How to Prune mullein 'Rosie'?
Deadhead the flowering stem

mullein 'Rosie' · Verbascum 'Rosie'
Grow in full sun in light, non-acidic, well-drained soil: will tolerate shallow chalky soils, dry soils, and poor soils, but will need staking in richer soils, and will not tolerate wet soils
Deadhead the flowering stem