Answer: Bearded iris are usually divided in late summer. Dig them up, trim the foliage down to about six inches, separate the rhizomes, and replant the more vigorous newer ones. Discard the oldest ones and any that show signs of borer damage. When you plant them, set them so that the rhizome is at the soil surface. Mulch between but not over top of them. You can also divide these in early spring but this will usually mean no blooms that year.
Hemerocallis or daylilies can be divided in fall or late summer, you could probably still do it now. Dig up the clump, trim the foliage back by about half, then pull or cut it into sections. A single fan will grow into a new plant, but a nice piece about four to six inches across will make a nice plant by next summer. Replant at the same depth as they grew before and keep well watered and mulched while they re-establish.
Hosta is treated much the same way, with now and very early spring being the best times to divide them. I do not usually cut mine back when I divide them though, as I feel the foliage helps shade the roots and thus helps keep the soil moist.
With all of these, keep in mind the mature size of the plant and space them accordingly. Iris tend to need division about every three years to keep them growing vigorously. Daylilies can sometimes go as long as five years between dividing, you would wait until blooming seems decreased. Hostas can live almost indefinitely without being divided unless they become too crowded. I hope this helps!
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