Key to Wisconsin Oenothera

• 1a.Petals white, fading to pink or red, less than 15 mm long; fruits indehiscent, hard and nut-like, with 1--6 seeds; inflorescence often becoming a long, slender spike with numerous flowers 2

• 1b.Petals yellow, or if pink or white, more than 15 mm long; fruit a dehiscent capsule, with many seeds; flowers solitary in leaf axils or many in a terminal raceme or spike 5

• 2a.Strong perennials usually with a cluster of branches arising from a thick taproot; cauline leaves linear to narrowly-lanceolate, most 4--8 mm wide; fruit strigose-puberulent, pyramidal above, constricted below to a terete stipe-like base O. suffrutescens

• 2b.Annuals or biennials, usually with a single unbranched stem; cauline leaves oblong to ovate-lanceolate, up to 5 cm wide; fruit glabrous or with spreading hairs, ellipsoid or fusiform, tapered at both ends 3

• 3a.Flowers nearly actinomorphic; fruit usually glabrous; petals 1.5--3 mm long; stamen filaments without minute scales at base O. curtiflora

• 3b.Flowers zygomorphic; fruit pubescent; petals 6--15 mm long; stamen filaments with minute scales at base 4

• 4a.Plants up to 2 m tall, villous and glandular-puberulent, not strigillose, with many straight hairs O. gaura

• 4b.Plants up to 4 m tall, strigillose with all hairs curled or appressed O. filiformis

• 5a.Petals white, 15--40 mm long, fading to dingy pink or slighty yellow-white in pressed specimens; stems glabrous, the thin pale white epidermis exfoliating near base; flower buds nodding, solitary in leaf axils; seeds in one row in each locule O. nuttallii

• 5b.Petals yellow or pale yellowish-white, fading to pale pink or turning red, orange, or purple, of varying lengths; stems pubescent, not exfoliating; flower buds erect; seeds in 2 rows in each locule 6

• 6a.Floral tube (hypanthium) funnelform, prolonged less than 1 cm beyond ovary; stigma barely lobed; sepals with midrib keeled; leaves usually folded lengthwise, with sharply serrate margins O. serrulata

• 6b.Floral tube slender, prolonged more than 1 cm beyond the ovary; stigma deeply 4-lobed; sepals not keeled; leaves mostly flat, not sharply serrate 7

• 7a.Ovary 4-angled or narrowly 4-winged; fruit clavate to obovoid, sharply 4-angled or winged; plants perennial 8

• 7b.Ovary and fruit more-or-less terete or obscurely 4-angled; plants annual or biennial 9

• 8a.Petals 4--9 mm long; floral tube (hypanthium) 4--8 mm long; stems strigose-puberulent; leaves nearly entire O. perennis

• 8b.Petals 13--30 mm long; floral tube 13--25 mm long; stems spreading-hirsute, the hairs 1--2 mm long; leaves denticulate O. pilosella

• 9a.At least some (usually middle or lower) leaves coarsely pinnately lobed to pinnatifid or sinuate-laciniate; flowers solitary in leaf axils; plants annual O. laciniata

• 9b.Cauline leaves linear, lanceolate, oblong, or ovate, with margins entire to serrate or denticulate, not coarsely pinnate; flowers several to many in a terminal spike or raceme; plants biennial, usually forming a basal rosette the first year 10

• 10a.Capsule linear, slender, 2--3.2 mm thick, more than 5 times as long; seeds not sharply angled, finely pitted, ascending in the locules; flowers often numerous in long, dense spikes 11

• 10b.Capsule lanceoloid, 4--8 mm thick at base, tapered upward, less than 5 times as long; seeds sharply angled, not pitted, horizontal in the locules; flowers often relatively few in short spikes 12

• 11a.Petals 5--16 mm long; stigmas surrounded by anthers at about the same level O. clelandii

• 11b.Petals 15--35 mm long; stigmas exserted above the anthers O. rhombipetala

• 12a.Floral tube and petals each more than 3 cm long; styles 2--6 cm long, stigmas exserted above anthers O. glaziovianna

• 12b.Floral tube and petals each less than 3 cm long; styles less than 2 cm long, stigmas surrounded by anthers at about the same level 13

• 13a.Plants (or at least the inflorescences) appearing glabrous to the naked eye O. parviflora

• 13b.Plants (including inflorescences) conspicuously pubescent 14

• 14a.Subulate sepal tips in unopened buds contiguous at base, in reflexed sepals separated from the flat sepal by only a transverse line; inflorescence erect at anthesis; leaves lanceolate to ovate-lanceolate, or elliptical, 15--60 mm wide; capsules dull-green or gray-green 15

• 14b.Subulate sepal tips slightly separated at base, slightly deflected outward in reflexed sepals, separated from the flat sepal by a small protuberance or shelf; inflorescence usually slightly arching or nodding at anthesis; leaves narrowly lanceolate, ovate, or oblong, 5--30 mm wide; shallowly repand-denticulate; capsules rusty brown or greenish-black 16

• 15a.Plants densely strigillose; leaf blades thick, dull-green to gray-green; inflorescences gray, usually with dense, appressed non-pustulose pubescence, the internodes longer than the capsules O. villosa

• 15b.Plants sparsely strigillose and villous, often also glandular puberulent; leaves thin, bright green; inflorescence green, dense with capsules longer than the internodes O. biennis

• 16a.Plants densely minutely canescent-strigillose, often also with long appressed white non-pustulate hairs; leaves grayish-green narrowly oblanceolate to elliptic, 5--15 mm wide; capsules rusty brown O. oakesiana

• 16b.Plants sparsely strigillose and villous with pustulate or non-pustulate hairs; leaves bright green, 10--30 mm wide; capsules greenish-black O. parviflora