Herbs annual or perennial, caulescent; from fibrous roots or a taproot, sometimes somewhat fleshy, or sometimes producing rhizomes. Stems usually erect or ascending, sometimes decumbent, branched or unbranched. Leaves in a basal rosette and cauline, cauline 3–10(–13) cm; blade margins entire, subentire, denticulate, or coarsely dentate. Inflorescences solitary flowers in axils of distal leaves. Flowers opening near sunrise, faintly scented; buds erect, terete, with free tips; floral tube 3–25 mm; sepals splitting along one suture, remaining coherent and reflexed as a unit at anthesis, or separating in pairs, or all separating individually; petals yellow, fading pale pink or lavender, or orangish pink or yellow, or not changing color, obcordate to obovate; stigma deeply divided into 4 linear lobes. Capsules leathery, straight, usually clavate or oblong, sometimes ellipsoid, angled or winged, apex rounded to truncate or weakly emarginate, tapering to a sterile, pedicel-like base (stipe), valve midrib raised, initially apically dehiscent, eventually dehiscent nearly throughout; sessile. Seeds usually numerous, clustered in each locule, ovoid, surface minutely papillose. 2n = 14, 28, 42, 56.

Distribution

North America.

Discussion

Species 6 (6 in the flora).

Section Kneiffia consists of six species (seven taxa) widely distributed in the eastern half of the United States and adjacent Canada, at 0–1900 m elevation (G. B. Straley 1977). Oenothera spachiana is the only annual species and occurs in open fields, prairies, rocky or sandy sites, and along roadsides; the remaining species are all perennial and occupy diverse habitats, including fresh and brackish swampy areas, wood margins, meadows, prairies, and sandy sites. Three species are self-incompatible (O. fruticosa, O. pilosella, and O. riparia) and the other three are self-compatible (O. perennis, O. sessilis, and O. spachiana). The flowers are diurnal, opening near sunrise and closing near sunset; in some populations of outcrossing species they may reopen for several days. In the outcrossing taxa, the flowers are pollinated by bees (Halictidae and Bombus). Oenothera spachiana and O. perennis are autogamous, the former often cleistogamous and the latter a PTH species. Taxonomy of the section here differs somewhat from Straley in that molecular data (K. N. Krakos et al. 2014) indicate that O. sessilis should be recognized at the species level since the closest relative is O. fruticosa, not O. pilosella as previously thought. Similarly, O. riparia is separated from O. fruticosa since it apparently is most closely related to O. perennis despite the morphological resemblance to O. fruticosa.

Selected References

None.

Lower Taxa

Oenothera fruticosa, Oenothera perennis, Oenothera pilosella, Oenothera riparia, Oenothera sessilis, Oenothera spachiana

Key

1	Herbs annual, from a taproot; flowers in leaf axils in distal 1/2 of plant.	Oenothera spachiana
1	Herbs perennial, from fibrous roots, sometimes producing rhizomes; flowers in axils of distalmost few nodes of plant.	> 2
2	Stigmas surrounded by anthers at anthesis; petals 5–10 mm; inflorescences nodding; pollen 40–70% fertile.	Oenothera perennis
2	Stigmas exserted beyond anthers at anthesis; petals (8–)15–30 mm; inflorescences usually erect, sometimes nodding; pollen 85–100% fertile.	> 3
3	Plants usually exclusively hirsute, rarely glabrous, from fibrous roots and producing rhizomes.	Oenothera pilosella
3	Plants usually strigillose, glandular puberulent, glabrate, or glabrous, sometimes villous, usually with fibrous or fleshy roots, not or rarely producing rhizomes.	> 4
4	Capsules ellipsoid, 8–10 mm; plants densely strigillose, glabrate proximally.	Oenothera sessilis
4	Capsules clavate to oblong-clavate or oblong-ellipsoid, (5–)10–17(–20) mm; plants usually strigillose, and/or glandular puberulent, or glabrous, sometimes villous.	> 5
5	Plants usually moderately to densely strigillose throughout, sometimes and/or glandular puberulent or villous; cauline leaf blades 0.1–1(–1.7) cm wide, margins entire or weakly and remotely denticulate.	Oenothera fruticosa
5	Plants glabrous or sparsely strigillose, sometimes also glandular puberulent distally; cauline leaf blades (0.2–)0.8–3(–5) cm wide, margins dentate to remotely denticulate.	> 6
6	Roots fibrous; leaf blade margins dentate to remotely denticulate; petioles 0.1–2(–6) cm; inland upland meadows, stream margins, edges of woods.	Oenothera fruticosa
6	Roots somewhat fleshy, with adventitious roots where submerged; petioles 0–1.5 cm; leaf blade margins remotely denticulate; coastal marshes, slow-running rivers.	Oenothera riparia