Clematis species and varieties fall into three groups and the group tells you how and when to prune them. The label should tell you which group your variety falls into or you can search online.

Caption: Clematis like 'Bourbon' (group 2) are a big and beautiful group of plants

SPRING-FLOWERING CLEMATIS (GROUP 1)
When: April to May, once they’ve finished blooming.

What to do: Little pruning is needed. To keep the plant closely hugging its support, cut it back with shears.

Why you prune this way: Stops the plant turning into a mass of tangled stems. This group of clematis flowers best on growth produced the previous year, so pruning later in the year would prevent the plant flowering the following year.

Examples of species and varieties in this group: C. alpina; C. armandii; C. cirrhosa; C. macropetala; C. montana

LARGE-FLOWERED CLEMATIS THAT BLOOM IN MAY AND JUNE (GROUP 2)
When: January to February.
What to do: Prune back the top of each stem to a pair of healthy buds.

Why you prune this way: To tidy the plant and remove any weak growth. The pruning should be kept light, otherwise you run the risk of removing the flower buds. 

Examples of species and varieties in this group: 'Beauty of Worcester'; 'Bourbon'; 'Nelly Moser'; 'Rebecca'; 'Royal Velvet'; 'The President'

LATE-FLOWERING CLEMATIS (GROUP 3)

When: February to March.
What to do: Cut back all the stems to healthy buds, 15-30cm above soil level.
Why you prune this way: Left unpruned, these types of clematis quickly become a tangled mess. They flower on stems produced in the current year, so pruning will encourage lots
of vigorous growth that will flower well.

Examples of species and varieties in this group: 'Abundance'; 'Etoile Violette'; 'Jackmanii'; 'Princess Diana'; 'Royal Velours'

Caption: Group three clematis should have all their stems cut back to 15-30cm above ground level