Iris suaveolens (for Siris)

does it happen after a cold night? Or do they get strong early sun?

I was going to say the same as SBG, heat exhaustion. Have you divided your clump or has it expanded to cover that space, good going for a littlie.

They don't get full sun till after 11 Sbg and same thing happened last year in the warm spring we had with no cold nights.
It spread itself Siris. The gap in the middle is from piece I dug up for you although after it's finished flowering I'm going to repot all the pans. Time for a change I think.

Mine hasn't flowered this year, but still might as it's had virtually no water deliberately. It's been covered all winter with the Perspex Alpine cover.

My pans are on a shelf outside all winter. Middle shelf so a bit of protection but still get a bit of rain from the shelf above.