image by Kconnors: Morguefile.com, Fern Fischer (all other images)
Bearded iris (Iris germanica) add a somewhat exotic look to the spring garden. They multiply rapidly by spreading rhizomes. For healthy, uncrowded plants that will produce larger flowers, you should divide and transplant bearded iris every four to five years. Divisions and transplants should be made no later than mid-August in areas with hard winter freezes. Later plantings are susceptible to winter heaving because the rhizomes are so near the surface of the soil and the roots do not have time to grow and anchor the plants securely.
Lift and Divide Your Iris
Dig straight down along an iris rhizome, and then lift the rhizome and any attached baby plants from the soil. True roots of iris may be 6 to 7 inches deep or more. Try to get all the roots.
Clean soil and plant debris away from the rhizome. Carefully clean soil away from the roots. Inspect the rhizome and make sure it is healthy. It should feel like a potato and should not have soft spots of rot or places where insects have bored into it. Dispose of unhealthy rhizomes.
Slice the rhizome apart to separate the plants with a sharp knife on a cutting board. Each set of leaves should have its own set of true roots, and these will become your new transplants. Include a section of healthy rhizome with each new plant.
Trim away the old leaves on the mature sections of rhizomes. Use scissors to trim the leaves into a fan shape. If new, small leaves are growing among the old ones, leave the new leaves untrimmed. Baby plants with only new, small leaves are ready to transplant just as they are.
Prepare the planting bed by tilling the soil fine, mixing in compost and raking it smooth. Create a planting ridge by making a furrow on each side of it. The top of the ridge is where you want the soil level to be after you transplant your iris. Arrange each iris plant so that the rhizome is lying on top of the ridge, leaves pointing up, with the roots spread out down both sides of the ridge. Space the rhizomes about 16 inches apart.
Cover the roots completely with soil. The soil should come up to the rhizome, but do not bury it.
Transplant the baby plants the same way. Water the bed thoroughly so the soil settles and makes good contact with the roots.