A magnification of 15× or more is recommended to see the nature of the barbs on the perianth bristles; look toward the apex of the bristle to see the barbs to best advantage.
1. Body of achene ca. 4.5–5 mm long, with a stout beak about 3 times or more as long exserted beyond the scale; anthers ca. 3.5–4 mm long.
1. Body of achene less than 2.5 mm long, the beak or tubercle shorter; anthers ca. 0.5–2.5 mm long.
2. Perianth bristles absent; annuals, typically of ± bare soil such as lake and pond shores and drying swales.
3. Tubercle depressed-triangular, much wider than long.
3. Tubercle elongate-triangular, much longer than wide.
2. Perianth bristles present; perennials of meadows, fens, bogs, and other more densely vegetated habitats.
4. Achenes prominently transversely ridged or wrinkled; tubercle ± broadly triangular, ca. 0.5–0.7 mm wide and nearly as long; bristles shorter than body of achene, minutely antrorsely barbed.
4. Achenes smooth or nearly so; tubercle more elongated; at least the longer bristles about equaling or surpassing tubercle, antrorsely or retrorsely barbed or smooth.
5. Perianth bristles minutely antrorsely barbed; tubercle usually green, basally serrulate; scales deep brown; filaments broadly flattened; plants with slender rhizomes.
5. Perianth bristles minutely retrorsely barbed or smooth (in R. alba, ascending-villous at base); tubercle and scales various; filaments narrower; plants tufted, not rhizomatous.
6. Bristles ca. 9–12, ± antrorsely villous at base; scales whitish (when fresh) or pale brown.
6. Bristles 6 (very rarely 9 in R. capillacea), not hairy at base; scales brown.
7. Spikelets mostly several and spreading in fan-shaped clusters; achenes broadly obovoid, 1.1–1.7 mm long (not including tubercle), with prominent whitish margins; leaves flat.
7. Spikelets mostly 1–4 (–7) in each ± narrow, ascending, ellipsoid cluster; achenes oblong-ellipsoid to narrowly obovate, 1.8–2.4 mm long (including a prominent stalk-like base, but not the tubercle), often with a less prominent pale margin; leaves involute-setaceous.