Herbs, perennial, cespitose, dark olive green to bronze or blackish when dry, to 4.5 dm, not glaucous. Stems branched, with 1-2 nodes, 2.3-5 mm wide, glabrous, margins often minutely denticulate especially basally, similar in color and texture to stem body; first internode 10-30 cm, usually longer than leaves; distalmost node with 1-3 branches. Leaf blades glabrous, bases not persistent in fibrous tufts. Inflorescences borne singly; spathes usually green, obviously wider than supporting branch, glabrous, keels denticulate to entire; outer 18-38 mm, 2-9.5 mm longer than inner, usually tapering evenly towards apex, margins basally connate 4-6 mm; inner with keel evenly curved or straight, hyaline margins 0.1-0.3 mm wide, apex acuminate to acute, ending 0.2-0.7 mm proximal to green apex. Flowers: tepals pale blue to violet, occasionally white, bases yellow; outer tepals 7.7-12.5 mm, apex rounded or emarginate, aristate; filaments connate ± entirely, stipitate-glandular basally; ovary similar in color to foliage. Capsules dark brown or black, sometimes with purplish tinge, ± globose, 4-7 mm; pedicel spreading or ascending. Seeds globose to obconic, lacking obvious depression, 0.5-1.2 mm, rugulose. 2n = 96. Flowering spring--early summer. Moist meadows, stream banks, swamp edges, sandy meadows, moist open woods; 0--800 m; Nfld. and Labr. (Labr.), N.S., Ont., Que.; Ala., Ark., Conn., Del., D.C., Fla., Ga., Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Nebr., N.H., N.J., N.Y., N.C., Ohio, Okla., Pa., R.I., S.C., Tenn., Tex., Vt., Va., W.Va., Wis. Sisyrinchium membranaceum E. P. Bicknell probably belongs here; Bicknell indicated that its relationship was 'with S. graminoides' and his description falls within that of S. angustifolium, except for slightly shorter spathe bracts. In previous floras, Sisyrinchium angustifolium often has been confused with S. montanum, especially when S. graminoides was segregated. Branching seems to be the primary point of confusion. The original descriptions of S. angustifolium and S. graminoides clearly indicated branching while that of S. montanum indicates it to be single-stemmed. There is some slight similarity between S. montanum var. crebrum and S. angustifolium with respect to spathe connation and dry color, and chromosome counts indicate that both have 2n = 96, but there is some indication that breeding barriers may exist (D. B. Ward 1959).
From Flora of Indiana (1940) by Charles C. Deam
Infrequent to rare in the northern part of the state, becoming frequent in the southern part. This species always has yellow roots, prefers a slightly acid soil, and is generally found in dry places in open woodland and clearings, along fence rows, and infrequently in open places with herbs and grasses of equal height.
Bright green, turning darker in drying; stems 1.5-5 dm, somewhat spreading, mostly not geniculate, thin and flattened, broadly winged, (2-)2.5-4 mm wide, the wings individually wider than the central part and minutely denticulate on the margin; lvs mostly 2-6 mm wide; spathes evidently peduncled from the axils of lf-like bracts, mostly not geniculate, the outer bract commonly 2-4 cm, its margins connate for (2.5-)3.5-6 mm at base, the inner bract evidently shorter, commonly 1.3-2.2 cm; tep blue, 7-10 mm; fr 4-6 mm; 2n=48. Meadows, grassy places, and damp woods; Nf. to Minn., s. to Fla. and Tex. (S. graminoides; S. gramineum)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.