There are numerous clematis species, hybrids and cultivars, but for pruning purposes they are split into three distinct pruning groups based on the time of flowering and the age of the flowering wood.

Clematis in group three flower in late summer on growth made in that season. They make new growth from the base each year, so can be cut back hard on a regular basis.

Examples of clematis in pruning group three include:

'Abundance' AGM
'Alionushka' AGM
'Étiole Violette' AGM
'Ernest Markham' AGM
'Duchess of Albany'
'Gravetye Beauty'
'Lady Betty Balfour'
'Polish Spirit' AGM
'Princess Diana' AGM
'Royal Velours' AGM
'Ville de Lyon'
'Sir Trevor Lawrence'
C. × triternata 'Rubromarginata' AGM
C. viticella
C. viticella 'Purpurea Plena Elegans'

Prune in late winter or early spring when buds show signs of growth – typically in February.

Initial pruning and training

If young clematis plants are left unpruned they often produce very long single stems with the flowers produced only at the very top.

Unless the plant already has three or four healthy stems growing from the base, all newly planted clematis should be pruned back hard the first spring after planting. Cut back to just above a strong pair of leaf buds about 30cm (1ft) above soil level. This will encourage multiple stems which can be trained in to supports to give a good coverage.

During the spring and summer, tie in new growth, spacing stems evenly on the support.

Pruning established plants

• In February or March, cut back all the old stems to the lowest pair of healthy buds 15-30cm (6in-1ft) above soil level
• Small-flowered clematis with attractive seed heads (such as C. 'Bill MacKenzie', C. 'Helios', C. orientalis, C. tangutica and C. tibetana subsp. vernayi) can just be thinned out and trimmed back to the main framework of branches, leaving the seedheads to be enjoyed

If left unpruned, this group will continue growing from where the growth ended the previous season, becoming top heavy, flowering well above eye level, and with a bare base. 
 
If desired, they can be left unpruned to scramble over pergolas where space is not limited.

Some mid- to late summer flowering clematis can be treated as either pruning group two or pruning group three, as desired. Examples include:

'Comtesse de Bouchaud' AGM
'Gipsy Queen' AGM
'Hagley Hybrid'
'Jackmanii' AGM
'Jackmanii Superba'
'John Huxtable' AGM
'Perle d'Azur' AGM
'Rouge Cardinal'
'Star of India'

Herbaceous clematis

Prune herbaceous clematis such as C. heracleifolia, C. integrifolia and C. recta to near ground level in early spring or late autumn.

Links

Clematis pruning: group one
Clematis pruning: group two