That depends on what type of Clematis it is. Clematis vines are divided into three groups, depending on the plant's growth habits. Group A is comprised of the early-flowering species, Group B contains the early to mid-season blooming large flowered cultivars, and Group C includes the late-flowering cultivars and species. So the trick to knowing when to prune your Clematis is to know which group your Clematis belongs to.
The ever-popular Clematis 'Nelly Moser', with its large pink and white open-faced flowers, is an example of a Group B Clematis. Because 'Nelly Moser', like all Group B Clematis, blooms in late spring and early summer on shoots from the previous year's growth, it needs a light pruning in early spring before growth begins to keep it vigorous. Remove all dead and damaged shoots first. Then prune each stem back to a point where a strong pair of buds are showing.
Clematis 'Jackmanii' is also a favourite with many. Who can resist its large, velvety purple flowers? However, pruning 'Jackmanii' is a very different proposition than pruning C. 'Nelly Moser', because C. 'Jackmanii' is a Group C Clematis, which blooms mid through late summer. Like all the Group C Clematis, it needs to be pruned severely in early spring before growth begins. Cut back all its stems to a pair of strong buds six to eight inches (15 to 20 cm) above the level of the soil.
If your Clematis is a member of Group A, however, such as C. alpina, which sports such lovely blueish-purple bells, or C. armandii, which bears saucer-shaped white flowers, you shouldn't be pruning it in the early spring at all. Instead, you need to prune Group A Clematis immediately after flowering, removing all damaged or broken stems and basically giving them a cosmetic going over rather than a severe pruning.
If you buy a Clematis from a B.C. nursery, it will have a care tag that tells you which group it belongs to, simplifying your pruning task. If you have an unidentified Clematis in your garden, your best bet is to wait until it blooms. Then you'll be able to make an educated guess about when and how to prune it.
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