There is scant information about seed germination and seedling emergence of common evening primrose (Oenothera biennis L.), even though it is a common and widespread weed in North America. This study was conducted to determine the influence of several environmental factors on its seed germination and seedling emergence from three North American populations in autumn. In alternating light (12 h)/darkness (12 h), maximum germination (89.7-97.7%) of freshly matured seeds occurred at alternating temperature regimes ≥ 25/15 C, and germination was lowest (0-80.3%) at the coolest temperature regime of 15/5 C. In comparison to seeds incubated with an alternating light/dark photoperiod, germination was lower when seeds were exposed to continuous darkness, indicating that seeds were positively photoblastic. Cold stratification at 4 C enhanced germination, with seed dormancy alleviated after 4-8 wk depending on the study population. For freshly matured seeds, germination exceeded 67% in test solutions ranging from pH 3 to 10, and was highest (80-100%) at neutral or near-neutral pH. Germination exceeded 84% in solutions with osmotic potentials ranging from 0 to -0.4 MPa, and germination was observed at osmotic potentials as low as -0.8 to -1.0 MPa depending on the study population. Seedling emergence was only observed for seeds sown on the surface of soil or buried to depths ≤ 2 cm. Thus, seeds of O. biennis are positively photoblastic and exhibit germination characteristics associated with type 2 non-deep physiological dormancy at maturity, with seeds being capable of germinating under a variety of climatic and edaphic conditions.