How to ID Louisiana Irises from Southern Blue Flag

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. Note that I'm located in Louisiana, so I'm not personally familiar with the Prairie Blue Flag (Iris savannarum) or Prairie Iris (Iris hexagona). I do see lots of misidentifications of Southern Blue Flag with either of those in Florida also. The distinction between the Southern Blue Flag and the blue Louisiana irises is important because they're in two different clades of beardless irises. The Southern Blue Flag is in the clade that includes Yellow Flag while the rest are in the Louisiana iris clade. This project is dedicated to compiling observations of the Louisiana iris clade (nothing personal against the Southern Blue Flag).

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. Here's an observation of a Southern Blue Flag with photos from the top and side. Below are photos comparing side views of Southern Blue Flag (top) and Giant Blue Iris (bottom).

1. Usually more than 2 flowers originating from the same point at the top of the stem.

1. Leaves have a slight midrib.

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 5, 2020 02:54 PM by ilouque