The best time to transplant Irises is after blooming.
They do best in a loamy soil not clay. You should add as much humus and coir to the soil as you can!!!
I have to question your comment about the soil type for irises. I have solid clay and while it may have taken a couple more years for the isires to settle in, they bloom like crazy and have to be thinned every 3 -4 years. Nor have I amended the soil, except for a small amount of peat (a 3 sq ft bag dug into a 8 ft by 4 ft plot).
I th>The best time to transplant Irises is after blooming.
So what?
They would still do much better in a loamy soil. Loam is also much better for you to work than clay because it isn't hard when dry nor sticky when wet.
Coir is a better choice as a soil additive than peat because it does not require destroying the few remaining peat bogs to produce it.
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