Two to four fan divisions are recommended for transplanting, and the roots must be kept moist while the plants are out of the ground. Plant the rhizomes one inch deep (slightly deeper in sandy soils). Siberian clumps can grow undisturbed for several years, dividing being necessary when either the clumps become crowded or when vigor declines and blooms get smaller. COLD CLIMATES: Spring is the best time to plant or divide, with August as second choice. This gives the plant a chance to establish a good root system before winter rolls in. WARM CLIMATES: Avoid the hot weather periods; many prefer the cooler fall period. If unable to obtain plants at the best time for your area, you may pot them up and put in a protected area. If wintering over in cold areas in pots, be sure to set the pots in the ground, with the tops at soil level. Use of gallon size pots is best for this procedure. In any situation, keep newly transplanted plants well watered at all times, with one inch per week a minimum, and mulch for their first winter.
~The above information adapted from The Society for Siberian Iris