In the garden, Bearded Irises are heavy feeders and need regular fertilization to bloom strongly and multiply. I recommend top dressing with a mix of high-quality compost, Yum Yum Mix, and some soft rock phosphate or bat guano in the spring, and again about a month after they have bloomed. Just scatter several cups of this soil-building mix in a ring around the clumps. Avoid high nitrogen chemical fertilizers, as it degrades the soil over time and promotes leaf growth at the expense of flowering.
In terms of maintenance, the best time to divide your Iris clumps is in August, every 3-5 years. When they stop blooming or the flowers are sparse, this is an indication that they are crowded and the soil is low in nutrients and needs to be enriched.
Just dig the plants out of the ground, shake off the soil, and cut the rhizomes (fleshy roots) into pieces with a sharp clean garden knife. Be sure to include a fan or two of leaves on each piece of rhizome. If you have some dusting sulfur, dip the cut ends into it and let the rhizomes dry for a day or two in the shade so the cuts callus over. Replant as outlined above.