1. Hydrangea arborescens L. (American hydrangea, smooth hydrangea, sevenbark)

Pl. 427 a, b; Map 1907

Plants sometimes colonial by rhizomes or stems that become prostrate and then root at the nodes. Stems 1–2 m long, erect to arching or spreading. Bark tan to grayish tan on older branches and peeling in long thin strips or sheets. Twigs light brown to brown, rarely reddish-tinged, sparsely to moderately hairy, the axillary buds small, with 3 or 4 scales visible, partially hidden by the U-shaped petiole bases. Leaves usually long-petiolate. Leaf blades 6–16 cm long, 3–12 cm wide, narrowly to more commonly broadly ovate or elliptic-ovate, tapered to a sharply pointed tip, mostly rounded to shallowly cordate (less commonly narrowed) at the base, the margins relatively coarsely toothed, the upper surface darker green, glabrous or sparsely hairy along the main veins, the undersurface light green, sparsely to densely hairy, sometimes only along the veins. Inflorescences terminal, much-branched, flat-topped to somewhat dome-shaped, umbellate panicles, with at least some of the peripheral flowers sterile or staminate and showier than the fertile flowers. Calyces of the fertile flowers of (4)5 sepals, these free, 0.3–0.6 mm long, triangular, glabrous, green; those of the sterile flowers of 3 or 4 sepals, these fused at the base, 6–12 mm long, broadly elliptic-ovate to angular-circular, glabrous, petaloid, white to greenish white. Corollas of 4 or 5 petals, these free, 1.0–1.6 mm long, oblong-elliptic, white, sometimes absent on sterile flowers. Stamens (8)10. Pistils 2 carpels, these united to the tip. Ovary completely inferior, 2-locular. Styles 2, the stigmas capitate to club-shaped. Fruits 2.0–2.5 mm long, broadly obconic to nearly globose, the surface with 8 or 9 prominent longitudinal ribs, dehiscing by a terminal slit developing between the styles. Seeds 0.6–0.8 mm long, oblong ellipsoid, more or less tapered at each end, the surface with several longitudinal ribs, brown, shiny. 2n=36. May–June.

Scattered in the Ozark and Ozark Border Divisions and Crowley’s Ridge (eastern U.S. west to Kansas and Oklahoma). Mesic upland forests, bases and shaded ledges of bluffs, and banks of streams, rivers, and spring branches.

Hydrangea arborescens was the first species described in the genus and was introduced into cultivation in England as early as 1736 (McClintock, 1957). Individuals within each subspecies with inflorescences consisting entirely of sterile flowers having enlarged petaloid sepals have been found rarely in various portions of the species range, including Missouri (f. acarpa H. St. John, f. grandiflora Rehder, f. sterilis (Torr. & A. Gray) H. St. John). Although they are of horticultural interest, these forms are not treated further here. The dried roots contain hydrangin, an alkaloid used medicinally as a diaphoretic and diuretic.

Hydrangea arborescens is quite variable in such characters as the relative presence or absence of sterile flowers, leaf size and shape, degree of pubescence, and hypanthium size, and several subspecies, varieties, and forms have been recognized (McClintock, 1957; Spongberg, 1972; Pilatowski, 1982). The present treatment follows that of McClintock (1957), who recognized three subspecies based upon differences in leaf pubescence patterns and trichome morphology, two of which grow in Missouri. The third taxon, ssp. radiata (Walter) E.M. McClint, is endemic to portions of the southeastern United States and differs in its leaf undersurfaces white or gray with dense matted hairs. Pilatowski (1982) noted that each of the three taxa has a distinctive flavonoid profile and maintains its morphological characteristics in the southern Appalachians where they grow together. He thus treated them as separate species. In our area, the distinctions between var. arborescens and var. discolor are not as clearcut, the two taxa occupy similar habitats in nearly the same geographic range, and intermediates are known.