A difficult genus of variable plants. Inflorescences have an outer spathe-like or sometimes leaf-like bract, concealing either 2 (In Sisyrinchium albidum) or 1 inner spathes. Some species have the spathes (and bracts) sessile and solitary on the stem. In others, the spathes are on a peduncle and usually 2 or more per stem; the stem thus appearing branched. Unfortunately, rare stems with peduncled spathes may occur in species in which spathes are usually sessile; and similarly, sessile spathes may rarely occur in usually peduncled species. One should always examine the population to make sure the variation is noted, and be sure to include mature fruit, if possible. The flowers of our species range from violet to blue, with white forms being frequent in some species.

Measurements of stem width in the key always refer to the approximate middle of the stem—the widest portion. Measurements of the fused portion of the outer spathe bracts are from the end of the central portion of the stem (not the wing, which, when broad, may extend over the base of the spathe) to the end of the fused portion. The fused portion may tear open with rough handling and needs to be sampled carefully. Measurements of the length of the spathe are also from the end of the central or proper stem, not the wing.

1. Spathes on peduncles arising from a leaf-like bract, usually more than one, the upper portion of the stem thus appearing branched.

2. Base of plant with very prominent dense tufts of light brown, rather straight fibers, sometimes as long as 6-7 cm but very fragile and breaking back to shorter tufts resembling a worn broom; corolla less than 7 mm long; spathes 15-21 mm long, the bracts subequal, the outer one with margins fused ca. 2.5-4 mm; capsule pale brown or tan, 3-5 mm long.

S. fuscatum

2. Base of plant without dense fibers, at most with a few rather lax fibers prominent only on lower 1-3 cm of plant; corolla often over 7 mm long; spathes various; capsule dark or pale, 2.5-5 mm long.

3. Stems 2.5–3.5 (–5.5) mm wide, inner spathe and outer bract usually very unequal, the outer bract longer or even leaf-like; peduncles and spathes not geniculate at base; plants nearly always turning dark green in drying.

S. angustifolium

3. Stems 0.5–2.5 mm wide; inner spathe and outer bract subequal; peduncles and spathes geniculate at base; plants drying pale green.

4. Stems minutely denticulate on margins [30–40×]; inner spathe 15–22 mm long; pedicels (at least some of them) wing-margined basally more than half their length.

S. strictum

4. Stems completely smooth-margined; inner spathe 10–13 mm long; pedicels merely 2-edged or slightly winged basally.

S. atlanticum

1. Spathes sessile or nearly so at the end of a simple stem.

5. Spathes 2, surrounded at base by an outer leaf-like involucral bract with margins not fused beyond the wing (if any) of stem.

S. albidum

5. Spathe 1, the outer (usually ± leaf-like) bract with the margins slightly to moderately fused at the base beyond wing of stem (except in S. campestre).

6. Margins of outer bract distinct nearly to the base, spathes greenish, very little purple tinged; stems less than 2 mm wide.

S. campestre

6. Margins of outer bract fused for (0.8-) 1-5 (-6) mm, spathes often strongly purple tinged or stems often more than 2 mm wide (or both).

7. Plant very slender, the stem 0.5-1.7 mm wide (usually 1 mm or less) and barely margined or narrowly winged; largest leaves at most 1.5 mm wide; capsule ca. 2.5-4 mm long.

S. mucronatum

7. Plants stout, the stem usually 1.7-3 (-3.5) mm wide (most often 2-2.5 mm), winged; largest leaves usually 2-3 mm wide; mature capsule ca. 5-7 mm long.

S. montanum