A successful method of dividing irises is to dig up the iris clumps with a spade or garden fork, gently lifting and shaking each clump to remove excess soil. All leafless, soft and rotting tubers are discarded, and the remaining young, healthy rhizomes are replanted. For best results, divide bearded iris in late summer after the iris have bloomed but while soil temperatures are still warm enough to promote root growth in the rhizomes.
To prevent them from becoming overcrowded and to maintain optimum plant health, bearded iris are divided and transplanted every three to five years. This maintains a large number of quality iris blooms and helps to prevent the occurrence of bacterial soft rot, a common disease in bearded iris that are not regularly thinned.
In the clump division method, only young rhizomes containing fans of 3 to 4 leaf blades are replanted. The gardener trims each fan to a 4- to 6-inch height to reduce transplant shock and stimulate root growth in the iris rhizomes. Smaller fans re-bloom in two years, while larger fans bloom the following spring.
Rhizomes are best transplanted in triangular groups of three, 12 to 24 inches apart, with rhizome "toes" facing inward towards each other at a distance of 8 inches. Always position Iris rhizomes at, or slightly below, the soil surface. The addition of a low-nitrogen fertilizer, bone meal or super phosphate assists in maximum root development for the young divisions, as does regular watering every week to 10 days until autumn rains begin.
A less aggressive method of dividing iris without removing the entire clump is to carefully thin them by cutting out old divisions at the centers of the clumps, leaving new growth undisturbed.