I think that's 12" off the pot too make it bush out but that's not set in cement . it depends on what you like . if it gets too big in the pot it will need a bigger one . some one who knows will advise you because an expert im not .

Your Clematis is a named alpina and in Group 1 there is no need to prune this but if you do it should be immediately after flowering in mid to late spring. As yours is a new clematis you might want to follow the RGS advice:

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 (12in) above soil level. This will encourage multiple stems which can be trained to supports to give good coverage.

As the clematis is in a pot make sure it has a cool root run and keep moist in summer. A clematis will do better in the ground rather than in a container.

alpina Pamela Jackman

Latin Name
alpina
Colour
Purple/Blue
Pruning
No Pruning - Group 1
Aspect
Any Aspect
Height
6 - 8ft (2 - 2.5m)
Flowering
March and April
Aspect
Will Tolerate shade
Species
Clematis - Alpina

Thanks Moon growe this was on the the page of the company I bought it from it also said it was suitable for containers and had a garden merit award so I asume the flowers will still come back on those stems on my trellis ? I chose it as it sounded robust but thought it was looking a little bit sad at the moment and wondered if I had mistreated it .

My clematis look sad right now Kidsgran but will leaf up and flower come spring. All the Award of Garden Merit means is that one of the RHS plant committees thinks it is worth growing to be honest.