Mathematically, division is the opposite of multiplication. Horticulturally, they are the same. Digging and splitting overgrown perennials to propagate them is known as ‘division’ because it divides many rooted stems or rhizomes of one plant into many new plants. Division is a form of propagation; and propagation is commonly known as multiplication. So, we divide plants to multiply them.
Autumn is generally the best time for panting. It is after most of the warmest and driest weather, and just before the cool and rainy weather that keeps newly planted plants from getting too dry. It seems obvious that autumn would also be the best time for division. However, a few perennials that are dormant or mostly dormant by the middle of summer can be divided now for an early start.
Bearded iris may not look dormant with their leaves still green, but they are about as dormant now as they will get. If divided now and allowed to slowly disperse roots through the remainder of summer, they can prioritize the production of new foliage when they come out of dormancy in autumn, like they would do naturally. Like many perennials in mild climates, they grow through winter.
Once dug, the plump rhizomes need to be separated from the old shriveled rhizomes that they grew from. For most, the best segments of rhizome are between the leafy tips and the stalks of the flowers that bloomed earlier this year, although it may not be easy to see where floral stalks were attached. The older sections of rhizome behind the flower stems are probably shriveled already.
The freshly divided segments of rhizome should then be groomed of deteriorating old leaves. Remaining green leaves can be cut in half to remove drying tips. Rhizomes then get replanted just below the surface, with the perpendicular fans of foliage standing upright. The main difficulty with dividing iris now is that they will need to be watered until the rain starts late in autumn. Bergenia, lily-of-the-Nile and a variety of perennials with big rhizomes get divided in a similar manner.
Tony Tomeo 
So it might be the right time in February here. I’ll have a few to divide at that time. Thanks for the info, Tony.
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February sounds about right, since it is half a year from our season. I tend to do mine later in the season because I get the impression that it is more comfortable for them in cooler weather, even if it does not rain right away.
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Thanks Tony, super useful article, and covering Bergenia – one of my difficult spots plants. Today I was describing to a farmer how to divide large Phormium using two excavators with bucket forks to drag them apart.
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Well, that would be for a bit phormium! In the production nursery, we separate them more meticulously.
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My irises need a good division or two, that’s for sure. Not sure it will happen this year, but it could!
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My Iris pallida could double annually if it got divided on schedule! Each rhizome blooms and splits into a pair!
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I did not know that. I have places to put them; it just takes a good half day to do it.
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Not all of the bearded iris bloom and divide so reliably, and even the Iris palida will not double if crowded. With adequate space, water and sunlight, it is pleased to do so!
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Well, mine spread, but not always in the directionI’d like, so maybe that’s a task for the next week or two…
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This math teacher is happy to hear about division and multiplication.
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tee hee
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Yes, it’s all true that division is a great boon to gardeners. It also happens to be my least favorite gardening chore, so I do it as little as possible.
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