Most weeding must be done by hand, since the irregular growth of the rhizomes rules out the use of a hoe. Under normal conditions, iris clumps need to be divided every third or fourth year, after the rhizomes have become crowded and inhibit flowering. If you must divide your irises, first cut back the fans of foliage to 4 or 5 inches, then lift the clumps with a spading fork inserted several inches away from the rhizomes, to avoid damaging them, and treat the cut ends with bulb dust or a fungicide to prevent decay. Any rhizomes that show border damage or disease obviously should be discarded. Make sure each small fan of leaves to be replanted is attached to a plump rhizome with strong feeder roots adhering to it.
Good drainage is a requisite.
The soil can be improved in this respect by mixing some peat, sand or compost into it when spading prior to planting. One planting method is to place the rhizomes on a peaked mound of soil or ridge in a shallow trench, with the feeder roots spread down on either side. When soil is firmed over the roots, the plant will be adequately anchored. Another way to plant is to dig a trench deep enough to accommodate the roots, set the rhizomes at ground level against one side of the trench and then fill the trench with soil. Whether planted in massed beds or given space among other perennial flowers, irises make more effective displays when several are planted in clumps.