Know Your Natives – Showy Evening Primrose

Showy Evening Primrose (Oenothera speciosa) of the Evening-Primrose family (Onagraceae) is a colonial perennial with large pink to white flowers. The etymology of the genus name, used by Linnaeus, is ambiguous: It is apparently from a Greek word for wine seeker or sleep inducer or, quite on the other hand, for ass catcher; originally, it was the name of a species of Epilobium or willow-herb, a related plant in the Onagraceae. The specific epithet is from the Latin for beautiful or showy. In the US, Showy Evening-Primrose occurs, as a native element, from south Texas (extending into Mexico) north into Nebraska and Missouri, east to Mississippi, and west to portions of Arizona and New Mexico. Additionally, naturalized populations are found eastward to the Atlantic and Gulf Coasts, as well as in southern California and a few sites in central California and Utah. In Arkansas, it now occurs statewide but likely was originally native principally to the Blackland Prairie region of the southwestern portion of the state and perhaps to the glades and prairies of western and northern Arkansas. It has since been spread by extensive cultivation and roadside plantings. Sunny habitats include sandy to rocky, dry to well-drained upland prairies, woodland glades, disturbed areas, and rights-of-way. Other common names include Pinkladies, Mexican Evening Primrose, and Pink Buttercups.

Plants have twisty tough taproots and, at various depths, wide-spreading lateral roots from which clonal shoots emerge. In favorable sites and where competition is not intense, a clonal plant may cover an area of several square feet.

Photo 1: Plant on left has a clonal stem (at center of photo). Root-runner at right (not connected to plant on left) has produced 5 stems. All stems bore flowers. Photo – April 23.

Plants have green basal leaves in late winter. New stem growth becomes evident in early spring. Branches develop from axillary buds along the lower half of the stems, while distal buds, especially on sunny sites, develop into flowers. Stems are weakly erect and, without support, tend to sprawl as they mature to a length of 2+ feet. They are covered with a dense, appressed, minute pubescence. With new shoots growing annually from crowns and runners, individual plants may have 100+ stems that may be tightly clustered or well-spread, depending on the presence of competing plants.

Photo 2: On this south-facing slope of a road right-of-way, new stems began to develop in late winter. Cold weather caused some leaves to be red. Photo – March 10.
Photo 3: New erect stems grow quickly. Photo shows the same colony as in in Photo 2. Photo – April 3.
Photo 4: This leafy group of stems has grown from a single root crown. A previous year’s dead stem is still attached on left (dead stems are typically absent).

Alternate stem leaves, to 2-3 inches long and ¾ inch wide, are elliptic to oblanceolate below, grading to lanceolate––and decreasing in length––above. Blades of the lower leaves tend to be pinnately lobed proximally, unlobed distally. Petiole length, like leaf blade outline, varies with leaf position: to 1 inch long below, while the upper leaves subtending the flowers may be sessile. With a hand lens, appressed pubescence can be seen on the upper surface.

Photo 5: Lower leaves (shown individually and on stem) are elliptical to oblanceolate in outline. Lower surface of 3 leaves shown on right. Lower leaf on left is 2½ inches long. Photo – March 13.
Photo 6: Lower leaves of this stem mostly lack lobes. Smaller, toothed, lanceolate leaves can be seen at top of photo. These lanceolate leaves are subtending seed capsules. Photo – May 18

Primary bloom-period is April into May with occasional flowers later during the growing season, as allowed by weather conditions. Single flowers, with a prominent inferior ovary, grow directly from the upper leaf axils. In Arkansas, flowers open in the morning for one day* as the previous day’s flowers drop off. With a single flower in bloom per stem on a nodding stem apex, flowers give the false appearance of being terminal. The inflorescence, to about a dozen flowers, is racemose. Flower pedicels grow to about the same length as the inferior ovaries.

Photo 7: Each blooming flower is on a separate stem. As a flower opens around sunrise, the previous day’s flower on the same stem quickly wilts. Photo – April 25.
Photo 8: Flowers of this multi-stemmed plant are nearly white. Flower color is plant specific.

Pale green flower buds consist of tightly rolled petals, enclosed by a tight-fitting calyx, above a floral tube attached at the apex of an inferior ovary. The spindle-shaped, 8-ribbed ovary, measuring ½ inch long and about ⅛ inch wide, tapers to a stalklike (pedicellate) base. Ovaries may be highlighted with red between their ribs.

Photo 9: In this early morning scene, some flowers have opened as the previous day’s flowers wilt. Two buds at right are poised to open. Flower at upper right is 2⅝ inches wide.
Photo 10: Stems, stalked ovaries, floral tube, and exterior of calyx are minutely pubescent. Note ovary ribs and, on right, inverted calyx and lack of reddish color along margins of sepals.

The cylindrical floral tube––requiring a pollinator with a relatively long proboscis––has a diameter of about ⅛ inch. With anthesis, sepals separate along one or more of their fused margins, and their bases invert to fully expose the rolled-up corolla. The overlapping broadly rounded petals, to 1½ inches long and 2 inches wide, have slightly indented tips and wide-cuneate bases. They may be white to pale pink between dark pink primary veins. The pink coloration tends to become more intense toward the petal’s apical margin. Near the floral tube, petals are colored with bands of vibrant yellow and green. Each plant or colony has the same flower color.

Flowers have 8 stamens (filaments + anthers) and 1 pistil (ovary + style + stigma). Slender stamens (¾ inch long) have long delicate filaments to which slender anthers (⅜ inch long) are attached in see-saw fashion. Stamens are about half as long as the petals. The style (1 inch long), extends from the floral tube, disposing the 4 stigmatic lobes (each to 5/16 inch long) well beyond the anthers. Anthers split longitudinally to release pale yellow pollen.

Photo 11: The stigma (above, left of center) of this 3-inch pink flower has not yet divided. Anthers are releasing pale yellow pollen. Knobbed bases of filaments form a ring around entrance to floral tube.
Photo 12: Abaxial side of same flower as shown in Photo 11. Margins of sepals may be reddish. Growing stem extends off lower side of photo.
Photo 13: This split flower retains the pistil, 5 stamens, and 2 petals. Style extends from the summit of the inferior ovary, through the floral tube, to well beyond the long-exerted anthers. Note the manner in which the calyx has become inverted.
Photo 14: These stems, at or near the end of the blooming period, show the racemose character of the inflorescence. Branched stem at center is 26 inches long. Photo – May 18.

With fertilization, the corolla quickly drops off as ovaries enlarge into fruiting capsules. Initially, capsules may be totally green or may be highlighted with red between the ribs. Mature capsules to ¾ inch long (including the short stalks) become dark brown and dehisce along the angles. Capsules produce numerous ovoid brown seeds. Placentation is axile.

Photo 15: Stalked capsules are axillary to lanceolate leaves. On this plant, the zone between the 4 angles is red. Photo – May 16.
Photo 16: Capsule has been split to show tightly packed developing seeds. Placentation is axile, i.e., seeds attached to a central placenta. The longest appressed minute hairs of the plant are on the stalked ovary/capsule.
Photo 17: Capsules, containing numerous seeds, split along seams and gradually disintegrate. Some leaves have already dropped. (Squares equal ¼ inch.) Photo – June 19.

Showy Evening Primrose has a showy floral display and is fairly easy to establish from root cuttings or seed. It will grow in almost any soil that is not too wet or too dry. Where competition is limited, it may develop into a dense colony with numerous spreading root-runners. Plants may form a leafy groundcover overwinter and into spring. A sunny site is necessary for flowering. Best use may be in a sunny confined space or a prairie-like setting.

At least seventeen species or subspecies of the genus Oenothera (as traditionally and narrowly circumscribed––excluding members traditionally treated in the genera Gaura and Stenosiphon) are reported to occur in Arkansas. The large white to pink flowers of Showy Evening Primrose are the species’ most distinguishing characteristic; all other Oenothera species in the state (excluding the Gaura and Stenosiphon species, which are also white to pinkish, but smaller) have yellow flowers. One member of the genus that has been previously addressed in this series of articles is Sundrops (Oenothera fruticosa).

*Farther north, flowers open in the evening and fade the next morning. In Steyermark’s Flora of Missouri, author George Yatskievych states that plants with pink corollas open in the morning, those with white corollas at dusk.

Article and photographs by ANPS member Sid Vogelpohl

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