Extension Educator, Horticulture
A common question in our U of I Extension office is "When do I prune (fill in the blank)?" Or as I read between the lines, the question is "I have this plant and I can't stand the way it looks. Can I hack it back now?" The old adage is you prune when your pruners are sharp. In general, it's best to prune after - not before - stressful environmental conditions such as heat, drought and cold. For flowering plants, however, we need to think beyond just plant health if we want to maintain good flowering.
Pruning flowering plants depends on how the plant grows and if it produces flowers on old or new wood. As a general rule plants that bloom before June 15 bloom on old wood and plants that bloom after June 15 bloom on new wood. Some plants such as the new 'Endless Summer' hydrangeas bloom on new and old wood. Forsythia and lilac bloom in early spring then as the season progresses the next year's flower buds are produced on old stems. Pruning them now translates into no flowers next year.
For plants that bloom on old wood, it is best to prune right after they flower. For other plants, as a general rule, it is best to wait and do severe pruning just before the plant's active growth season. Therefore for summer blooming plants, prune in late February or early March when the severe part of winter has passed and spring is not too far away.
When to prune clematis seems to be a bit of a mystery, partly because there are many different types of clematis. In the 1950's a somewhat loose grouping was put together to help explain pruning and flowering periods.
Richard Hawke, director of the plant evaluations gardens at the Chicago Botanic Garden, did a nice job of outlining the groups in his clematis evaluation publication. For nice pictures and descriptions of clematis that received high marks in the evaluations, check out http://www.eplants.org/cgi-bin/plantsearch.cgi
Group 1 is the early flowering species that bloom in late April to late May. These include Clematis alpina 'Constance', Clematis alpina 'Pamela Jackman', Clematis macropetala 'Lagoon' (nice deep blue with double petals) and Clematis montana 'Elizabeth'. The Group 1 clematis bloom on last year's wood. These should be pruned after bloom, but only if necessary.
Group 2 clematis are early double and semi-double mid-season cultivars. They bloom mid to late May and if healthy will repeat bloom in September into October. These bloom on the new growth that is produced from old buds. If they are pruned to the ground, they will bloom but will miss the early flower period. These require minimal pruning to remove any dead stems in late March just as buds swell. These include 'The President' a nice rich purple or 'Vino' a deep red. 'Mrs. Cholmondeley' is light lavender blue and seems to tolerate more shade than other clematis.
Group 3 are the late large flowered cultivars and other clematis species. They flower in mid to late June into July. Clematis viticella cultivars are vigorous plants that bloom July through September. 'Madame Julia Correvon' is a rich red that blooms all summer. These bloom on new wood so you can't go wrong however you prune. Generally these should be cut to a living bud at about 18-24 inches from the ground in spring as the buds swell. If these are never pruned, the flowers are produced way up at the top of the plant leaving their skinny legs showing. Group 3 also includes some of the clematis that do not climb such as the indigo blue flowered Clematis 'Durandii'.
An amazing number of clematis are now available from vigorous house-eating ones to delicate ramblers for the flower border.