Campanula persicifolia

Hi gardeners,

I was given several Campanula persicifolia plants (all growing very well) and told to cut off the spent summer blooms to encourage a second flowering.

QUESTION: Do you cut off just the flowers on the long stalks or should the whole long stalk be cut off?

On my Campanula mediums I cut off only the faded flower heads and always get more but the C. persicifolia grow differently.

Many thanks in advance
B