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This genus is well known as a difficult one, especially in the subsection Oenothera (Dietrich, Wagner & Raven; 1997), which can be called the O. biennis complex, including in Michigan O. biennis, O. glazioviana, O. oakesiana, O. parviflora, and O. villosa. Distinctions in this group are subtle and poor specimens may be almost impossible to place. Even the sepal tip character distinguishing the O. parviflora group from the O. biennis group is not as clear-cut as most keys imply. The sepals are connate in bud, separating at anthesis and becoming reflexed, when the narrow apical appendages are most easily seen. These are more clearly subterminal in the O. parviflora group, with a tiny projecting shelf-like apex of the sepal proper. In the O. biennis group, there is generally a transverse (convex) line, or even a boat-shaped tip, marking the end of the sepal proper; so experience (not to mention intuition) is helpful in interpreting this character. The petals in the O. biennis group tend to be longer (ca. 14–20 mm in our material) than in the O. parviflora group (ca. 10–17 mm) and leaves in the O. biennis group are also generally broader than in O. oakesiana, which is rather distinctively narrow-leaved (although O. villosa may approach it).

Three old specimens (Ingham, Washtenaw, and Genesee Cos., 1879–1911) in poor condition and/or with scanty data are the more southeastern O. nutans Atkinson & Bartlett [= O. biennis subsp. austromontana Munz]. They are glabrous or glabrate throughout the inflorescence and lack glandular hairs (except on the calyx). This taxon has larger flowers (ca. 2 cm or more) than O. biennis. If not simply cultivated, the specimens probably represent non-persistent waifs. Oenothera speciosa, from the southeastern United States with large pink flowers is often cultivated and spreads aggressively, at least vegetatively; no records as an escape are known yet.

1. Petals white, turning pink, less than 1 cm long; fruit indehiscent, 2–6-seeded.

2. Ovary and fruit with appressed to ± incurved hairs; fruit abruptly contracted just below the middle to a thick stalk-like, truncate base.

O. suffrutescens

2. Ovary and fruit with mostly wide-spreading hairs; fruit ± equally tapered to both ends.

3. Main stem with all hairs (whether long or short) ± curled or appressed.

O. filiformis

3. Main stem with numerous ± straight hairs (overtopping shorter or appressed hairs).

O. gaura

1. Petals yellow (if white to pink, then over 1 cm long); fruit dehiscent, many-seeded.

4. Ovary 4-angled; fruit sharply 4-angled or -winged, tapered to the base.

5. Petals (3–) 4.5–9 (–10.5) mm long; tip of stem usually nodding when bearing buds; anthers ca. 1.6–2.7 (–3.8) mm long; body of capsule ca. 8–11 mm long.

O. perennis

5. Petals (11–) 12–27 (–30) mm long; tip of stem erect or nearly so; anthers 3.7–7.1 mm long; body of capsule various.

6. Calyx glabrous or nearly so, the free tips at most 2 mm long (often less than 1 mm); capsules ca. 5–10 mm long.

O. fruticosa

6. Calyx copiously long-hirsute, at least on the free tips, which are (1.7–) 2–3.5 mm long; capsules ca. 12–13 mm long.

O. pilosella

4. Ovary terete (or with several low ridges); fruit terete to obscurely 4-sided, abruptly rounded at the sessile base.

7. Floral tube ± funnel-shaped, about half as long as the ovary (or even shorter); stigma broad, with 4 rounded (not linear) shallow lobes; leaves linear-lanceolate, less than 7 mm broad, sharply but remotely toothed.

O. serrulata

7. Floral tube linear, longer than the ovary; stigmas with 4 linear lobes; leaves various.

8. Petals white (turning pink); anthers ca. (7–) 9–10 mm long; stems silvery white, glabrous (except for fine glandular pubescence toward the inflorescence).

O. nuttallii

8. Petals yellow (sometimes turning reddish in age); anthers 3.5–8 mm long; stems green to brownish or reddish, usually pubescent.

9. Capsules nearly or quite linear, slender, 2–3.2 mm thick, 5–12 (–14) times as long; seeds finely pitted but not strongly angled; cauline leaves either pinnately lobed or linear to narrowly lanceolate (less than 1 cm broad).

10. Leaves pinnately lobed (even ± pinnatifid); flowers few, in the axils of upper and middle leaves (not a distinct bracted inflorescence); plant with long spreading hairs much overtopping the fine pubescence.

O. laciniata

10. Leaves (except in basal rosettes) linear to lanceolate and entire or nearly so; flowers numerous, crowded in the axils of reduced leaves (bracts) in a terminal spike; plant with only appressed hairs.

11. Petals 0.5–1.5 cm long; stigma surrounded by the anthers at anthesis.

O. clelandii

11. Petals 1.5–2.5 (–3.5) cm long; stigma well elevated above the anthers at anthesis.

O. rhombipetala

9. Capsules tapered upward from near the base, ca. (4–) 5–7 mm thick and 3.5–5 times as long; seeds sharply angled but not pitted; cauline leaves unlobed, elliptic-lanceolate, often at least 1 cm broad.

12. Petals ca. 3.5–4.5 cm long.

O. glazioviana

12. Petals ca. 1–2 cm long (rarely longer).

13. Subulate sepal tips subterminal, their bases slightly separated in bud and with a distinct protuberance within (visible at anthesis); tip of stem often bent at early anthesis, with another bend “correcting” it to become erect.

14. Calyx, ovary, capsule, and upper leaves or bracts ± densely pubescent with appressed whitish non-glandular hairs; largest leaves typically less than 15 mm broad and denticulate.

O. oakesiana

14. Calyx, ovary, capsule, and other parts glabrate to ± sparsely pubescent, often with some long spreading hairs as well as shorter glandular hairs; largest leaves various, typically at least 15 mm broad and nearly entire.

O. parviflora

13. Subulate sepal tips terminal, their bases contiguous in bud and with at most a mere transverse ridge within at anthesis; tip of stem straight at anthesis.

15. Plant green in aspect, with mostly spreading long hairs and often shorter glandular ones.

O. biennis

15. Plant, especially the distal portion and inflorescence, gray in aspect, with rather dense appressed non-glandular pubescence.

O. villosa