Know Your Natives – Two-Wing Silverbell

Two-wing silverbell (Halesia diptera) of the Storax (Styracaceae) family is one of several understory trees in the family with pendant showy white flowers. The genus name recognizes English botanist Stephen Hales who authored Vegetable Staticks* in 1727. The specific epithet is based on Greek words for “two-winged” in reference to fruit structure. In the U.S., two-wing silverbell occurs primarily from southeastern Texas, across Louisiana, southern Mississippi, most of Alabama, the Florida panhandle, and southern Georgia, with scattered occurrences in Arkansas and South Carolina. In Arkansas, it occurs naturally only in Nevada County. Habitat preference is sandy, wet to mesic soils of streambanks and shady bottomlands.

Two-wing silverbell, a deciduous species, occurs naturally either as a large shrub with multiple trunks or a medium-size, single-trunk tree. Trees may be broad and open in more shady sites or, in sunnier sites, more densely branched with a rounded outline. Branches are slender, with lower branches tending to be oriented horizontally, while upper branches are ascending and spreading. Shrubs and trees attain most of their length from terminal buds with lateral buds providing infill. Trees may reach a height of 15 to 30 feet with a similar width.

New branches are medium green with short dense pubescence. By the end of the growing season, new branches have lost their pubescence and are smooth and reddish brown. In subsequent growth years, bark develops grayish and reddish streaks. Over time, the streaks on lower branches and trunks evolve into ridges and furrows. Bark of mature trunks is moderately textured with blocky longitudinal flat ridges and narrow cluttered furrows.

Photo 1: Lower branches of this 6-year-old tree are oriented horizontally while upper branches are ascending. The main trunk is fairly smooth with grayish and reddish streaks.
Photo 2: This 25-year-old tree has two trunks. The larger one, 16 inches in circumference, has developed a moderately textured bark with flat ridges and narrow furrows.

The petiolate alternate leaves are broadly elliptic to obovate. A few small ovate leaves occur at the base of current year’s leafy branches and at the base of floral twigs (see below). While most leaves are medium green, the ovate leaves tend to be darker. The off-set pinnate yellowish green veins are recessed on the upper side and expressed on the lower side. Larger leaves may be 6¼ inches long, including a ¾ inch petiole, and 3¼ inches wide. Blade margins are irregularly serrate. Early in the growing season, upper leaf surfaces are short pubescent and puckered between main veins. With age, leaf pubescence is lost and the blade surface flattens. Leaves become yellowish in fall.

Photo 3: Leaf size and shape varies from ovate leaves at the base of branches (and floral twigs) to more-distal elliptic to obovate and oblanceolate leaves. This leafy branch lacks floral twigs. Photo – April 4.

Floral twigs reach the bloom stage in late March into April at the same time that leafy branches are developing. Floral twigs grow from lateral axillary buds along one-year-old branches. A twig bears a cluster of 2-10 white bell-shaped flowers on long (ca ¾ inch) drooping pedicels emerging from a compressed raceme. While the lowermost one or two flowers of a twig may be subtended by an ovate leaf, more distal flowers are subtended by small bracts with additional tiny bracts occurring along the pedicels. Newly grown floral twigs are clothed with short dense pubescence that extends onto the sepals and petals.

When flower buds first appear, they are already pendant and white with an elongate football shape. A floral tube or hypanthium is fused to the ovary, so that the ovary is wholly inferior, rimmed with 4 indistinct triangular sepals. The snow-white, bell-shaped corolla comprises 4 petal-like lobes slightly united at their base.

Photo 4: Floral twigs and leafy branches develop simultaneously. Flowers and fruits are pendant throughout their development. Floral twigs drop off after fruiting. Photo – April 3.

At anthesis, the corolla of the ¾-inch-long flowers remains pendant, so that the 8 (twice as many as the corolla lobes) stamens and style remain somewhat hidden. Stamens are inserted on the base of the corolla, their white filaments united below and closely adjacent above, forming a column or tube. Elongate anthers face inward. Anthers split along their length to release pale yellow pollen within the stamen column. The slender style has a gentle taper from a relatively broad base to a flat stigma at its pointed apex. The style extends well beyond the anthers with that portion within the stamen column densely pubescent. Ovaries have up to four ovules. The persistent styles are retained as prominent beaks on the fruit.

Photo 5: Exterior of corolla lobes is pubescent. Connate stamens terminate in elongate anthers which appear as extensions of the filaments. Style is exserted well beyond stamen column.
Photo 6: Display shows a separated flower along with an additional stamen column at lower left. Interior of stamens (see split column) and style are pubescent. Note the short triangular sepals atop the inferior ovary. (Black object is an insect.)
Photo 7: Small oval leaves occur at base of leafy branches and floral twigs. Floral twig at bottom of photo bears developing fruits with styles retained. Squares = ¼ inch.

Ovaries form large, oblong, dry, dangling and indehiscent fruits with an opposite pair of large longitudinal fin-like wings and a much-reduced pair of wings (more like ridges) set at 90⁰ to the larger wings. Fruits are to 1¾ inches long, including a ¼ inch spiked beak (the persistent style). Wings and ridges are widest at mid-fruit, tapering to both ends. Fruits are dark green and fleshy during development, becoming tan to brown in fall as the wings and surrounding soft tissue become corky. The corky tissue surrounds a hard woody stone with longitudinal ridges. The 1-4 white seeds within the stone are spindle shaped. Mature fruits, on dry pedicels 1-1½ inches long, may drop-off (abscise) in the fall or remain attached until spring. Fruits may be dispersed a short distance by wind or float away in flowing water. Floral twigs are entirely deciduous, dropping off after fruiting.

Photo 8: This slender woody branch shows the smooth reddish growth of the current year and the roughened gray growth of the previous year. The flowering twigs (now bearing fruit) are axillary to the previous year’s leaves. The current leafy branch emerged from the previous year’s terminal bud. Photo – August 11.
Photo 9: With fruits mature, these floral twigs (now fruit-bearing) have died. Twigs may remain on the plant into spring. Photo – November 23.
Photo 10: A cross-section of a fruit. The corky wings surround a hard woody stone which may contain up to four seeds. Inset (squares = ¼ inch) shows pieces (due to difficulty of removing seeds from stone) of several elongated seeds.

Two-wing silverbell is an excellent plant for a garden with mesic acidic soil. It is attractive as a multi-trunk shrub or single-trunk tree. It should do well in a woodland edge or a sunny urban setting with supplemental watering until well-established. Halesia diptera var. magniflora is available in the nursery trade and has larger and more numerous flowers. Flowering is better in sunny sites. Green fruits may be eaten by wildlife.

A second species of Halesia occurs in Arkansas, the common, mountain, or four-wing silverbell (Halesia tetraptera) of the Interior Highlands, which matures to 40 feet tall. It has sometimes been called Carolina silverbell (Halesia carolina) in Arkansas, although some authorities consider this more restricted in range farther to the southeast, from Mississippi to South Carolina and Florida, and differing in corolla size, degree of style and anther exsertion, fruit shape, and width of fruit wings. Two-wing silverbell can be distinguished from four-wing silverbell by 1) the pubescent abaxial surface of the corolla lobes and style rather than glabrous, 2) stamens 8± instead of 15±, 3) petals united only at the base as opposed to most of their lengths, and 4) fruit with 2 prominent wings instead of 4. The two species of the other genus of the Storax family in Arkansas, American snowbell (Styrax americanus) and big-leaf snowbell (Styrax grandifolius), are shrubs with slightly similar-looking leaves and bell-shaped white flowers. However, the snowbells have 5-lobed corollas, shorter petioles, and non-winged round fruits.

* The 376-page book can be read at https://library.si.edu/digital-library/book/vegetablestatick00hale

Article and photographs by ANPS member Sid Vogelpohl

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