Answer: I'm glad you asked before planting your iris and installing landscaping fabric over the top of the bed. Iris grows from rhizomes. The rhizomes elongate and sprout new fans of leaves at each end, then the older center portion of the rhizome dies out. This is how the plants muliply and the rhizomes renew themselves. If you were to plant the rhizomes and then put landscape fabric over the top the new foliage would either penetrate the fabric (surprise!), or grow beneath it and die. Since the older portions of the rhizomes die out after developing new growth at the ends, you'd eventually kill your iris if you planted them beneath landscaping fabric. If weeds are your concern, plant the iris and then place a 3"-4" layer of organic mulch over the bed. The mulch will suppress weeds, help the soil hold moisture, and make digging and dividing your iris (every 3-4 years) a much more pleasant experience. When it comes time to divide your iris bed you can dig the organic mulch into the soil, replant your iris, and topdress with another few inches of organic mulch material.
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