How to Prune iris 'Supreme Sultan'?
Cut down old flower stems after flowering, and remove any dying foliage in autumn

iris 'Supreme Sultan' · Iris 'Supreme Sultan'
Plant in full sun in well-drained soil, and with the top surfaces of the rhizomes exposed so that they get baked in summer sun, which encourages flowering. See bearded iris cultivation
Cut down old flower stems after flowering, and remove any dying foliage in autumn