Answer: Iris is usually divided in mid- to late-August. Carefully dig the plants (I use a garden fork) and separate them, then replant. I find them easier to handle If I trim the foliage down to about 6" before digging. You'll find the rhizome will have both live and dead parts; cut or break away the shriveled up parts and replant the plump, healthy pieces. Each piece you plant should have a fan of foliage attached. Plant so the tops of the rhizomes are just barely below the level of the soil. Be sure to water them in well after planting. Daylilies can be dug and divided after the first frost. Again, cut away any dead foliage; cut the remaining foliage 4-6" for ease of handling. Bury the divisions 3-4" beneath the soil surface. Hostas should not be divided until late winter or early spring, as soon as the new leaves begin to emerge. Hostas tend to form a solid root mass so dividing is easier if you wait until they begin to sprout. Simply cut through the root mass with a shovel (I have actually used a hatchet!), making sure that each division has at least one new shoot (leaf) emerging from it. Replant the divisions 3-4" deep.
Best wishes with your garden!
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