More than 200 species belong to the genus Iris, including common varieties such as bearded iris (Iris germanica) and dwarf Dutch iris, also called reticulata iris (Iris reticulata). Most cultivated varieties grow from thick, gnarled rhizomes while species such as dwarf Dutch iris grow from bulbs. Both types require division and transplant every three to five years to invigorate their blooming and prevent common illnesses associated with crowding. It is a simple process that ensures the irises will have long, productive lives, but it must be timed properly according to the type of iris to have a successful outcome.
Plan to transplant rhizomatous iris species any time after they bloom, from summer until early autumn. Transplant in early summer if your climate has a short growing season and harsh winter. Wait until early autumn in a more moderate climate with a long, hot growing season and mild, rainy winter.
Select a sunny garden bed with fast-draining soil, and prepare that planting site. Break up the soil, and remove all stones and other debris that could impede drainage. Amend the top 8 inches of poor, sandy or clay-based soil by incorporating a 3-inch-thick layer of plant-based compost with a pH between 6.0 and 7.0.
Scrub the blade of a sharp garden knife with a sponge, removing all old plant matter. Combine 1 part bleach with 3 parts water in a bucket while wearing waterproof gloves. Soak the knife's blade for five minutes in the bleach-water solution. The solution eliminates pathogens on the blade. Rinse the solution off the blade by using clean water, and allow the blade to air-dry.
Lift a clump of rhizomatous iris from the ground by using a garden fork, and move it to a shaded location. Put on garden gloves, and keep them on while handling iris rhizomes. Prune back the iris foliage to one-third of its original length. Doing so limits moisture loss. Cut apart the rhizomes by using the garden knife and ensuring each rhizome division has a fan of leaves, a length of rhizome and plenty of roots.
Inspect the rhizomes, and discard all of them that have visible signs of illness or damage, such as wrinkled patches, soft spots, mold or discoloration. Wash the garden knife if it contacts diseased rhizomes.
Dig a hole for each rhizome in the prepared planting site, spacing the holes 12 to 24 inches apart. Make each hole wide enough to accommodate its designated rhizome and deep enough to hold the roots. Form a small mound of soil in the center of every hole.
Transplant one iris rhizome into each hole. Set each rhizome on its hole's soil mound, with the roots trailing down the mound's sides. Place soil around the rhizomes, filling the holes, covering the roots well and keeping the rhizomes just beneath the soil surface.
Water the soil to a 3-inch depth immediately, settling the soil and hydrating the roots. Water the site again after the top 1 inch of soil dries. Keep the soil well-watered until cold weather arrives.
Cover the planting site with a 3-inch-thick layer of straw mulch if extreme cold weather is common in your location. Remove the mulch in spring. Watch for renewed iris growth after the soil temperature warms.
Plan to transplant bulbous irises after their current season's foliage dies back completely and turns yellow. Wait until autumn if you are in a moderate climate, or transplant them in summer if your area has a short growing season.
Weed a sunny, fast-draining garden bed. Amend the top 12 inches of soil that is poor, sandy or clay-based by incorporating a 3-inch-thick layer of plant-based compost.
Lift the iris bulbs with a garden fork. Examine the bulbs, and discard all of them with damage or signs of infection, such as mold, discolorations or cracks. Prune off old, dead foliage by using pruning shears.
Transplant the iris bulbs into the prepared garden bed. Space the bulbs 3 inches apart in clusters of five, and space the clusters 6 inches apart. Plant the bulbs at a depth of 2 to 3 inches.
Water the soil to a 2-inch depth immediately, settling the soil. Water sparingly at a rate of 1 inch every seven days until the first autumn rain, and afterward discontinue watering until the following spring. Watch for the first iris leaf sprouts in late spring or early summer.