How to Prune Canterbury bells?
Do not cut back after flowering if self-seeding is desired. However, removing of the spent flower spikes may encourage second flush of flowers

Canterbury bells · Campanula medium
Also known as: Canterbury bell; Coventry bells; Coventry rapes; cup and saucer; fair-in-sight; gingerbread bells; mariettes; Mercury's violets; St Thomas's bell; lady's nightcap
It prefers fertile, moist but well-drained soil. It will tolerate wide range of soil types if well-drained. Grow in sun or partial shade. Suitable for container cultivation
Do not cut back after flowering if self-seeding is desired. However, removing of the spent flower spikes may encourage second flush of flowers