Bearded irises (Iris germanica), which earn their name from the small fuzzy patch at the base of each flower petal, grow in U.S. Department of Agriculture plant hardiness zones 3 through 9. Like most iris varieties, bearded irises grow best in dry, well-drained soil and rarely require irrigation. Mulch can benefit the plants if it's used properly so that it doesn't retain too much soil moisture.
Bearded irises prefer dry soil. Mulch will conserve soil moisture, which can lead to rot. The mulch also can harbor insect pests and disease organisms, so it shouldn't be allowed to touch the leaves or roots. Planting iris rhizomes shallowly so that the top of each rhizome is at or slightly below the soil surface helps minimize moisture issues. Mulch is primarily used only for weed management or winter protection in iris beds, and it isn't recommended for moisture conservation.
Mulches that insulate well, such as straw, work well for winter protection. Straw generally contains no seeds, so it won't introduce weeds into the garden bed. More attractive mulches, including bark and pine straw, help with weed suppression without detracting from the beauty of the planting. Generally, winter mulches are laid 2 to 4 inches thick, depending on the severity of your weather. Areas with dry but cold winters require heavier mulches than those that receive snow, because snow cover helps insulate the soil. Summer mulches are spread no more than 2 inches thick.
A winter mulch insulates the iris roots from extreme cold and temperature fluctuations. Winter freeze and thaw cycles can cause the ground to shift, which pushes the rhizomes out of the soil. When applying winter mulch, spread it over the iris bed only after the ground has begun to freeze. Spreading it earlier may cause the soil to retain too much moisture, causing the iris roots to rot. Established plants may survive without a winter mulch, but irises planted in fall usually need winter protection for the first year.
Winter mulch requires removal in spring after the soil thaws, but you can replace it with a lighter summer mulch layer. Summer mulch is necessary only if weeds are a major problem in the iris bed. The mulch shouldn't cover the roots, because it will retain too much moisture, which results in overly damp soil. Spread the mulch around the perimeter of the clump of irises, but don't cover the rhizomes. Pull the mulch back if it touches the foliage, to minimize disease and rot issues. Any weeds that penetrate the mulch layer require immediate pulling to prevent them from becoming established.
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