How do I divide and transplant Bearded Iris?
Iris need to be thinned or divided before they become overcrowded, generally every 3-5 years. If Iris are allowed to become too crowded the bloom will suffer, some varieties may crowd others out and disease problems may be aggravated. Depending on your location, July through September is the time to divide and transplant Bearded Iris.
Transplanted Iris should be planted a minimum of six weeks before the first hard frost in your area.
Old clumps may be thinned by carefully cutting out the old divisions at the centers of the clumps and leaving new growth in the ground. In the case of very old and compacted clumps, the process of thinning might be easier if you dig up the entire clump, remove the old "spent" rhizomes, trim the foliage of the new rhizomes and replant them. Smaller shoots may take two years to produce blooms, but larger shoots should bloom the following spring. 
We recommend supplementing the soil with a low-nitrogen fertilizer, super phosphate or bone meal when transplanting. These extra nutrients help the new shoots to have the best chance of success in establishing their root systems. Water newly planted rhizomes well initially, and if dry conditions continue then once every 7 to 10 days until the autumn rains begin, to help the new roots become established.
