Flowering stems 10–60 cm long, the flowers 10–30 in a loose to dense spike. Main stem leaves 2–5. Sepals 2–4 mm long, green. Lateral petals 2–4 mm long, ovate to elliptic, the tips pointed, yellowish green. Lip 3–6 mm long, ovate, the tip broadly rounded, the margins somewhat irregular to entire, unlobed but with a pair of teeth at the base and a low tubercle in the center, green to yellowish green. Spur 5–6 mm long, not strongly curved. Column 1.5–2.0 mm long, yellowish green, the viscidia 1 mm apart, positioned so as to face more or less forward (parallel to each other). 2n=42. May–September.

Uncommon, but widely scattered nearly throughout Missouri, although not yet known from the southwestern quarter of the state (eastern U.S. and adjacent Canada west to Texas). Mesic bottomland forests and wet‑mesic prairies.

The flowers of this inconspicuous orchid are pollinated by mosquitoes and butterflies. The species is divided into 2 varieties by most botanists. These occur throughout the species’ range, but var. flava tends to be more common in the southern portion, whereas var. herbiola is more common to the north. In most states, the varieties are relatively easily separable, but in Missouri they are far less distinct. The varieties generally grow together, and intermediate plants may reflect interbreeding between them. Homoya (1993) discussed the range of morphological characters used to distinguish the varieties of P. flava, as well as habitat and flowering time differences in Indiana, but users of the present volume should be aware that the key below will not adequately determine some specimens.