Plants 13-65 cm. Leaves (1-)2-3(-4), spreading to ascending, gradually to rather abruptly reduced to bracts distally; blade lanceolate to elliptic-oblong, 5-35 × 1-7 cm. Spikes lax to dense. Flowers resupinate, inconspicuous, yellowish green; lateral sepals porrect to reflexed; petals oblong-orbiculate to ovate, apex somewhat crenulate; lip descending to commonly reflexed, 2-6 × 1.8-5 mm, base auriculate, with prominent central tubercle, distal margin entire to slightly crenulate; spur cylindric to strongly clavate, 4-8(-11) mm, apex rarely 2-lobed; rostellum lobes mostly parallel to slightly diverging, directed downward, minute, obscure; pollinia remaining enclosed in anther sacs; viscidia mostly quad-rangular, oblong-quadrangular, orbiculate, or broadly asymmetric, commonly canaliculate to nearly cylindric and difficult to assess; ovary rather slender to stout, mostly 8-10 × 2 mm.
Stem 3-7 dm, ±leafy; well developed lvs lance-linear to lanceolate or lance-elliptic, to 20 נ5 cm, the upper much reduced or bract-like; spike loose or compact, 5-20 cm, 1.5 cm thick; bracts lance-linear, shorter to much longer than the fls; fls sessile, greenish-yellow or green, 5-6 mm wide; lip deflexed, 4-6 mm, often irregular on the margin, bearing a conspicuous, fin-like protuberance on the upper side just below the middle, and usually with a small lateral lobe on each side at the base; spur 3-6 mm; 2n=42. Boggy or swampy ground and flood-plains; N.S. and s. Que. to Minn., s. to Fla. and Tex. June-Sept. (Platanthera f.; Perularia f.; Perularia scutellata) Variable in aspect. Northern plants (including most of those in our range) usually have a leafier stem and a congested spike with elongate bracts. These have been distinguished as var. herbiola (R. Br.) Ames & Correll, in contrast to the more southern var. flava, extending n. into our s. margin, with loose spike, shorter bracts and broader lip.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.