Hartmannia Spach, Hist. Nat. Vég. 4: 370. 1835; Oenothera [unranked] Hartmannia (Spach) Endlicher; Oenothera subg. Hartmannia (Spach) Reichenbach; Oenothera [unranked] Xylopleurum (Spach) Endlicher; Oenothera sect. Xylopleurum (Spach) Walpers; Oenothera subg. Xylopleurum (Spach) Reichenbach; Xylopleurum Spach

Herbs perennial, caulescent; from a taproot, sometimes producing new shoots from rhizomes. Stems decumbent to ascending or erect, usually branched, sometimes unbranched. Leaves in a basal rosette and cauline, cauline 1–6(–10) cm; blade margins subentire, weakly serrate to weakly sinuate-toothed, sometimes sinuate-pinnatifid. Inflorescences solitary flowers in axils of distal leaves. Flowers opening near sunrise or sunset; buds erect, terete, without free tips or free tips minute (except to 4 mm in O. speciosa); floral tube 4–26 mm; sepals splitting along one suture, remaining coherent and reflexed as a unit at anthesis or, rarely, separating in pairs; petals usually pink or rose purple, fading darker, rarely white, fading pink, obovate to obcordate; stigma deeply divided into 4 linear lobes. Capsules hard and leathery, straight, clavate or narrowly obovoid to rhombic-ellipsoid, angled or narrowly winged (to 0.5 mm, apex attenuate to a sterile beak, proximal part tapering to a sterile, pedicel-like base (stipe), valve midrib raised (prominent in O. speciosa), dehiscent at apex or nearly throughout fertile part; sessile. Seeds numerous, clustered in each locule, narrowly obovoid, surface glossy, appearing granular, but minutely papillose under magnification. 2n = 14, 28, 42.

Species 5 (4 in the flora): w, c United States, Mexico, West Indies, Bermuda, Central America, South America; introduced widely in tropical and subtropical regions.

Section Hartmannia consists of five species, mostly diploid (2n = 14), but Oenothera speciosa is diploid and also has polyploid populations (2n = 28, 42). All of the species are morphologically very similar, characterized by purple to pink (or white in O. speciosa) petals, leaves often lobed toward the base or distally, capsules often straight, with the fertile portion of the capsule relatively short, angled or narrowly winged (less than 5 mm), apex attenuate, tapering to an acute beak. All species form a definite rosette, which persists at least until the onset of flowering, and ascending to decumbent stems. Most of the species occur in an area from Arizona and Texas south into Mexico, but O. speciosa extends to the Central Plains in the United States, and O. rosea also ranges farther to Central America, the Caribbean (Bermuda, Hispaniola, Jamaica), and northern South America to central Chile. Only Oenothera deserticola (Loesener) Munz occurs outside the area and is restricted to high elevations in the Trans-Volcanic Belt of Mexico. Oenothera rosea is widely naturalized worldwide in tropical and subtropical areas. Four species form bivalents in meiosis, and are self-compatible, except for O. speciosa, which is self-incompatible. Oenothera rosea is a PTH species and forms a ring of 14 chromosomes in meiosis, and is self-compatible and autogamous. Oenothera speciosa has mostly diploid (2n = 14) populations with vespertine or diurnal white flowers in the northern part of its range; many of those from central Texas southward are morning-opening, pink-flowered, and mostly tetraploid (2n = 28) or hexaploid (2n = 42). Pollinators are primarily bees, noctuids, and very few hawkmoths (in O. speciosa), and sometimes secondarily skippers and other butterflies (summarized by W. L. Wagner et al. 2007).