Eriophorum virginicum is rather different from all our other species in flowering and fruiting quite late in the season, and also having the inflorescence dense, sometimes appearing head-like, even though composed of more than one spikelet. However, as in all other species with multiple spikelets, there are one to several elongated bracts associated with the inflorescence. Species with a single terminal spikelet lack elongate bracts.

1. Spikelet solitary, erect, without any involucral leaves; base of spikelet with several sterile scales.

2. Plants densely cespitose; achenes abruptly contracted to a tiny apiculus.

E. vaginatum

2. Plants colonial from long-creeping rhizomes; achenes with a gradually tapered beak 0.5-0.6 mm long.

E. russeolum

1. Spikelets 2 or more (very rarely 1), spreading or ± nodding (or erect in E. virginicum), subtended by 1 or more slender involucral leaves; base of spikelet with at most 1-2 sterile scales.

3. Cauline leaves with blades slender (up to 2 or rarely 3 mm wide), 3-angled (often channeled) their entire length; involucral leaf 1 (very rarely 2), shorter than the inflorescence; anthers 1-1.8 mm long; achenes 2.5-3.2 mm long.

4. Uppermost cauline leaf with blade distinctly shorter than its sheath; scales usually suffused with blackish or lead color; culms smooth.

E. gracile

4. Uppermost cauline leaf with blade about equaling or longer than its sheath; scales greenish brown or reddish brown (or occasionally becoming leaden toward edges); culms minutely scabrous above.

E. tenellum

3. Cauline leaves with blades flat, at least in their lower half, often wider (up to 6 mm); involucral leaves 2 or more, often exceeding inflorescence; anthers and achenes various.

5. Scales rather thick, brownish to reddish (sometimes a green band centrally), with midnerve inconspicuous (the scales either obscurely nerved or the lower ones ± equally 3-7-ribbed); anthers 0.7-1.5 mm long; mature achenes ca. 3-3.5 mm long; spikelets maturing (bristles elongating) after the middle of July (usually in August), ± congested, the pedicels short and the spikelets therefore erect or slightly spreading.

E. virginicum

5. Scales thin, drab to blackish or lead colored, at least toward apex, usually with an evident midnerve (or if this obscure, then the anthers longer); achenes often shorter; spikelets maturing in June or early July (though persisting into or even through the winter), the pedicels usually elongating, the longer ones at length ± nodding.

6. Midnerve of scales prominent only below the tip; anthers (2-) 2.2-4.5 (-5) mm long; summit of leaf sheaths with dark purplish border; mature achenes mostly ca. 2-2.3 (-3) mm long; plants colonial from long-creeping rhizomes.

E. angustifolium

6. Midnerve of scales prominent, even somewhat enlarged, to the very tip; anthers 0.8-1.5 mm long; summit of upper leaf sheath without dark border; mature achenes mostly ca. 3 (-3.4) mm long; plants cespitose or solitary.

E. viridicarinatum