Siberians are usually said to be best divided in fall also, though it can be done in spring. They don't appreciate division and may sulk for a year or more.

I divide my siberians as soon as they are through blooming....they don't sulk...they have lots of time to settle in before they have to put out new blooms.
In fact, I divide all iris as soon as they are through blooming. Then they can settle down and settle in and hopefully put out bloom the next year.
Linda C

Pallida and pumila are bearded iris. Fall is too late here. I do it late July-August. They need enough time to settle in before winter, so the roots can handle the moisture buildup.

Siberians can be done pretty much any time. Japanese don't like me, so they sulk whenever I do it.

I divided my Siberians last September. No blooms this year, but lots of healthy foliage.

I normally divide my dwarf bearded toward end of summer (sometime in August), but I've done it even later than that with no problems - they're pretty tough.

I've had very good success dividing siberian iris in early May, well before they bloom. As long as it's raining or you give them water, they have rebounded and flowered for me.

Bearded iris I've divided after they bloom.

I dug up two divisions from my four year old clump last Sept. and they have a couple blooms each on them right now. I knew it would be iffy, but so glad it's turning out well.

OK, seems like end of the summer/early fall won the popularity vote for I.pallida, I.pumila and I.sibirica, so let it be. :-))
What about I.ensata?
It's a jungle out there just in 2 full years of growth. Planted 2 x2G plants in June of 2006 and now could count about 20 or more flower stalks forming. Those 2 plants and their babies already took at least 10sf of space and still going :-((

The iris will do OK if you divide them in late summer or early fall....but they will do better if you divide them right after they bloom.
All I know says that spring is the best time to plant all varieties. But you are wanting to know when is best to dig up what you have...
Probably they would be happiest in their new home if you dug them in early spring, but you would sacrifice this year's bloom.
So...divide them right after they bloom for best results.
Linda C

i am on the bus that linda c. is driving.
and george, congrats on your new colony!with your green thumb, i'm not surprised at their growth rate!
best,
Mindy

I divided mine early spring this year. It was one of the short iris. I pulled out 6 divisions from it in mid April and potted them up. The clump I left in the garden bloomed without skipping a beat and so did those that were potted up.

Here is what the clump looked like after dividing it on April 30th....

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And here it is blooming on May 21st...

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I've moved them before in bloom :) Tough things they are. In an ideal world, move them right after blooming. Might as well divide them too, then.