Scirpus

1. Perianth bristles ± straight (or some with 1–2 kinks), slightly if at all exceeding achene (less than twice as long), minutely retrorsely barbed, sometimes rudimentary or absent; spikelets grouped into numerous hemispherical to spherical heads, each consisting of 5–25 or more spikelets (or some on a plant as few as 3); plants mostly solitary or few together, or colonial from long-creeping rhizomes; base of involucre not blackened.

2. Leaf sheaths all green; plants without long creeping rhizomes, in small clumps or solitary; bristles (often rudimentary) barbed only on apical half or third; styles 3-cleft; midrib narrow and green, excurrent on most scales as a short but distinct mucro; summit of culm smooth or slightly scabrous.

3. Perianth bristles absent or 1–3 and vestigial or much shorter than the achenes, with barbs lacking or only at tips.

S. georgianus

3. Perianth bristles 4–6, slightly shorter than to somewhat exceeding the achene, barbed for much of their length.

4. Bristles ± straight or stiffly curved to follow the achene outline, at least the longer clearly slightly exceeding the achene; lower leaf sheaths and blades distinctly septate-nodulose.

S. atrovirens

4. Bristles weak and contorted, all shorter than the achene; lower leaf sheaths and blades weakly septate-nodulose.

S. hattorianus

2. Leaf sheaths strongly tinged with red, at least toward base of plant; plants colonial from long-creeping rhizomes; bristles barbed nearly to their bases; characteristics of styles, scales, and culms not combined as above.

5. Styles 3-cleft; achenes 3-angled; bristles (3–) 6; summit of culm usually scabrous.

S. expansus

5. Styles 2-cleft; achenes 2-sided; bristles 4 (–5); summit of culm usually smooth.

S. microcarpus

1. Perianth bristles much crinkled or curled at maturity, exceeding the achene, smooth; spikelets mostly solitary or in groups of 2–5 on pedicels; plants usually in dense, many stemmed clumps; base of involucre blackened or not.

6. Bristles inconspicuous, scarcely exserted beyond the scales, about twice as long as achene; spikelets cylindric, mostly solitary on pedicels of various lengths; scales orange-brown with conspicuous green midrib excurrent as a very short awn.

S. pendulus

6. Bristles conspicuously exceeding the scales and achenes at maturity; spikelets short-ovoid to nearly spherical (very rarely cylindric), solitary on pedicels or in groups of mostly 2–5; scales various but midrib not conspicuously excurrent.

7. Spikelets all or mostly all sessile in clusters of (2–) 3–7 or more.

S. cyperinus

7. Spikelets mostly pediceled, the ultimate branches of the inflorescence typically bearing 1 central, sessile spikelet with 2–3 pediceled ones.

8. Scales and bases of bracts dark blackish green; plants slender with leaves 2–5 mm wide.

S. atrocinctus

8. Scales and bases of bracts brown or gray-brown; plants more robust with leaves 3–10 mm wide.

S. pedicellatus

All species found in Scirpus

Scirpus atrocinctusWOOL-GRASS 
Scirpus atrovirensBULRUSH 
Scirpus cyperinusWOOL-GRASS 
Scirpus expansusBULRUSH 
Scirpus georgianusBULRUSH 
Scirpus hattorianusMOSQUITO BULRUSH 
Scirpus microcarpusBULRUSH 
Scirpus pedicellatusWOOL-GRASS 
Scirpus pendulusBULRUSH 

Citation:

MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. December 13, 2017. http://michiganflora.net/genus.aspx?id=Scirpus.