Scirpus rubricosus

I have become increasingly aware and concerned about what appears to be a distinct species that is largely ignored/lumped in recent literature and completely unknown to most botanists. Several years ago when I first started working in wetland communities I was introduced to Scirpus cyperinus and its fitting misnomer “wool grass” (why not “wool sedge”?). Initially this was an easy species to recognize since it differed from other Midwestern Scirpus in having the following combination of morphological characters; average height of nearly two meters, folded (V-shaped) leaves (as opposed to the pleated M-shaped leaves of S. atrovirens), large diffuse inflorescences and achenes subtended with very long flocculent bristles that give the mature inflorescence a woolly appearance. And life was good……until I started to notice that there was something wrong. There appeared to be two very different morphologies within what I was calling S. cyperinus. Here they are side by side:

The photo on the left is typical S. cyperinus. The plant on the right is what used to be called S. rubricosus (or at least S. cyperinus var. rubricosus). The few authors that even bother mentioning both entities describe how most of the individual spikes of S. cyperinus are sessile or nearly so while those of S. rubricosus are often long pedunculate (but a few can also be sessile). Here is a close up of each (S. cyperinus on top; S. rubricosus on bottom);

And that’s it. Most also say that there is too much intermediacy to recognize S. rubricosus even at the varietal level. I have seen both of these entities in the field numerous times throughout the Midwest and have never had the slightest difficulty distinguishing them. That being said, I have also reviewed nearly 100 herbarium sheets and found this practice to be much more tenuous because something is lost in the pressing process. But what? Architecture. Here is the first photo again:
Notice the major difference in architecture between these two. Scirpus cyperinus (left) is broccolini-like or maybe like an elm tree festooned with pompoms. Notice also how the base of the inflorescence is somewhat obscured by the inflorescence. Now compare to S. rubricosus(right). Notice first how much larger it is. This is consistent. Now notice how the branches of the inflorescence fountain to one side and droop conspicuously. This is also very consistent. Lastly notice how leggy and conspicuous the long basal branches of the inflorescence are. When these characters are zoned in on in the herbarium you can sort a hundred of these things into two clean piles in a matter of minutes.

Interestingly, both often grow sympatrically. In such cases, armed with the architectural difference, one can stand and identify them from many meters away. It becomes a fun game where you check the sessile/pedunculate spikes to see how you did. Do this and you will quickly find that these two are as distinct as an elephant is from and elephant seal.

I have not noticed any ecological difference between the two. That being said, S. cyperinus is the only one I see in wet depressions in agricultural fields and is the most common one I see in road ditches. However, I have seen S. rubricosus in pretty degraded wetlands.

One more thing; there is a much more conservative and uncommon species, at least uncommon in the central and southern Midwest, that looks like these two. Scirpus pedicellatus differs in having pretty much all the spikes pedunculate, the scales are lighter brown in coloration and it fruits a month earlier. Architecturally it is very much like S. rubricosus. I collected dates off of 41 specimens at the Missouri Botanical Garden herbarium and found the following:

Species Phenology(bloom to fruit) Average

S. cyperinus June 25 to September 16 mid-August

S. rubricosus July 1 to October 7 mid-August

S. pedicellatus July 3 to August 5 mid-July

Keep an eye out for this one!


  1. Interesting stuff. I've seen two different Scirpus cyperinus-like things, and I thought I'd figured it out this year that one was S. cyperinus and one was S. pedicellatus. Now I wonder if I've seen any S. rubricosus, though Reznicek and Voss seem to consider it as a more southern plant that may barely reach into southwest Michigan. From FNA...

    Scirpus cyperinus is extremely variable. A form common in the northern part of its range, south to Iowa, northern Ohio, Maryland, and (in the Appalachians) North Carolina and Tennessee, has bases of the involucral bracts and the involucels blackish, the spikelets sessile or nearly so in glomerules, and the scales relatively short, ovate, and brownish. This form has often been treated as S. cyperinus var. pelius. A more robust southern form, extending north to southern Missouri and Illinois, Kentucky, Virginia, and (along the coast) New Jersey and Massachusetts, has the bases of the involucral bracts and the involucels reddish brown, the spikelets mostly solitary, and the scales relatively long, narrowly elliptic, and reddish brown. This form has often been treated as a distinct species, S. rubricosus (or under the illegitimate name S. eriophorum Michaux). These two morphologies intergrade so extensively that it is not practical to recognize them taxonomically at any rank.

    Scirpus cyperinus often hybridizes with S. atrocinctus and S. pedicellatus, forming hybrid swarms. Some plants appear to have characteristics of all three species; the names Scirpus atrocinctus var. grandis Fernald and S. atrocinctus forma grandis (Fernald) D. S. Carpenter are based on such a specimen.

    So it sounds like the authors of this treatment found too many intermediate specimens to keep S. cyperinus and S. rubricosus separate. They clearly appear different in your photos.

  2. Hey Scott. Thanks for your comments. I saw both entities growing in the same wetland last winter when I did a workshop for some folks in Bloomington, Indiana. I haven't really been north of there since.

    Thanks for the FNA quote. I have read it and several other sentiments of the same chord. Intergradation or not, there ARE two morphologies. I would think that would warrant SOME level of taxonomic status. Of the specimens I reviewed at MO last year I could find no intermediates. I know we have had this discussion before, but here is yet another example of a morphospecies based system being mistaken for biological species based system. Yet, somehow S. atrocinctus, S. pedicellatus and S. cyperinus can form "hybrid swarms" and maintain their taxonomic ranks. I'm running into this a lot these days and it is driving me nuts.

  3. I have to add another point to my comment above. Too often botanist forget that the characters in keys to species (or other taxa) are not the species themselves. There are many solid species out there that just defy key characters (extensive morphological overlap with other species) yet maintain their biological reality. Dichanthelium, Carex, Rubus, Quercus and other large genera are full of such conundrums. A great example is the Dichanthelium werneri post before this one. Here is a case where the key characters were just wrong, yet everyone cut and pasted it into a succession of "treatments" until someone said "hey, I can't key this thing therefore it doesn't exist". I know that you know this, but I wanted to clarify the point for anyone that might be reading.... Knibb High Football Rules!!


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