Ripe achenes are essential for positive determination of species. Several species of Eleocharis approach closely to Michigan and should be looked for within the state. Eleocharis mamillata is a circumboreal species known from Ontario and Wisconsin which resembles E. palustris but has typically 5 or more perianth bristles, these longer than the tubercle, and soft, very easily compressible stems. Eleocharis wolfii approaches from the south and has strongly flattened stems, but with white or pearly achenes, with prominent longitudinal ridges connected by numerous minute cross-bars, like those of E. acicularis. Eleocharis tenuis approaches Michigan from the east and south. It resembles E. elliptica, but has culms only 4–5-angled and achenes do not persist in the inflorescence after the scales have fallen.

1. Mature spikelet scarcely if at all thicker than main portion of culm; scales persistent; culms quadrangular, triangular, or terete and cross-partitioned.

2. Culms terete, cross-partitioned, appearing as if jointed (with narrow, light-colored bands on surface).

E. equisetoides

2. Culms angled, not clearly partitioned nor appearing jointed.

3. Culms sharply 4-angled, stout (3–5 mm thick); spikelet (1.2–) 2–5 cm long.

E. quadrangulata

3. Culms 3-angled, not over 2 mm thick; spikelet 1–2 (–2.5) cm long.

E. robbinsii

1. Mature spikelet decidedly thicker than culm, with scales usually deciduous; culms terete (or sometimes flattened or many-ridged), not cross-partitioned.

4. Tubercle appearing as if a slender or minute conical continuation of the body of the achene, slightly differentiated in texture or color, not separated by a constriction nor appearing as a distinct apical cap; stigmas 3; summit of leaf sheath without a prominent tooth.

5. Fertile culms 20–70 cm tall, flattened, stout; vegetative culms often as long or longer and rooting at their tips; spikelets (7–) 9–17 mm long.

E. rostellata

5. Fertile culms up to 35 cm tall, but some if not all in a tuft less than 20 cm tall, very slender; culms not rooting at tips; spikelets (2–) 3–7 mm long.

6. Plants not over 5 cm tall; achenes ca. 1–1.3 mm long, including tiny tubercle; spikelets 2–3 mm long.

E. parvula

6. Plants (at least most culms) usually over 5 cm tall; achenes 2–2.5 mm long; spikelets 4–7 (–8) mm long.

E. quinqueflora

4. Tubercle differentiated in shape as well as texture, and usually separated from body of achene by a narrow constriction, forming a distinct apical cap; stigmas 2 or 3; leaf sheaths in several species (especially those likely to be confused on tubercle characters) with a prominent tooth at summit.

7. Achenes 3-sided (the angles sharp, or obscure and the achene plumply rounded); styles 3-cleft; surface of achene normally ridged, reticulate, roughened, or in a few species only minutely cellular or punctate.

8. Achenes white or pearly, with prominent longitudinal ridges connected by numerous minute cross-bars; basal scales of spikelet fertile.

9. Culms not spongy; culms and scales deep green, the culms often with reddish basal sheaths and the scales with a reddish brown band on each side; anthers 0.6–1 (–1.2) mm long; bristles equaling achene, shorter, or absent.

E. acicularis

9. Culms conspicuously spongy; culms and scales rather pale green, lacking reddish tinting; anthers not over 0.6 mm long; bristles overtopping tubercle.

E. radicans

8. Achenes greenish, yellow, golden, brown, black, or, rarely, whitish, and reticulate, smooth, or roughened, but not as described above; basal scales of spikelets sterile.

10. Plants tufted to densely cespitose, without rhizomes; achenes whitish, greenish, olive, or black, smooth to finely reticulate.

11. Culms stout, flattened, mostly all over 20 cm long; achenes very dark glossy brown to black, the pale depressed tubercle completely covering the summit of the body; anthers ca. 1.3–2 mm long; bristles scarcely reaching summit of achene.

E. melanocarpa

11. Culms very thin or even hair-like, unequal in length, the longest up to 18 (or occasionally 35) cm tall; achenes whitish, light greenish yellow or olive, the tubercle not over a fourth the width of the achene; anthers less than 0.5 mm long; bristles often equaling or overtopping the tubercle.

12. Lowermost (sterile) scale shorter than to about the same length as the fertile scales, and with a similar texture; achenes ca. 0.6–0.7 mm wide, tubercles much longer than wide; widespread.

E. intermedia

12. Lowermost (sterile) scale much longer than the fertile scales, strongly differentiated and often appearing like a small involucral bract; achenes ca. 0.3–0.5 mm wide, tubercles about as long as wide; rare in southwestern Michigan.

E. microcarpa

10. Plants with very stout rhizomes; achenes yellow, golden, or brown, the surface strongly papillate-roughened or honeycombed.

13. Achene yellowish to dark brown, the 3 angles narrowly keeled or at least strongly ribbed.

E. tricostata

13. Achene yellow to orange or golden-brown, bluntly angled or nearly terete.

14. Culms very strongly flattened and often ± twisted, with obscure ridges; scales at middle of spikelet reddish brown with narrow, deeply bifid scarious whitish tips mostly 0.6–1 mm long.

E. compressa

14. Culms slightly or not at all flattened, prominently ridged; scales at middle of spikelet deep reddish brown to nearly black, with short, entire, lacerate, or bifid tips mostly not over 0.6 mm long.

15. Culms usually 10–50 cm tall, 0.4–0.8 mm wide, scales 1.9–3.4 mm long.

E. elliptica

15. Culms ca. 3–10 (–15) cm tall, ca. 0.2–0.3 mm wide; scales 1–1.4 mm long.

E. nitida

7. Achenes 2-sided (lenticular or biconvex); styles 2-cleft (or often 3-cleft in E. engelmannii, E. flavescens, E. obtusa, E. ovata); surface of achene smooth (sometimes minutely cellular), usually ± shiny.

16. Summit of leaf sheaths thin and membranous, cleft on one side, usually whitish; achene olive green to brown, ca. 1–1.5 mm long, including the green tubercle; anthers ca. 0.6–1 mm long.

E. flavescens

16. Summit of leaf sheaths thin to firm, obliquely to ± squarely truncate, not split (sometimes with a tooth); achenes and anthers various.

17. Achenes shiny dark purplish to black, 0.4–1 mm long including the tiny tubercle; anthers ca. 0.3–0.4 mm long; plant delicate, with culms ca. 0.5 mm thick or less.

18. Achenes 0.4–0.5 mm long; scales 0.8–1.2 mm long; perianth bristles absent (in our material).

E. atropurpurea

18. Achenes 0.5–1 mm long; scales 1.2–1.7 mm long; perianth bristles present.

E. geniculata

17. Achenes yellow to brown, 1.1–2.8 mm long including the tubercle; anthers and stature various.

19. Plants perennial, with stiff culms and rhizomes; scales acute to acuminate at apex (or somewhat obtuse); achenes 1.5–2.8 mm long, including tubercle; anthers ca. (1–) 1.2–2.7 mm long.

20. Basal sterile scale of spikelet solitary, completely or nearly encircling spikelet at the base; fertile scales ovate, acute to somewhat obtuse at apex; culms slender, usually not over 0.7 (rarely as much as 1.1) mm thick.

E. erythropoda

20. Basal sterile scales of spikelet 2 or 3, the lower one not fully encircling the spikelet; fertile scales ovate to ± lanceolate, often with more acuminate (and sometimes recurved) tips than in E. erythropoda; culms often stouter, 0.5–4 mm thick.

E. palustris

19. Plants annual, with soft, easily compressed, densely tufted culms; scales broadly rounded at apex; achenes 1.1–1.5 (–1.7) mm long, including the strongly flattened tubercle; anthers (0.3–) 0.4–0.7 mm long.

21. Base of tubercle slightly less than 2/3 as wide as the broadest part of the mature achene; scales purplish brown.

E. ovata

21. Base of tubercle at least 2/3 as wide as broadest part of mature achene; scales brown or reddish brown (rarely flushed with purple).

22. Tubercle very depressed, not over 1/4 of the total length of the achene, nearly or quite as wide as the truncate body, on which it appears as a flattish cap.

E. engelmannii

22. Tubercle ± broadly triangular, more than 1/4 the total length of the achene and about 3/4 to nearly as wide as the broadest part of the body.

E. obtusa