Oenothera hartwegii subsp. hartwegii
Herbs sparsely to densely strigillose throughout, more densely so on distal parts. Leaves 1–3.5 × 0.05–0.4 cm, fascicles of small leaves 0.2–1.5 cm usually present in axils; blade narrowly lanceolate, sometimes linear, base attenuate, margins entire or shallowly and sparsely serrulate, sometimes undulate. Flowers: buds with free tips (1–)2–6 mm; floral tube (18–)30–50(–60) mm; sepals 8–20 mm; petals 13–30 mm; filaments 5–10 mm, anthers 5–9 mm; style 30–65(–75) mm. 2n = 14, 28.
Phenology: Flowering Feb–Oct.
Habitat: Rocky or gravelly soil, sometimes limestone, grasslands, conifer woodlands.
Elevation: 900–2300 m.
Tex., Mexico (Aguascalientes, Chihuahua, Coahuila, Durango, Nuevo León, San Luis Potosí, Zacatecas).
Subspecies hartwegii is the most southerly distributed among the taxa in sect. Calylophus, occurring widely from western Texas south into northern Mexico. It is often found in canyons and high plains in the northern part of its range, and reaching pine forests at its southern limits. It is weakly distinct from subsp. maccartii.