For new or replanted beds (to which commercial fertilizer and organic material have been added at planting time), a light dressing of a complete fertilizer (2-4 lbs. Per 100 square feet) is sufficient about two months prior to bloom (late January in Louisiana). Beds which have not been replanted in late summer are generally given two applications, a fairly heavy application at the start of the growing season (with good watering in), and then the light dressing just before bloom. Complete fertilizers such as 8-8-8 are preferable to high nitrogen fertilizers. High nitrogen can cause the plants to produce leafy growth while suppressing flowering; it can also make the plants more susceptible to disease.
~The above information adapted from The Society for Louisiana Iris website