Contrary..It only stands to reason to dead head any spent flower..because the plant will then start to focusing on producting seed heads if you don't deadhead...If you don't plan on gathering the seeds to plant then don't let your plant waste its energy producing seedheads when it could be focusing on growing and producing more flowers...Your Clematis "Prince Charles" is a pruning group 3 which means it will only flower on the new vines produced in a growing season..I see you are in zone 7 and IF if were me..I would hard prune and fertilize him and he will grow back up and flower again for you...this will also make him grow a stronger root system...I just finished hard pruning several of my pruning group 3's because their first flush of flowers are over....Hope this helps...Jeanne

Thank you, Jeanne, that helps a lot. I certainly don't want seeds, so I'll do as you suggest and hard prune it.

Mary