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);
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!