White four-o’clock (Mirabilis albida) of the Four-O’Clock (Nyctaginaceae) Family is an herbaceous perennial that blooms from evening into the next morning. The genus name is from a Latin word meaning “wonderful”. The specific epithet is from Latin for “whitish”*. Another common name is pale umbrella-wort, in reference to the plant’s large floral bracts. This plant occurs throughout the central and in scattered areas of the eastern U.S. In Arkansas, it is found across much of the Interior Highlands of the northwestern half of the state, as well as a few scattered occurrences in the sandhills of the Coastal Plain. Habitat includes dry to well drained prairies, glades, and openings in deciduous woods.
White four-o’clock develops several lanky, erect to leaning stems that grow from terete (round in cross-section), white (externally tan) rambling roots. Stems, growing from several shallow points of the root, are a yellowish-green to medium green and may have purplish streaks or blotches. Lower portion of larger stems becomes brown-fissured and persists into the next year. Stems may reach 4 feet or more tall, with a few scattered lower branches and many upper branches bearing inflorescences. Terete stems and branches have widely spaced swollen nodes from which leaves and additional branches and leaves grow. Stems and branches exhibit various degrees of pubescence, ranging from glabrous to puberulent (covered with fine down).
Photo 1: This young plant has several stems with the stem on the right bearing a floral bract (encloses flowers) near its apex.
White four-o’clock bears opposite, linear cauline (stem) leaves. Plants appear leafy early in the growing season but soon become open or even scraggly. Lowermost leaves may be 5½ inches long and ¾ inch wide while leaves approaching the inflorescence become increasingly small to tiny. While lower leaves have short (to ¼ inch) partially winged petioles, upper leaves are sessile. Leaves are entire (smooth margins) with a uniform width between their tapering rounded apices and wedge-shaped (cuneate) bases. Larger leaves are ascending and arched with a gentle up-fold along their midribs. Leaf venation is pinnate.
Photo 2: Display shows coloration of terete stems with swollen nodes and underside of a leaf.
Flowers bloom from early summer into fall. Inflorescences of young plants are axillary along the unbranched main stems. On multi-branched plants, inflorescences occur near the ends of branches. As flowers bloom, branch apices continue to grow to allow production of additional leaves and flowers. With a multitude of flowering branches, the upper portion of the plant may become twisty and intertwined.
A leaf axil produces a single spindly peduncle to about 5/8 inch long which supports a single calyxlike floral bract that typically bears three flowers. The loosely up-folded large ruffled bract hides three ovaries which appear topped by flower buds–actually the showy calyces still unexpanded. (There are no true petals in four-o’clock flowers.) As a flower approaches maturity, the flower expands and is exserted (see below) from the floral bract. Then, as the flower fades, it shrinks into a ball that withdraws back into the enclosing bract. Peduncles and bracts (joined as a single unit), at first light green to purplish and lightly to densely pubescent, open wider and become dry, light tan and tissue-thin when ovaries have matured into achene fruits. Dry bracts are up to 1 inch wide (when flattened). The peduncle-bract unit, now very lightweight, falls off with fruit still adhering. With sequentially up-branch detachment of peduncle-bract units, branches become naked except for pairs of “subtending” small leaves.
Photo 3: Flowers bloom near ends of branches as growth continues at their apices. As fruit-bearing, peduncle-bract units drop off, branches become naked except for small leaf pairs.
Flowers of any particular floral bract, typically 2-5, bloom and fade within several days. Fully opened flowers (versus those that do not open fully, as noted below) become exserted from the floral bract, displaying five light pink to rose, spreading, thin, deeply notched calyx lobes extending from a bowl-like base. Each calyx lobe is ½+ inch wide.
Flowers are perfect and about ½ inch across. Five long slender stamens and a long slender style have coloration similar to the calyx. Stamens and style are strongly exserted and may be in a central straight group or be widely separated and twisty. Stamens bear round masses of yellow pollen and the style has a white, round (capitate) stigma. Style, stamens and calyx are attached at the top of a single ovary, which is one of several ovaries attached to the floral bract.
Photo 4: Lavender calyx, stamens and style of this half-inch-wide flower are attached to top of one of three ovules within a floral bract.
In addition to flowers that open fully, some flowers open partially. With those, the stamens and style are slightly exserted outside the rolled-up calyx. Additionally, some flowers are apparently self-pollinating so that the buds do not attempt to develop (cleistogamous flowers). Viable fruits may be produced by flowers fully or partially open as well as by cleistogamous flowers. Often, some ovaries on a bract do not develop into viable fruit.
Photo 5: As a flower fades on the right, fruits attached to a lower floral bract (left) mature. Third object within bract is a drying detached flower.
The ovary of fertilized flowers changes from a light green color to whitish before drying and shrinking from about 3/16 inch long to form a single, dry, hard, oddly-shaped, yellowish-brown, one-fifth-inch-long fruit, an achene. The fruits are rounded in cross-section, elongate, and covered with rounded knobs (tubercles) along five longitudinal ribs and along sunken inter-rib areas (sulci). Fruits taper to broad rounded tops and small tapered bases (hot-air balloon shape). Fruits have a circular pattern at the apices where calyx, stamens and style were attached. Lines of spiky hairs cover the fruit. As fruit matures, light green floral bracts become more open and change to light tan to whitish as they become dry. These dry peduncle-bracts drop off with fruits still attached.
Photo 6: This floral bract, with attached peduncle and fruit, is poised to drop off. Note fruits’ texture, shape of bract and its five radiating veins. Black spot marks where a third fruit had been attached.
Photo 7: With floral bracts partially cut-off, this display shows fruits produced by self-pollination. An undeveloped flower bud at lower left.
Two additional species of the genus have been recorded in Arkansas. Wild four-o’clock (a.k.a. heart-leaf four-o’clock) (Mirabilis nyctaginea), which is native, is shorter and sturdier than Mirabilis albida and has larger cordate to triangular leaves. The inflorescences of Mirabilis nyctaginea occur in large, dense terminal clusters. Garden four-o’clock (a.k.a. marvel-of-Peru) (Mirabilis jalapa), native to South America, is also shorter and sturdier than Mirabilis albida and has larger trumpet-shaped flowers in various solid shades of red, pink, purple, yellow, white or may even be variegated (with potentially different colored flowers on the same individual plant).
- Reference to “whitish” is not certain. Roots, immature fruit and dry floral bracts are all whitish.
Article and photographs by ANPS member Sid Vogelpohl