Description of Oenothera sect. Kneiffia:
Distribution: This section consists of five species (seven taxa) widely distributed in the eastern half of the United States and adjacent Canada, from sea level to 1,900 m elevation (Straley 1978). The annual O. spachiana occurs in open fields, prairies, rocky or sandy sites, and along roadsides; the remaining species, all perennial, occupy diverse habitats, including fresh and brackish swampy areas, wood margins, meadows, prairies, and sandy sites (Straley 1978).
Habit: Perennial or annual herbs, roots fibrous or a taproot, sometimes somewhat fleshy, or occasionally producing rhizomes; stems erect or ascending, sometimes decumbent.
Leaves: Leaves in a basal rosette and cauline, the cauline ones 3-10 (-13) cm long, linear, lanceolate to ovate, subentire to coarsely dentate.
Flowers: Buds erect, terete in cross section, with free tips, floral tube 3-25 mm long, scarcely flaring at the apex, sepals splitting along one suture, remaining coherent and reflexed as a unit at anthesis or separating in pairs or all separating individually, petals yellow, not changing color or fading pale pink or lavender.
Fruit: Capsule with the proximal part sterile, hollow and cylindrical, the body leathery, clavate or obovoid, angled or winged throughout the capsule body, the wings 0-2 mm wide, the distal face of each valve with a rib, apex rounded to truncate or weakly emarginate, dehiscing apically initially, but eventually nearly the full length of the capsule.
Seeds: Seeds usually numerous, clustered in each locule, ovoid, angled from crowding within the capsule, surface minutely papillose.
Chromosomes: Chromosome numbers: n = 7 (in O. perennis (14)), 14, 21, 28; x = 7.