If you have several plants, plant them at least a foot and a half apart, “facing” the same way. The rhizomes will then increase in the same direction, without crowding each other too soon. In about 2 or 3 years, the new rhizomes will begin to crowd each other and you will want to divide the plants.
You will have so many new rhizomes that you can share them with your friends and neighbors. Perhaps you received your first rhizomes from a friend. When digging, keep all plants carefully labeled with their names, for sure identification. It is wise to keep diagrams of your planting area to double check individual labels on the plants.
The digging and separating is best done between one and two months after bloom season, usually in July or August for this Idaho area. Soon after this the irises grow roots which help to hold the plant firmly during the winter in areas where freezing and thawing can result in heaving the rhizome out of the ground. Some member’s mulch in the winter but you will want to remove the mulch early in the spring to prevent rotting of the rhizomes.