Answer: Daylilies (Hemerocallis) are best divided in early spring just as they begin to grow, or in late summer. However, if you make large divisions you could do it almost any time as long as you keep them well watered. Iris are usually divided in mid to late summer, this allows their foliage to grow and mature after they bloom and coincides with their active fall growth period. Bulb lilies can be gently split and immediately replanted in the late fall as they go dormant.
In general, it is better to leave perennials in individual containers so they can grow without crowding from competing plants and so you can store them over the winter without disrupting their roots when you remove the annuals at the end of the year. Iris in particular must not have their rhizomes shaded or crowded by neighboring plants as this can lead to rot problems.
I think this season of bloom problem results in most container gardeners using a predominance of annuals rather than perennials. The annuals bloom all summer or, if you use early annuals such as pansies can be pulled and replaced with summer bloomers to get that continuous display. I'm sorry I can't be more encouraging about mixing the perennials.
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