1. Humulus japonicus Siebold & Zucc. (Japanese hops)

Pl. 333 f, g; Map 1419

Plants annuals, with taproots. Stems 0.5–5.0 m or more long, with dense, 2-armed hairs on the ridges, rough and prickly to the touch, minutely hairy or glabrous between them. Leaves with petioles 4–20 cm long, mostly longer than the blades, usually sparsely to moderately pubsecent with stiff, 2-armed hairs. Leaf blades 3–15 cm long, 4–18 cm wide, broadly ovate to depressed-ovate in outline, with 5–9 relatively deep palmate lobes (except on very young stems), the margins with stiff, bulbous-based, prickly hairs, the upper surface sparsely roughened with stiff, bulbous-based, prickly hairs, the undersurface roughened with stiff, spreading, bulbous-based, prickly hairs mostly along the veins and with yellowish sessile glands. Staminate panicles 15–30 cm long, 2–4 cm wide. Pistillate spikes 1–2 cm long at flowering, elongating to 1.5–3.0 cm long at fruiting, the bracts 7–12 mm long, ovate to broadly ovate, the margins densely hairy, the outer surface sparsely to moderately hairy, not glandular. Sepals 2–3 mm long, lanceolate, hairy. Stamens with the anthers lacking glands. Fruits 3–4 mm long, 2.5–4.0 mm wide, the surface smooth, light brown to yellowish brown, the persistent calyx often darker-mottled. 2n=20. July–October.

Introduced, widely scattered, locally common in some counties along the Missouri and Mississippi Rivers (native of Asia; introduced in the eastern U.S. west to North Dakota, Kansas, and Arkansas; Canada). Bottomland prairies, bottomland forests, and banks of streams and rivers; also ditches, roadsides, railroads, and moist, open, disturbed areas.

The bracts of the conelike fruiting structures of H. japonicus do not possess glands, and consequently the species is of no use in brewing. It has occasionally been grown as an ornamental climber (variegated forms exist), but it can spread aggressively in the garden. It appears to be increasing its range in the floodplains of some streams and rivers and has become a problem weed in some constructed or restored bottomland prairies.