Kneiffia chrysantha (Michaux) Spach; K. depauperata Jennings; K. perennis (Linnaeus) Pennell; Oenothera chrysantha Michaux; O. perennis var. rectipilis (S. F. Blake) S. F. Blake; O. pumila Linnaeus var. chrysantha (Michaux) Gordinier & Howe; O. pumila var. minima Lehmann; O. pumila var. pusilla (Michaux) Torrey & A. Gray; O. pumila var. rectipilis S. F. Blake; O. pusilla Michaux

Herbs perennial, sparsely to moderately strigillose, glandular puberulent distally; from fibrous roots. Stems usually erect to slightly decumbent, unbranched to few-branched distally, (3–)15–30(–75) cm. Leaves in a basal rosette and cauline, basal 2–4 × 0.2–1.2 cm, petiole (0.2–)0.5–1.2(–2.5) cm, blade oblanceolate to obovate; cauline 3–7 × 0.2–1.2 cm, petiole 0.1–1 cm, blade oblanceolate to obovate, margins entire or weakly and remotely denticulate. Inflorescences nodding, flowers in axils of distalmost few nodes. Flowers opening near sunrise, nearly unscented; buds with free tips to 1 mm, connivent; floral tube 3–10 mm; sepals 2–4 mm; petals bright yellow, fading pale yellow, or orangish yellow to pale pink, 5–10 mm; filaments 3–4 mm, anthers 1–2 mm, pollen 40–70% fertile; style 3–4 mm, stigma surrounded by anthers at anthesis. Capsules clavate, 4-angled or narrowly 4-winged, 5–10 × 2–3 mm, stipe 1–2 mm; sessile. Seeds 0.7–0.8 × 0.2–0.3 mm. 2n = 14.

Flowering May–Jul(–Aug). Fields, open woods, boggy areas; (0–)150–900(–1400) m; St. Pierre and Miquelon; B.C., Man., N.B., Nfld. and Labr. (Nfld.), N.S., Ont., P.E.I., Que.; Conn., Del., D.C., Ill., Ind., Iowa, Ky., Maine, Md., Mass., Mich., Minn., Mo., Nebr., N.H., N.Y., N.C., Ohio, Pa., R.I., S.C., Tenn., Vt., Va., W.Va., Wis.

Oenothera perennis, a PTH species that forms a ring of 14 chromosomes in meiosis, is self-compatible and autogamous (G. B. Straley 1977). It is disjunct in Nebraska from the rest of its range in eastern North America, occurring in Garfield, Holt, and Rock counties (R. Kaul, pers. comm.). It is introduced in British Columbia.

Oenothera pumila Linnaeus is an illegitimate substitution based on O. perennis Linnaeus, while Kneiffia michauxii Spach is an illegitimate substitution based O. pumila, as is K. pumila Spach, and the three pertain here.