Extension Educator, Horticulture
What they say, "When do I prune (fill in the blank)?" What I hear, "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". Many plants can be pruned in the spring; however, if flowers are the goal we need to step away from the shears a moment to determine how the plant grows and if it produces flowers on old wood (last year's stems) or new wood (this season's new stems).
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 are sort of pruning fool-proof such as 'Endless Summer' hydrangeas and reblooming roses. They bloom on new and old wood so no matter when you prune they still flower.
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 summer blooming plants can be pruned now.
When to prune clematis is a bit of a mystery due to the many different types of clematis. In the 1950's a somewhat loose grouping was put together to help explain pruning and flowering periods.
They are designated as A, B, C or 1, 2, 3, or little pruning, half pruning or hard pruning, depending on the author. Usually the plant label will list the pruning category.
Group 1 or A is the early flowering species that bloom in late April to late May and generally require little pruning. These plants flower on "old wood" or last season's stems. For these varieties spring pruning should be reserved to removing only dead stems. If necessary they can be lightly pruned again right after blooming.
Group 1 includes Clematis alpina 'Constance' and 'Pamela Jackman', Clematis macropetala 'Lagoon', and Clematis montana 'Elizabeth'.
Group 2 or B 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 flower on both old and new wood. Prune lightly in spring when buds begin to swell, removing dead and weak stems and reducing size if needed. The largest flowers will be produced on the old wood while new growth will provide bloom for late season. Group 2 can be pruned again immediately after flowering if needed.
Group 2 clematis include: 'The President', 'Vino', 'Anne-Louise', 'Arctic Queen', 'Bees Jubilee', 'Crystal Fountain', and 'Rosemoor'.
Group 3 or C clematis are the late large flowered cultivars and other late blooming clematis species. They are the easiest to prune but also the group in greatest need of a hard annual prune. These vigorous vines bloom on new wood so it's hard to go wrong. Cut to a pair of healthy strong buds at the base of the plant in spring as the buds swell. If these go unpruned, the flowers are produced way up at the top of the plant exposing their bony legs. Group 3 also includes some of the non-vining clematis such as Clematis durandii.
Group 3 includes: 'Comtesse de Bouchaud', 'Rouge Cardinal', 'Duchess of Albany', C. tangutica, C. viticella cultivars such as 'Etoile Violette','Polish Spirit' and 'Madame Julia Correvon'. The poster child for this group is the sweet smelling, late blooming Sweet Autumn clematis. Its rampant growth requires a heavy spring prune.
The good news is incorrectly pruned clematis usually don't die from pruning mistakes. Worst case scenario is flowers are delayed until later in the season or until next year.
Check out Chicago Botanic Garden's publication "Clematis for Northern Landscapes" at http://www.chicagobotanic.org/research/plant_evaluation/index.php or stop by our office for a copy.