How to Fertilize white flag iris?
Diluted iris-specific feed every four weeks spring through summer

white flag iris · Iris albicans
Grow in well-drained, fertile, neutral to slightly acidic soil in full sun. Do not cover the rhizomes with mulch, or allow other plants to shade them; sun on the rhizomes encourages flowering. Give a high potash feed in spring and divide every three to four years. See also bearded iris cultivation
Diluted iris-specific feed every four weeks spring through summer