Biennial or short-lived perennial, 0.5-2 m; lvs lanceolate to oblong, mostly 1-2 dm, acute or acuminate, entire to repand-dentate, often crisped on the margin, sessile or short-petioled, glabrous to canescent, fls in stiff, terminal, simple or branched, ±leafy-bracteate spikes; hypanthium 2-5 cm; sep 1-3 cm, their appendages terminal, 1-5 mm, connivent for most of their length before anthesis; pet yellow, 1-2.5 cm; anthers 4-7 mm; fr 1.5-4 cm, stout, tapering upwards; seeds 1.2-1.8 mm, angular-prismatic; 2n=14, a complex heterozygote. Fields, roadsides, prairies, and waste places, throughout most of the U.S. and s. Can. July-Sept. More or less divisible into 3 vars.:
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
[Deam recognized Oenothera pycnocarpa, O. nutans, O. canovirens, and O. cymatilis. Currently these are known as O. biennis, O. nutans, O. villosa, and O. parviflora respectively.] He notes that O. pycnocarpa is the common form of the Oenothera biennis complex in Indiana. It is found throughout the state and in all kinds of habitats. It and the next three species are regarded as obnoxious weeds. Each plant bears a great number of seed and self-sown seedlings will appear many years afterward. The status of this and the next three species is not yet definitely determined. Some authors regard them simply as varieties of Oenothera biennis but I am regarding them as species as did the authors who described them. The plants are exceedingly variable and only an expert can name them with any degree of certainty. I have a large number of specimens which I am not including in this treatment because I can not satisfactorily name them.