‘Early Sensation’ and the monstrous C. armandii.
These are pruned in spring, immediately the flowers drop. You can prune away most of the flowered stems if you want to keep the size of the plant constant. You can prune less hard if you are happy for the plant to increase in size. If you do not prune then the base of the plant will become bare, congested with dead growth, and the plant will potentially become large. We have all seen sheds ‘consumed’ by montana clematis. Do not prune after August (through till April) or you will cut off all the flowers for the following spring.
These are the large-flowered clematis that bloom around June. These flower on short shoots produced from the previous year’s growth rather than directly from these old shoots like Group 1. These are usually pruned in March, depending on the weather, just as the buds are breaking, and you tidy them, cutting off the ends of the previous year’s shoots back to a pair of fat buds. In theory you also cut out thin, weak growth but it can be tricky. Clematis shoots are very brittle and one of the most frustrating jobs in the garden is to try to untangle and sort clematis shoots only to break them or trip over them. If you do it every year it is manageable but if you have left it for a few years it can be a real mind-teaser.
If it all gets too much, or if the plant is very neglected, you can cut the whole ‘bird’s nest’ back to 30cm. You will forfeit flowers that summer but the plant will grow with renewed vigour and you may get a few flowers in late summer. And there will be lots the following year.
These are the late-flowering kinds that include the Viticellas and Texensis hybrids as well as the herbaceous kinds such as C. heracleifolia and hybrids including ‘Arabella’ and ‘Durandii’. Also the ‘orange peel’ kinds such as C. tangutica.
These bloom at the end of the new shoots so they are pruned hard, possibly back to 30cm, in March. The new shoots will then grow and flowers are produced at the end of the stems in late summer – think of buddleias. They are the easiest of all to prune and to control and it is these that I would recommend to interplant with other shrubs or climbers.
Training is also important after pruning to space out the shoots of all, so they are spread out rather than just make a column and flower in a bunch at the top.
Just to confuse things, plant breeders have introduced new styles such as the ‘Bijou’ range (top pic) for patio pots, that have moderately large flowers on compact plants, but have a much longer flowering time and can be pruned harder than the normal Group 2. If the flowering time of your plant is given as May to September then compromise with the pruning and prune in March by doing a moderately hard prune and tidy up.