An iris begins as a rhizome, which looks like a short, thickened stem with attached roots. Don’t plant too deep. Only the roots should be covered, leaving the rhizome on top of the soil. Plant your rhizomes in the fall. Plant in groups of three to five, spacing the groups about 18 to 24 inches apart. For a big display, plant en masse. Great companion plants include Cheddar pinks and other low-growing perennials, such as thrift. Divide irises every three or four years to give them plenty of room to grow.