This is the advice I found on the net:

"Little or no annual pruning is needed, other than the removal of frosted or diseased stems and thinning out congested growth to near ground level. Overly long or wayward shoots can be shortened at the same time. Neglected plants will respond well to hard pruning and so can have all stems cut back to near ground level. If this is too drastic for you, cut out one-in-three stems, starting with the oldest instead".

This needs doing in March.

I hope that helps.

I have 2, although they do not require pruning? it is best to keep them under control as they are fast growers and spread wide/fast especially if cut right back. Also I found if not pruned eventually they do not flower as much and for as long. If left unpruned they can also grow so fast that the weight is not supported and they tear off leaving a damaged area.