Real Dictionary

evening primrose
variable noun: Evening primrose is a tall plant with yellow flowers that open in the evening. Its seeds are used to make medicine.

evening primrose in American English
any of a genus (Oenothera) of plants of the evening-primrose family, with yellow, pink, or white flowers, many of which open in the evening
noun: a plant, Oenothera biennis, having yellow flowers that open at nightfall

evening primrose in British English
noun: any onagraceous plant of the genus Oenothera, native to North America but widely cultivated and naturalized, typically having yellow flowers that open in the evening

evening-primrose in American English
designating a family (Onagraceae, order Myrtales) of dicotyledonous plants found chiefly in temperate America, including the fuchsia, sundrops, and willow herbs