I don't think a mixup of rhizomes is rare at all. You can practically count on it from the big box stores, and it happens readily enough from Iris growers/vendors. How could it not? If there's no blossom -- they all look alike!! Without PBF, there's hardly any identifiable difference. Mistakes happen. The biggest difference is that the growers/vendors will be embarrassed about it and happily correct the error -- the bb stores don't care. Their attitude is *you bought an Iris, you got an Iris*.
