Know Your Natives – Sundrops

Sundrops* (Oenothera fruticosa) of the Evening Primrose (Onagraceae) family has bright yellow flowers open during the day. The genus name may be from Latin (oenothera) for a sleep inducing plant or from Greek (onothera) a hypnotic plant added to wine. The specific epithet is based on Latin for “bushy”. In the U.S., the species occurs from eastern Oklahoma east to Florida and north to Michigan and New England. In Arkansas, it is primarily a species of the Interior Highlands. It is also known as narrow-leaf sundrops and southern sundrops. Habitats include dry to moist but well-drained, partially shady to sunny areas, such as open woodlands, woodland borders, and country road rights-of-way. 

Plants, with shallow, fibrous, somewhat thickened roots, develop a flat-lying basal rosette of leaves over winter, followed by one to over a half dozen erect slender stems that grow 1-2 feet tall. Young stems and branches are reddish and pilose, with dense, soft, white pubescence. Older stems become tan, harden, and may lose their pubescence. 

Stem leaves are mostly alternate, but may be opposite at the ends of branches and tightly clustered at and within the terminal inflorescence. Leaves have a blunt, rounded apex and a gradually tapering (acuminate) sessile base. Leaves are oblanceolate, those of the basal rosette to about 4 inches long and ¾ inch wide, the larger stem leaves to about 3½ inches long and ½ inch wide. Leaves are pilose on both surfaces, with hairs of the upper surface short, dense, and evenly spread, while hairs of the lower surface are concentrated along the midrib and leaf margins. Margins of smaller leaves are entire; those of larger leaves are obscurely wavy. Margins are revolute (turned under), especially on larger leaves. 

Photo 1: Basal leaves are present during winter, with stems arising in mid- to late March. With age, stems and undersides of leaves change from reddish to green. Photo – March 31.
Photo 2: On this multi-stemmed plant, axillary leaf tufts have not yet developed. Stem leaves are mostly alternate, but may be opposite to tightly clustered at or within the inflorescence. Photo – April 12.
Photo 3: This post-bloom stem has axillary branches and leaf clusters so that the appearance is somewhat bushy. Photo – June 12.

The inflorescence consists of single flowers growing from leaf axils, each blooming for one day. More vigorous plants tend to have a tight, terminal cluster of leaves, producing a large and showy flower cluster. Below the terminal flowers, upper leaves may subtend a flowering branch. With inflorescences blooming sequentially, the flowering period, primarily in May, may extend for a month.

Photo 4: This vigorous plant has a large terminal cluster of flower buds intermixed with subtending leaves. Photo – May 2.
Photo 5: Sepals are partially fused and retract from the petals in one or two units. Sepals, petals, and stamens are attached at the tip of a long floral tube. The inferior ovary is below the floral tube.

Flowers develop from cylindrical buds comprising 4 sepals, 4 petals, and 8 stamens attached atop a long, narrow, terete floral tube. The yellowish floral tube is attached to the summit of a pale green, thickened, “inferior” ovary (i.e., the ovary is below the insertion of the other flower parts). Here the club-shaped ovary is lined with eight ribs and tapers into a slender floral stalk, the pedicel. Sepals, floral tube, ovary, and pedicel are covered with long, soft, white hairs.

Sepals are partially fused to each other and retract from the petals, as the bud opens, either asymmetrically as a unit of four to one side of the flower or opposite each other in two pairs. Flowers have 4 large bright yellow petals, 8 yellow stamens, and a long yellow style. Petals are broadly rounded with an indented apex and translucent veins that serve as insect guides. Stamens have long delicate filaments to which slender anthers are attached in see-saw fashion, the better to accommodate contact with the pollinator, typically a lepidopteran. The slender style, extending from the ovary through and well beyond the floral tube, elevates the stigma above the anthers, where it divides into four widely spread receptive lobes. These showy, spectacular flowers may be to 2 inches across with a ¾ inch long floral tube and a ¾ inch long ovary.

Photo 6: Upper cauline leaves may subtend short branches which may produce flowers. The leaves are slightly folded along midribs. Photo – May 10.
Photo 7: Long, soft, white hairs densely cover flower buds. The bright yellow petals have translucent veins. Delicate filaments are tipped with see-sawing anthers. The slender style ends with a 4-lobed stigma.

After anthesis, flowers quickly drop from the ovary. With fertilization, the ovary matures to a hardened capsule about ¼ inch long with a stalk of similar length. When dry, capsules split along four seams from the top downward. The numerous seeds are tannish, smooth, and lopsided.

Photo 8: The eight-ribbed, club-shaped capsules split along four seams. Photo – June 7.

In a garden or natural area, this plant would be mostly unnoticed when not bloom. In bloom, it has striking yellow flowers. It is adaptable to various well drained soils and can do well in partial shade. In favorable sites, it may need to be managed to prevent excessive spreading by seed. It is eaten by deer.

Seventeen total species and/or subspecies of the genus Oenothera are known to occur in Arkansas. All except one of these have yellow flowers. The species most similar to O. fruticosa is O. pilosella (prairie sundrops), with two subspecies represented in the state. Oenothera pilosella differs from O. fruticosa by having shorter pedicels and thus nearly sessile capsules.

  • Day-blooming species of the genus are referred to as “sundrops”. Night-blooming species are referred to as “evening primroses.”

Article and photographs by ANPS member Sid Vogelpohl

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