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Keyed in multiple places:

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Key to Eleocharis, Key A: spikerushes with tip-rooting culms present; culms only vegetative, or a mix of primarily proliferous and some reproductive spikelets

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1 Plants producing only vegetative culms, no reproductive culms present.
  2 Culms usually firm or hard, 0.35-3 mm wide, compressed; plants tip-rooting in terrestrial wetland habitats.
    3 Culms 0.35-2 mm wide; bearing up to 8 subacute ribs; distal leaf sheath not bearing an apical tooth [calcareous brackish or tidal habitats, inland calcareous fens, other similar limestone-influenced wetlands; ME, ON, south to FL, TX, further westward]
    3 Culms 0.5-1.3 mm wide; usually with 10 blunt ribs, finely ridged and minutely granular (at 20x); distal leaf sheath nearly always bearing an apical tooth up to 2 mm long; [freshwater habitats, coastal plain ponds, often acidic; MA w. to MI, south to TX and FL]
  2 Culms wiry, soft, 0.1-0.7 mm wide, variously shaped but not conspicuously compressed; plants growing submersed or floating in aquatic habitats (e.g. creeks, ponds, rivers).
      4 Plants with rhizomes (0.5-2 mm thick); with submersed flaccid culms internally spongy bearing incomplete transverse septae (aquatic forms).
        5 Tubers absent; culms emerging from jointed elongate stolons; [Sinkhole ponds, depression ponds, sawgrass sloughs, lakeshores, creeks, canals, ditches; NC to FL, w. to TX]
        5 Tubers sometimes present; plants with rhizomes but typically growing more tufted; [Sinkhole ponds, depression ponds, blackwater impoundments, natural lakes, millponds, springs; typically fresh-water habitats; Ont. and Que. south to MS and FL]
      4 Plants without pronounced rhizomes (sometimes with short caudexlike rhizomes in E. vivipara), submersed or flaccid culms lacking transverse septae.
          6 Culms producing secondary or tertiary proliferations (proliferous spikelets branching more than once per culm).
             7 Base of whorl abruptly widened from culm, forming a distinct shoulder; whorl divisions many per whorl (commonly 20 or more); whorl divisions usually 0.2 mm or less wide, finely capillary (often < 0.1 mm, but some may approach 0.3 mm); surface texture of divisions obviously beaded (under dissecting microscope); plants ultimately appearing "fan-like" towards terminal whorls
             7 Base of whorl gradually widened from culm, vase-shaped, not forming a distinct shoulder; whorl divisions fewer per whorl (commonly 15 or less); whorl divisions usually 0.3 mm or more wide (0.5 mm or more, but the finest secondary or tertiary divisions as slender as 0.15 mm); surface texture of divisions not beaded (under dissecting microscope); plants appearing less "fan-like" towards terminal whorls
          6 Culms producing a single proliferous spikelet (not proliferating more than once per culm).
               8 Upper portion of sheath thin and scarious, the edge not differently colored (sometimes with a few red-brown spots or streaks, but not with concentrated red dots at the tips); sheath tip 1-2 mm long; plants usually filiform and capillary; without rhizomes
               8 Upper portion of sheath firm, the edge closely red-dotted; sheath tip < 1 mm long; plants soft or firm (emergent plants typically more robust); with thin caudex-like rhizomes
1 Plants with a few reproductive culms present; plants thus a mix of predominantly vegetative culms (bearing proliferous spikelets) and a few spikelet-bearing culms.
                 9 Culms wider, 0.35-2 mm in diameter, typically strong or firm, usually darker green colored with erect and arching culms, not spotted.
                   10 Leaf sheath summit with narrow tooth (up to 1 mm long); achene blackened, apex widened and truncate; [widespread, primarily in Coastal Plain, disjunct in Ridge and Valley province of VA]
                   10 Leaf sheath summit truncate, sometimes with callous swelling; achene green to medium or dark brown, apex rounded, not widened and truncate; [widespread northward within calcareous fens and brackish wetlands, scattered southward in brackish habitats]
                 9 Culms thinner, 0.1-0.4 (-0.6) in diameter, soft and wiry, often arching and light green colored, sometimes spotted or red-brown streaked.
                     11 Culms producing 2 or 3 proliferous whorls; plants usually growing aquatically or submersed under water in ponds, lakes, or depressions.
                       12 Spikelets with a single flower and 2 scales; achenes biconvex, with a beak, but lacking a tubercle (plants rarely reproductive)
                       12 Spikelets with 5-25 scales, spiraled; achenes trigonous with prominent angles, with a pronounced tubercle (plants often with some reproductive culms)
                     11 Culms only producing 1 proliferous whorl; plants growing submersed in water or emergent in drawdown zones of ponds and other aquatic or wetland habitats.
                          13 Achenes finely to coarsely honeycomb-reticulate or with an otherwise reticulate or porose surface.
                            14 Scales 2-6 per spikelet, arranged distichously; tubercle trilobed and decurrent on achene sides; [waif; Mobile, AL]
                            14 Scales 5-25 per spikelet, spiraled; tubercle trigonous, not decurrent on achene sides [widespread in southeastern Coastal Plain]
                          13 Achenes smooth to finely reticulate, but lacking a defined porose or textured surface.
                              15 Achenes often spotted, gray or greenish to red-brown; spikelets distichous or spirodistichous; sheath apex obtuse, acute, or narrowly acute; bristles clearly or sparsely retrorsely spinulose
                                16 Sheath apex obtuse or acute; spikelets distichous at maturity; bristles 3-7
                                16 Sheath apex narrowly acute, membranous, usually with torn edge; spikelets spirodistichous at maturity (appearing superficially spiraled); bristles 6
                              15 Achenes not spotted (or rarely spotted), usually white or light grey colored; spikelets spiraled; sheath apex narrowly acute; bristles not evidently spinulose
                                  17 Scale apices rounded throughout, typically more uniform white-colored; proximal scale tip appressed to basal scales; tubercle birettaform; bristles to 0.3 mm long (distinctly shorter than achene) or absent
                                  17 Scale apices subacute to acute throughout (sometimes a few obscurely rounded), typically with red-tinging on the scale margins; proximal scale free (i.e. not tightly appressed to basal scales); tubercle pyramidal, semicircular, or birettaform; bristles 0.2-1.0 mm long (shorter than or equaling achene, rarely absent)
                                    18 Perianth bristles 0.6-1 mm long, shorter than to equaling achene; proximal scale of spikelets 0.8-1 mm wide
                                    18 Perianth bristles 0.2-0.4 mm long, shorter than achene; proximal scale of spikelets 0.4-0.5 mm wide

Key to Eleocharis, Key C: spikerushes with cylindric spikelets about the same width as culms; fresh culms with incomplete septae not evident externally

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1 Culms acutely quadrangular (with four distinct sides)
1 Culms trigonous or terete, sometimes obscurely angled.
  2 Culms 0.5-1.5 mm wide; spikelets < 3 mm wide; achene < 1.4 mm wide; plants sometimes with aquatic, submerged forms bearing vegetative, filiform, flaccid culms.
    3 Achene body 0.65-1.5 mm long, whitish, stramineous, or pale green; scales mostly ca. 3.5 mm long; culms terete when fresh; tubers absent
    3 Achene body (1.5-) 1.9-2.6 mm long, stramineous or medium brown; scales mostly ca. 5 mm long; culms acutely trigonous when fresh; tubers sometimes present
  2 Culms 1.0-7.5 (-8.5) mm wide (culms mostly > 1.5 mm); spikelets 3.5-8 mm wide; achene > 1.4 mm wide; plants without submerged, vegetative aquatic forms.
      4 Achene apex gradually narrowed into a stout spongy region, confluent with tubercle; perianth bristles smooth (or with only minutely retrorse barbs distally); [widespread in coastal areas from NC to FL, w. to TX]
      4 Achenes lacking a stout spongy region at the apex, distinctly separate from the tubercles (or achene apex slightly constricted into a hard annular thickening, but still with a distinguishable transition zone as in E. mutata); perianth bristles distinctly retrorsely barbed; [coastal areas; s. FL and s. TX].
        5 Culms terete or obscurely 3-5 angled, not conspicuously trigonous throughout; culms (2- ) 3.6-7.5 mm wide [s. TX Coastal Plain]
        5 Culms trigonous throughout; culms (1.1-) 1.5-5.1 (-8.5) mm wide [s. FL].
          6 Scales of spikelet ovate-oblong; bristles irregularly retrorse (retrorse spinules shorter and more concentrated distally), most shorter than the achene body; achenes shiny dark brown at maturity; achene apex markedly constricted into a short neck; culms primarily 1.5-3.7 mm wide; [waif, s. FL]
          6 Scales of spikelet ovate to broadly ovate; bristles coarsely retrorse, mostly exceeding achene and tubercle; achenes lustrous olive-yellow to amber at maturity, achene apex slightly constricted into hard annual thickening (not forming a "neck" between achene and tubercle); culms primarily 2.6-5.1 mm wide; [potential waif; s. TX and s. FL]
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