I find that it's important to keep the iris free from fallen leaves and too much mulch to keep the rhizomes from rotting. Prior to application of mulch I cut back the foliage. In the winter some mulch of pine needles helps with soil acidity but I remove it asap in the spring while the ground is still frozen and well before growth starts to appear. Problem with mulching include rodent / insect damage and "mushiness" in spring. This can cause loss of bloom from well developed plants of certain varieties.