Mulching is usually a great organic method of weed control, but it can be devastating to your iris plants. Covering the rhizomes—the stalk of the iris—with organic matter can foster bacteria and will attract the destructive iris borer caterpillar.
Susceptible to Rotting
The iris is generally a robust plant that will even spring up from discarded trimmings in your compost. However, once infected, bacteria will quickly cause devastating rot and can even spread from iris to iris through the root systems. The rhizomes of the iris are particularly susceptible to rotting when covered with the moist organic matter. Keep this in mind during planting, as planting too deeply will have the same harmful effect.
In addition to creating a moist, decaying environment that the rhizomes can not survive, mulch also attracts the iris borer caterpillar. Iris borers are one of the few pests that bother the otherwise resistant irises. Bearded types of irises are at risk to damage from these pests.
Moths lay eggs in the mulch. In the spring, the larvae then chew down to the roots of your irises. The damaged rhizomes are then very susceptible to the bacterial infection that causes soft rot. For these reasons, plant the iris with the stalks completely above the soil, remove debris and do not use mulch.