Answer: Iris are traditionally grown alone in their own bed or are left a wide berth in the perennial or mixed border so that their rhizomes can enjoy sunshine and good air circulation. This is important because they are prone to rot. Poor drainage or an overlay of foliage from neighboring plants or a layer of mulch on top of them can all hold moisture close to the rhizomes. It is fine to mulch around them, just not overtop the rhizomes themselves. If you plant annuals in between them, make sure there is enough room that they do not overshadow or flop or spread out on top of the rhizomes.
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