1 Petals rhombic-ovate to elliptic, widest near midlength, acute to broadly rounded; seeds ellipsoid, regularly pitted, borne ascending in locules; leaf margins deeply sinuate-lobed to subentire; [subsection Candela].

..2 Petals 5-17 mm long; stigmas surrounded by anthers at anthesis; pollen ca. 50% fertile.

....3 Inflorescence dense, with > 2 flowers per spike opening each day; leaves gray-green; [midwestern, south to KY and AR, or as a waif southwards and eastwards]

....3 Inflorescence lax, 1-2 flowers per spike opening on each day; leaves green; [southeastern Coastal Plain]

..2 Petals 15-35mm; stigmas exserted well beyond anthers at anthesis; pollen 85-100% fertile.

......4 Pedicels, sepals, and capsules densely to sparsely appressed-strigillose, sometimes also with a few stipitate glands; mature flower buds not overtopping spike apex; free sepal tips 0.5-1.5 mm long; petals mostly bluntly acute to short-acuminate; [primarily of the Great Plains and Midwest, southwest to AK, IL, e. TX, and nw. LA]

......4 Pedicels, sepals, and capsules usually glandular-puberulent, spreading-villous, hirsute with pustulate-based hairs, or glabrate; mature flower buds overtopping spike apex; free sepal tips 1-6 mm long; petals mostly broadly rounded; [primarily southeastern, e. TX north to AK and east to AL].

........5 Inflorescences loose, unbranched; bracts 0.5-1.7 cm long, shorter than capsules; capsules hispid with pustular-based hairs; flower buds with free sepal tips 1-3 mm long, glandular-puberulent but not hispid; [endemic to se. TX]

........5 Inflorescences dense, often branched; bracts 1-3 cm long, longer than capsules; [more widespread].

..........6 Flower buds with free sepal tips 2-6 mm long, divergent; flower buds, ovaries, or capsules at least sparsely hirsute with spreading, pustulate-based hairs, often also glandular-puberulent; [e. TX east to LA and sw. AK, introduced in MO]

..........6 Flower buds with free sepal tips 1-3 mm long, erect; flower buds, ovaries, and capsules glabrate or glandular-puberulent; [endemic to AL and MS]

1 Petals obcordate, widest near the apex, deeply to shallowly emarginate (rarely truncate); seeds prismatic and angled, irregularly pitted, borne horizontally in locules; leaf margins remotely dentate to subentire, or weakly sinuate-dentate near base; [subsection Oenothera].

............ 7 Stigma well-elevated above the anthers at anthesis; petals (2.5-) 3-5 cm long; anthers 8-22 mm long; pollen 90-100% fertile (except O. glazioviana).

............ ..8 Cauline leaves 0.4-1.0 cm wide; apex of the inflorescence curved; free sepal tips subterminal, divergent; capsules spreading at nearly right angles to the stem, conspicuously curved upward, long-attenuate

............ ..8 Cauline leaves 1.5-6 cm wide; apex of the inflorescence erect; free sepal tips terminal, erect; capsules erect or slightly spreading.

............ ....9 Floral tubes (6-) 8-12 cm long, styles 9-17 (-20) cm long, buds 7-12 mm in diameter; capsules 6-12 mm in diameter; plants predominately densely appressed-strigillose (sometimes also glandular-puberulent in the inflorescence and sparsely appressed-villous), lacking or rarely with a few pustulate-based hairs; [TX, OK, and NE]

............ ....9 Floral tubes (2-) 3-4.5 (-5) cm long, styles 5-9 cm long, buds 5-10 mm in diameter; capsules 3.5-7 mm in diameter; plants usually with spreading, pustulate-based hairs in addition to the strigillose pubescence, or appearing nearly glabrous to the unaided eye.

............ ......10 Upper stems, ovaries, and floral tubes often appearing glabrous to unaided eye, sometimes glandular-puberulent, rarely somewhat strigillose; pustulate-based hairs absent or scattered and translucent; sepals yellow-green

............ ......10 Upper stems, ovaries, and floral tubes conspicuously strigillose and villous, also sometimes glandular-puberulent; pustulate-based hairs reddish-based (when fresh), often abundant; sepals often reddish-striped or flushed with red

............ ........11 Cauline leaves 2.5-4 cm wide, bright to dark green, usually with strongly crinkled margins; pollen ca. 50% fertile; ca. 50% of seeds abortive; [exotic; uncommonly cultivated, rarely naturalizing]

............ ........11 Cauline leaves 1-2.5 cm wide, dull green or gray-green, not crinkled; pollen 90-100% fertile; few seeds abortive; [native; TX and OK westward]

............ 7 Stigma surrounded by or below the anthers at anthesis; petals 0.7-2.5(-3) cm long; anthers 3-10 mm long; pollen ca. 50% fertile.

............ ..........12 Free sepal tips subterminal in bud, 0.5-5 mm long, usually divergent; inflorescences often curved at apex; capsules rusty brown or greenish black when dry; [primarily of the Northeast and Upper Midwest, south to n. GA and nw. SC in the mountains].

............ ............ 13 Stems densely silky-strigillose, the hairs appressed, sometimes also with scattered pustular-based hairs distally; leaf blades grayish-green; dry capsules rusty-brown; leaves 0.5-3 cm wide

............ ............ 13 Stems often nearly glabrous in inflorescence, predominantly villous and glandular-puberulent, sometimes also sparsely strigillose; leaf blades bright green; dry capsules greenish-black or dull green; leaves (1-) 1.5-7 cm wide.

............ ............ ..14 Petals 1.4-2.5 (-3) cm, fading pale yellowish-white to translucent; bracts caducous, pale green; free sepal tips terminal in bud; capsules usually dull green when dry

............ ............ ..14 Petals 0.8-1.5 (-2) cm, fading pale yellow or orange; bracts persistent, green; free sepal tips subterminal in bud; capsules usually greenish-black when dry

............ ..........12 Free sepal tips terminal in bud, 0.5-3mm long, erect or slightly divergent; inflorescences erect at apex; capsules dull greenish or gray-green when dry; [collectively widespread].

............ ............ ....15 Stems appearing nearly glabrous in inflorescence (sepals, ovaries, and corolla tubes often strigillose and/or glandular-puberulent), strigose and villous below with translucent (at least in fresh material) hairs

............ ............ ....15 Stems conspicuously and often densely strigillose, villous, and/or glandular-puberulent throughout, hairs with reddish pustular bases often abundant.

............ ............ ......16 Stems, leaves, ovaries, capsules, and corolla tubes densely strigillose, appearing exclusively appressed-pubescent without magnification (also with some appressed or subappressed pustular-based hairs and rarely glandular-puberulent in the inflorescence); leaves grayish-green; sepals usually red-striped or flushed red; [mostly west of the Mississippi River, scattered eastward as an introduction]