The Confusion of Carex rosea and C. radiata
Carex is a “groaner” among botany students.
With no petals or sepals, identification requires the small flower parts be magnified, and then measured, usually in millimeters. These small floral parts are scales and stamens in the male flowers, and scales and sacs (perigynia) with a single seed like achene enclosed as the female flowers.
Some Carex species are so similar that the student must return to the same plant and take notes and drawings until its maturity. Of course, finding the same plant again, or even intact, is part of the challenge, since deer often browse on the tufts or larger clumps.
Two Carex species that cause gnashing of the teeth are C. rosea and the similar C. radiata. Although both species may grow in the same forest, and can even be found growing near each other, C. radiata is more often found near the woodland wet spots with Quercus bicolor. C. rosea seems to grow in drier, more upland sites with Q. alba, Q. rubra, and Q.macrocarpa.
One of the prime keying characters of these sedges is the straightness or curliness of the red stigmas. This is best seen in April and May since the stigmas are just broken remnants when collected in late May and summer. The stigmas of C. rosea are curved to coiled, and when I first saw them, I recorded in my notes that they looked like red ram’s horns. The mostly straighter or twisted together stigmas of C. radiata were not always consistent, and a few curved to coiled stigmas were also seen protruding from the sacs. That’s why the word “mostly” was invented for use in keys for identification.
Keys are written descriptions of a few characteristics of plants. They are usually choices in pairs and the best fitting description leads to another set of choices and hopefully lead the “keyer” to the name of the unknown plant.
Some identification keys make the students roll their eyes, and give huge sighs. My favorite is the key that differentiates C. rosea from C. radiata by the thickness of the stigmas, being .07 mm and .06 mm wide respectively. Our ordinary centimeter rulers have no further submarkings in one millimeter, so determining that measurement frequently left them gasping in despair. The relationship of the stigmas to the length of the sac’s beak is another prize character. C. rosea’s stigmas are shorter than the beak, or nearly so, while those of C. radiata are longer than the beak, or barely so.
The good news in identifying Carex is that leaf width is fairly constant. While there is a whole group of Carex with leaves wider than 10 mm, there is another group that have narrow leaves 3 mm wide or less, and that’s where these species fit.
Unfortunately for the students, the measurements are close between these two related species. Somewhat like measuring the noses of your cousins and comparing with the members of your family. These sedges both have leaves around 2 mm wide, but in sorting through the many leaves in a clump, the gist is, if the leaves are mostly over 2 mm tending toward 3 mm, they belong to C. rosea, while in clumps of C. radiata they tend toward the narrower width and are mostly less than 2 mm wide.
After measuring many leaves and recording the widths, the students move on to the sacs in the star-like spikes, spaced at intervals on the culm. If the students are observing mature plants, then the sac length can be a clincher clue. C. radiata has 3 – 4 mm long sacs while sacs of C. rosea are slightly smaller, only 2 – 3 mm in length.
Even professional botanists despair of the similarity between species that previously were considered varieties of one species, and now they all have species status. This, of course, leads to a good discussion of “what is a species?”
Botanists are mere mortals, not you of course, and rename plants. These two species are a good example since the former name of plants in Illinois known as C. rosea are known now as C. radiata. And before that C. rosea was C. convoluta by some authors. Which is why, when asked the question of which botany manuals I use, I say “all of them.”
- Retired CLC Biology Professor
- Author of Aquatic Plants of NE Illinois
copyright © 2014 Linda Curtis, botanist