In many ways an attractive vine, but a very invasive one, and all the harder to get rid of because it is covered with sticky prickles. It can cover a huge area quickly, and seems to be found more and more commonly in the Pittsburgh area. Easily mistaken at first glance for a wild cucumber, but distinguishable by the greenish (rather than pure white) male flowers and the deeply five-to-seven-lobed (rather than more shallowly five-lobed) leaves.
Male flowers and female flowers are borne on separate plants; the female flower clusters are similar to the ones on the familiar domestic hops (H. lupulus) used in making beer. (Japanese hops are said to be poor for beer-making.) The male flowers, which stand up above the vines, are the ones you will notice in a large patch.
These vines were growing in Bird Park in Mount Lebanon, where they were blooming in the middle of September.
This plant had not yet commonly escaped when most of the standard references were written, but Britton’s Manual of the Flora of the Northern States and Canada includes it in an appendix:
HUMULUS L. (See Appendix.) Herbaceous perennial rough vines, with broad opposite thin petioled palmately veined leaves, lanceolate membranous stipules, and dioecious axillary flowers, the staminate panicled, the pistillate in ament-like drooping clustered spikes. Staminate flowers with a 5-parted calyx, the segments distinct and imbricated, and 5 short erect stamens. Pistillate flowers in 2′s in the axil of each bract of the ament. consisting of a membranous entire perianth, clasping the ovary, and 2 filiform caducous stigmas. Fruiting aments cone-like, the persistent bracts subtending the compressed ovate achenes. Endosperm fleshy. Embryo spirally coiled. [Name said to be the diminutive of the Latin humus, earth.] Two species, the following [H. lupulus] widely distributed through the north temperate zone, the other [H. japonicus, our current subject] native of northeastern Asia.
Humulus Japonicus Sieb. & Zucc. Japanese Hop. A twining vine, similar to the Common Hop, the leaves deeply pedately 5-7-cleft. Pistillate aments few-flowered, their bracts and bractlets deltoid, acuminate, hispid-pubescent at least on the margins, not glandular. In waste ground, Conn, to D.C. Introduced from Japan. Aug.-Sept.