Irises ... need to be divided

You don't have to dig them all up, but you can. The ones around the edges are the best ones to keep - they are newer.

Thank you very much for your advice Bathgate!

Bathgate is correct. If the clump is really congested, and you can't face thinning out the whole lot, perhaps 1/2 this year the other next.
I do like this one, an historic I think! Just visited Cayeux Iris fields in the Loire, really impressive.

You've read my mind Siris! I'm not up to dig up the whole clump although it's rather congested. New rhizomes grew from old ones around the middle of the clump grow on top of each other. Poor things.
Glad to know you're enjoying exploring the Loire!

Yes, I also visited the garden de la Source, Orleans, the holders of the Historic Iris collection in France, very disappointing, being renovated, and badly labelled/mislabelled/ not at all! But went to a municipal garden fair in Alencon ( by chance) bought some plants in pots, made up for Orleans.

It must have been upsetting to see the state of "la Source" especially when you had high expectations. I was amazed by the way about your knowledge about irises; you spotted some mislabeling there. Astonishing! It was lucky you found a nice place to visit accidentally after that and bought some plants. All in all, your trip was a very good one, I think and hope! We're driving down tomorrow night. Talk to you again later this month!