There aren't many good visual guides pointing out the differences between Southern Blue Flag (Iris virginica) and Giant Blue Iris (I. giganticaerulea). I've seen lots of irises in the South misidentified to one or the other recently. Because of that, I have spent some time the last few weeks cycling through iNaturalist observations to pick out the differences between the two species. First I started with research grade observations then moved onto needs ID observations. Once I felt comfortable with the differences I'd noticed, I started identifying observations. Today, that practice paid off when I stumbled on some Southern Blue Flag while measuring and recording Giant Blue Iris. I'm not a botanist, so I will try to avoid jargon that I'm a little fuzzy on.

Here are a few reliable field marks of Southern Blue Flag that help differentiate it from Giant Blue Iris:

1. Prominent veins on the underside of the sepals (falls), especially around the base. This is best seen in photos from the side of the flower (https://www.inaturalist.org/observations/21745551)
2. Multiple flowers originating from the same point at the top of the stem (https://www.inaturalist.org/observations/21745508).
3. Leaves have a slight midrib (https://www.inaturalist.org/observations/21745542).

The Giant Blue Iris (https://www.inaturalist.org/observations/21745584) has none of those features. The base of the sepals is covered by a green layer, so there aren't any veins worth mentioning. Flowers occur singly along the stalk with two usually at the top. There are several leaves along the stalk as well while Iris virginica stalks are usually pretty bare. Leaves are smooth with no midrib. Giant Blue Iris also tend to be taller with plain green stems whereas the Southern Blue Flag typically were shorter and their stems had a slight purple-ish coloration. This may be more of a local variation and heights are variable depending on nutrients, so I wouldn't consider those good field marks like the three already mentioned.

Posted on March 29, 2019 08:37 PM by ilouque