Evening primrose is a tall plant with yellow flowers that open in the evening. Its seeds are used to make medicine. Any of various North American plants of the genus Oenothera, characteristically having four-petaled yellow flowers that open in the evening. Also called sundrops. a plant with small flowers, whose seeds are used to make medicines. Any of various species of herbaceous plants of the genus Oenothera (family Onagraceae). They are noted for their showy flowers, especially the yellow-flowered biennial O. biennis, which is found widely throughout North America and and has been introduced to Europe. It has been grown by geneticists to determine certain principles of heredity. The true primrose belongs to the family Primulaceae