Sometimes the difference between two closely related species is pretty obvious, as in white trout lily and American trout lily (one is white, the other yellow). Sometimes the difference is pretty minor, as in the presence of auricles in American trout lily and dimpled trout lily.
When it comes to Iris versicolor and Iris virginica, there are numerous differences. After hours spent reading the literature, I was able to tease them out. They’re detailed in the chart below.
I. versicolor is mostly a northern species, while I. virginica is mostly southern (with a few disjunct populations along the Great Lakes). Maryland is one of the places where their ranges overlap. If I’d seen them in Massachusetts or North Carolina, I could identify them by location alone. But I found them in Maryland, in Montgomery County, where both species have been reported, which brings up another problem, but that will have to wait.
Many of the differences as described are relative rather than absolute. Also, some authorities differ, as you’ll see. Sources (and a few definitions) are at the bottom.
|I. versicolor||I. virginica|
|cauline leaves||seldom as tall as flowering stem (1, 3, 4, 6, 8)||taller than flowering stem (3, 4, 6, 8)|
|spathes||never foliaceous; thickly chartaceous to scarious (1)||herbaceous (1)|
|perianth||violet blue (1);
richly pigmented on the outer sepal margins, fading lighter towards the throat, the veins prominent but the throat a more poorly defined pale greenish yellow (6)
|lavender to violet (1);
lighter blue, less contrast between the darker colored sepals margins and throat; veins less prominent with a sharply defined, school bus yellow spot in the throat (6)
|signal||pubescent, greenish to greenish yellow (1);
greenish-yellow, papillate (5);
greenish-yellow, rather flat, with few to no hairs (4)
|green claw with yellow ground, yellow signal fine pubescent (1);
bright yellow, pubescent patch (5);
bright yellow, usually with a lot of obvious soft hairs (4)
|petal margin||rarely notched (1)||often notched (1)|
|petal size||1/2 to 2/3 as long as sepals (6);
lanceolate, short, narrow in proportion to the sepals, and oblanceolate to spatulate. (7)
|2/3 as long as sepals (6)|
|style arm||base not auriculate (1)||base inwardly auriculate (1)|
I sampled six plants at each of the two sites. Stand #1 is situated in a vernal pond, currently without standing water, with a tree canopy providing plenty of shade. Although there are a lot of irises in the stand, only six were blooming; these were all in the relatively sunnier part of the stand. The irises form a near monoculture, surrounded by a near monoculture of lizard’s tail (Saururus cernuus).
Stand #2 is situated along the C&O Canal, next to but not currently in the water. The site is almost fully sunny, with some dappled shade provided part of the day by a single large tree. The area is swarming with other plants, both native and alien. Most of them are kept small and short by occasional mowing of the bank above the irises.
In Stand #1, five of the six specimens had cauline leaves taller than the flowering stem; the sixth one was much shorter. Five of the specimens had notched petals, while one had flowers mostly spent, the petals all damaged. None of the spathes appeared to be thin, dry, membranous, or papery. I was only able to see a few style columns far enough inside to look for the auricles. I believe they were present, but wouldn’t swear to it. (I refuse to pick or dissect a flower found on federal park lands.)
In Stand #2, two plants had cauline leaves longer than the flowering stem, one had leaves equal to the stem, and three were shorter. One plant had all three petals notched while two plants had no petals notched. The other three plants each had one notched petal, with the other two missing or damaged. The spathes looked much the same as in Stand #1. I was unable to see far enough into the style arms to determine if there were auricles.
In both stands, all the signals had a bit of a fuzzy appearance when seen under a 10x magnifying lens. At that magnification I couldn’t say if they were pubescent or papillate, so I took a lot of shots with the macro lens, then zoomed and cropped the best shot to get this: The tiny round things are actually grains of pollen, but was I looking at papillae or hairs? I sent the photo to a botanist friend, who said that they are hairs. (This was later confirmed by another expert.)
The final couplets for these species in Weakley’s Flora read:
15 “Signal” a greenish-yellow, papillate patch, surrounded by an area of heavily veined purple-on-white; [of VA northward]………. I. versicolor
15 “Signal” a bright yellow, pubescent patch.
….16 Plants to 10 dm tall, usually with 1-2 well-developed branches; capsule 7-11 cm long ……………...I. virginica var. shrevei
….16 Plants to 6 dm tall, little or not at all branched; capsule 4-7 cm long……………………...I. virginica var. virginica
Based on my observations, I have to conclude that Stand #1 is Iris virginica. Stand #2 is trickier because there was variation, but since the signals were pubescent, I have to say that those plants are also I. virginica.
Also, I should be so lucky as to find significant stands of two similar but different species within a hundred yards or so of each other.
But… which variety are they?
In Stand #1, four of the plants had two branches, one had one, and one had three. Three of the plants also had axillary inflorescences (secondary flowering stems arising from a leaf axil). The plants were of fairly uniform height, around 80 to 90 centimeters. So these are probably I. virginica var. shrevei.
Stand #2 was again a little trickier. The plants I measured were 70 to 80 centimeters tall, but I can’t tell the height for the whole stand, as they were growing up a bank and I really didn’t want to stick my hands down in there among the poison ivy to measure them. Five of the plants had two branches, and one had one branch. Only one had an axillary inflorescence.
Again, should I be so lucky as to find two different varieties so close together? I doubt it. I’m going to make an educated guess that Stand #2 is more stressed by the presence of many different species of plants and a less uniform soil (along the edge of a canal, randomly rocky, versus uniformly wet, level soil with essentially zero competition from other forbs), and that these factors account for the variations. These plants are very likely the same species as Stand #1.
Told you I was obsessed.
tomorrow: a few more thoughts, some pretty pictures
cauline: pertaining to the stem
scarious: thin, dry, membranous
pubescent: covered in fine hairs
papillate: covered in small, rounded protruberences
auriculate: having an ear-shaped appendage
1 The Flora of North America I. versicolor; I. virginica
2 Illinois Wildflowers
4 Clemson University Cooperative Extension Rain Garden Plants
5 Flora of the Southern and Mid-Atlantic States
6 Minnesota Wildflowers
7 American Iris Society
8 University of Michigan Herbarium