Flowers: Four petals, usually yellow but sometimes white or pink. Individual flowers are short-lived but plants produce many flower buds. Flowers of many of the Oenothera species open in the evening (“vespertine flowering”) and are often called evening primrose. Other species’ flowers are open during the day and may be referred to sundrops.
Culture: Full sun or partial shade and well-drained soil. Usually drought tolerant.
Pests and problems: Root rot in wet soils is possible with some species.
Other facts of interest: Plants in the Onagraceae family bear flowers with 4 or 8 petals and 4 sepals, an uncommon number. The sepals are reflexed behind open flowers, resembling pennants in the wind, says herbaceous plantsman Allan Armitage.
Propagation: Seed, division.
O. biennis, Common Evening Primrose. Leafy, erect branched stems and large yellow flowers that open in the evening. Plants can reach 8 feet in height but are usually shorter. Biennials that often reseed. Native to Eastern North America. Have naturalized in Europe where they are called German Rampion.
O. fruticosa, Sundrops. Reddish, slender, hairy stems with lance-shaped, sessile, hairy leaves that are 1 to 3 inches long. Basal leaves are slightly larger. Plants can grow more than 1 foot tall. Flower buds are erect, and open to a terminal cluster of bright yellow flowers, 1 to 2 inches wide. Flowers are open during the day. The seed capsule is shaped like a club, tapering to a slender stalk. Native to Eastern North America.
O. missouriensis, Ozark Sundrops (shown in picture at top of previous page). Also known as O. macrocarpa. Short reddish stems with upright growing tips on spreading stems. Leaves have petioles and are 1 to 4 inches long. Plants are less than 1 foot tall. Flowers, open during the day, are bright to lemon yellow, up to 5 inches in diameter, and persist for many days. Sepals often have red spots in the bud stage and may retain the spots when the flowers open. The seed capsule is ellipsoid and winged. Not as heat tolerant as some primrose species, and should be mulched in winter. Native to Missouri and Kansas, south to Texas.
O. speciosa, Showy Evening Primrose/Pink Primrose. Rhizomatous and spreads more rapidly than many other primrose species. Leaves are linear, 1 to 3 inches long, pinnately lobed, and softly hairy. The plant can grow more than 1 foot tall. Flowers, open during the day, are 1 to 2 inches in diameter and appear in the axils of the upper leaves. Flowers open white but mature to a pink color. Native to the Southcentral United States.