Dee's right. That's exactly what Roundup drift does. Sometimes I actually prefer the Roundup mutation to the original bloom.
I think my mystery has been cleared up. I got the iris from Neil Thurston, so I sent him a photo and he said it must be 'Rose Teall,' which was once growing right next to 'Terre de Feu' in his garden. As it happens, I also have 'Rose Teall' growing in another part of my garden and it's ready to bloom, so I'll see whether they're identical.
