I've been thinking I'll go back to mulching with bark mulch again. I had always mulched until I found out bearded irises perform better without it (those I already had performed well, but I wanted to baby the newer varieties). Several of my news irises went into a new bed that is mulched, and those look the happiest of all the new additions. I think the moisture retention and weed prevention is making the difference here. I've always been careful to keep mulch off the rhizomes too.
