Clematis should only be pruned once during the year. Winter and early flowering ones including the C. cirrhosas and C. montanas (Group 1) don't need any pruning unless they're outgrowing their allotted space, then trim AFTER they've flowered.

Group 2 Clematis only need a trim, as well, in early spring. They're the ones that flower twice, in general, in May/June and September - like 'Nelly Moser'.

Group 3 - the later flowering ones, including the viticella and texensis groups need to be hard pruned in early spring - back to a healthy pair of buds about a foot up from the base.