Irises bloom from rhizomes that grow just below the surface of the soil. The rhizomes increase in size and need to be dug up and divided every 3 to 4 years. The best time to transplant iris is in early summer, right after they finish blooming.
Prepare the soil in the new planting bed. Spread a 2-inch layer of peat moss and a 2-inch layer of compost on the surface of the soil. Use a garden spade to turn over the soil and incorporate the peat moss into the soil. Rake the surface of the soil smooth.
Cut off approximately 2/3 of the length of the iris leaves.
Dig up iris rhizomes to be transplanted with a garden fork. Take care not to slice through them.
Divide rhizomes by removing the new, firm ones from the outside of the clump and discarding the old, tough centers. Make sure each division contains part of a rhizome and a fan of leaves.
Soak the rhizomes in a solution of household bleach for 30 minutes. Mix 4 oz. bleach with 1 quart water, or prepare a solution that is approximately 10 percent bleach. This helps prevent disease from infecting the iris plants.
Put the soaked rhizomes in a shady spot for 2 to 3 hours to dry and to help the cut ends heal.
Plant the iris rhizomes to the new location. Dig a shallow hole with your garden trowel. Lay the rhizome in the prepared hole, with the leaves above the surface of the soil. Back-fill with just enough soil so that the rhizome is just below the surface. Iris rhizomes that are planted too deep will not flower.
Water the entire bed after transplanting all of the rhizomes. Use water-soluble fertilizer mixed to half the strength recommended by the fertilizer manufacturer.
Apply a 2-to-4-inch layer of mulch, such as shredded bark or wood chips. Iris rhizomes grow close to the surface of the soil and are easily damaged when cultivating to remove weeds. A mulch will reduce the growth of weeds as well as the vigor of any weeds that do manage to sprout, making them easy to remove.
Shade the newly transplanted iris from the sun for a few days with shade cloth or a polyester row cover.