2. Parthenocissus tricuspidata (Siebold & Zucc.) Planch. (Boston ivy)

Pl. 580 b, c; Map 2715

Stems to 10 m or more long, the older stems often producing aerial roots. Young stems hairy. Tendrils short, with 3–7 branches, most of the tips with small circular adhesive discs. Leaf blades mostly simple and shallowly to deeply 3-lobed, usually toward the tip (rarely unlobed), occasionally a few of the largest leaves completely trifoliate, 3–15(–20) cm long, broadly ovate to obovate or obhemispheric in outline, broadly rounded or more commonly cordate at the base, the lobes tapered to sharply pointed tips, the margins sharply and finely to coarsely toothed, thickened and somewhat leathery, the upper surface shiny, glabrous, and dark green, the undersurface paler and often sparsely hairy along the main veins. Inflorescences wider than long to longer than wide, lacking a well-defined central axis, with 2(3) main branches at the tip of the stalk, each of these 1–3 times dichotomously (less commonly trichotomously) forked, the 10–90 flowers single or in small clusters at the branch tips. Fruits 6–8 mm in diameter. Seeds 3.5–4.0 mm long, the surface somewhat roughened or wrinkled, light brown to brown. 2n=40. May–July.

Introduced, known thus far only from the city of St. Louis (native of eastern Asia, introduced sporadically in the northeastern U.S. and adjacent Canada). Railroads and disturbed open areas.

Boston ivy has long been valued horticulturally as a cover for arbors, walls, and rocky areas. It was first reported as an escape in Missouri by Mühlenbach (1979).