Most campanula varieties should be pruned in early spring, at the start of the growing season, according to White Flower Farm. New growth that appears at the end of the season should be left in place to protect the plant during the winter.
One exception to this pruning rule is the campanula lactiflora, which is also known as a bellflower. This variety benefits from pruning by a third after its first flowering, which produces new growth for the later season. The bellflowers can also be cut back to 3 inches above the ground in the fall, according to White Flower Farm.