I'm not sure what I do is a good guide as I've only been growing for a few years. Per Polly's recommendation I fertilized in September when they're setting flower buds. I used a balanced time release, heavily. In early spring I pulled mulch back and applied composted cow manure, heavily, then re-covered w/mulch. Balanced fertilizer, per recommendations, early June. Being in New England I have acid soil and our Spring was fairly wet so supplemental watering was minimal, but more than 1" per week.
Edit: Check out "CULTURE FOR JAPANESE, BOG, WATER AND SPECIES BEARDLESS IRISES"
Irises (Iris)
