A Quick Guide to Midwestern Andropogon

Whenever one thinks of Tallgrass Prairie grasses, the “big four” often come to mind. The “big four” being Big Bluestem (Andropogon gerardii), Little Bluestem (A. scoparius), Switch Grass (Panicum virgatum) and Indian Grass (Sorghastrum nutans). Though they represent only a fraction of the grass diversity that prairie is capable of expressing, they often make up a significant portion of the vegetative biomass.

All four of these species are easy to identify by their flowering structures and many folks working in prairies can distinguish these species by vegetative (non-floral) means. It is really just a matter of stem bases; Panicum virgatum is the only one with a circular cross section, both A. gerardii and S. nutans have an oval cross section and A. scoparius has a very flat base. Andropogon gerardii and S. nutans can then be separated by ligule length where S. nutans has a prominent and cartilaginous ligule compared to the shorter membranaceous ligule of A. gerardii.

You may have noticed that I refer to Little Bluestem as belonging to the genus Andropogon instead of Schizachyrium. It isn’t that I am stubbornly hanging on to an old name because I fear change and scientific innovation, but rather, because I have read the literature involving the removal Little Bluestem from Andropogon and have found it extremely unconvincing. I’ll address this topic in a future entry.

In the Midwest, A. scoparius is often confused with other flat based members of the genus (basically all but A. gerardii) such as A. virginicus, A. ternarius and A. gyrans (=A. elliotii to some). From my experience, when folks learn to distinguish the similar species, they are surprised at how common the other species are.

In the photo above, you can see all four placed side by side. They are, from left to right, A. virginicus, A. ternarius, A. scoparius and A. gyrans.

Andropogon virginicus (below) is a species of old fields, highway medians and degraded grasslands. It is the most generalist of the group and holds little to no conservation value other than it being better than a field full of exotics. Though I have seen it in relatively intact natural communities, to me it doesn’t seem to have a precise niche in any. Rather, it seems like an ecological anachronism like Gleditsia triacanthos or Maclura pomifera.

To the experienced eye, it is quite distinct even from a distance. It has a more yellow-green cast to the summer foliage and a more yellow-orange cast to the cured stems in the winter (see photo above).

It differs from other Midwestern Andropogon in that the spikelets are included in leaf sheaths and small blades all along the stem (above). Other members of the genus express their spikelets on well exerted peduncles or, as in the case of A. gyrans, the subtending sheaths are large, spathe-like and distributed at the top of the stem. These differences will become more obvious with the descriptions and photos below.

Andropogon gyrans (below) is like a meek, polite and a little fancy version of A. virginicus.

I say this because it occurs in the same weedy habitats as A. virginicus but never in such density. Chances are if you have an abundance of A. virginicus in an area and you look around you will likely find a few stems of A. gyrans mixed in the population. Andropogon gyrans also seems to lack any real fidelity to a well defined natural community or soil type. In fact, all you can say about it and A. virginicus is that they are only found in full sun and moist to dry soils. Andropogon gyrans cures about the same color as A. virginicus but with deeper orange notes along the spathe-like sheaths that subtend the spikelets.

The winter stems of A. gyrans make it the most easily recognized member of the group. The spikelets are more or less congregated in the upper ¼ of the plant and are subtended by the afore mentioned large spathe-like bracts (above). These large bracts give the plant a similar silhouette to that of the tropical genus Heliconia. I have yet to find a consistent field character for distinguishing A. gyrans and A. virginicus without full grown stems. Luckily, one can usually find a remnant stem or two.

Andropogon scoparius (below) is the major player in the Tallgrass and Mixed Grass Prairie systems.

It also contributes significantly to the biomass of glades, woodlands and savannas. The standing, dormant vegetation of A. scoparius is typically more red-orange in color than the yellow-orange of A. virginicus and A. gyrans.

It also differs from A. virginicus and A. gyrans in that its peduncles are not included or otherwise associated with subtending leaf blades or sheaths. Rather, the peduncles are long, exerted and conspicuous (above). When flowering and/or fruiting material are present, A. scoparius can be distinguished from other Andropogon in the presence of a solitary spikelet on each peduncle (A. virginicus, A. gyrans and A. ternarius have two spikelets per peduncle). The best way to differentiate A. scoparius from the other three species in this article is by their ligules. The ligule of A. scoparius is more or less 1 mm long, while the other three have much shorter (less than 1/2mm) ligules. This may seem like a subtle character, but a good field botanist can easily distinguish 1mm from 1/2mm or even 3/4mm. It is all about practice and the perception of scale.

Like A. scoparius, A. ternarius (below) has exerted peduncles.

In fact, they are so exerted in this species that the plants take on a very leggy appearance. This leggy silhouette is further exaggerated by a lack of foliar density, compared to other species. Andropogon ternarius has the red-orange coloration of A. scoparius and is most likely to be misidentified as that species.

The easiest way to tell the two apart, in addition to the peduncle lengths, is that the tips of the peduncles of A. ternarius have a tuft of pubescence (see above and below) whereas A. scoparius is glabrous.

Caution must be used in conjunction with this character because the spikelets of BOTH are pubescent. Of course, A. ternarius has two spikelets (thus the name Splitbeard Bluestem) where A. scoparius has one (seem like a weak character to base a genus on doesn’t it?). Andropogon ternarius has the most southern distribution of the four species mentioned here and is the most conservative. It prefers acidic soils and is less tolerant of disturbance. Where you find it, it is a remnant or relic plant, not a colonizer as the other species mentioned here (less so for A. scoparius, of course). It also makes a nice subject for creative photography.

Lastly, as always, one must keep in mind that this is a diverse group and that along the southern fringe of the lower Midwest one is likely to run into members of the genus not mentioned here; some native, some exotic. That being said, all the other species are different enough that the cautious botanist will instantly know when he/she has stepped outside the comfort zone.