Japanese Hop

Humulus japonicus Siebold & Zucc.
Hemp family (Cannabaceae)

Origin: Temperate Asia (China, Japan, Korea, Taiwan and the Russian Federation) and tropical Asia (Vietnam)

Background
Japanese hop was originally imported to the U.S. in the late 1800s for use as an Asian tonic and ornamental vine. It is still sold for these purposes today. Common hop (Humulus lupulus) contains bitter acids and essential oils used for flavoring and as preservative in beer. The chemistry of Japanese hop is less desirable for that purpose.

Chris Evans, River to River CWMA

Leslie J. Mehrhoff, Univ. of CT

Distribution and Habitat
Japanese hop occurs in scattered locations from Nebraska to Maine to Georgia and is most common in the Northeastern U.S. and eastern Canada. It has been reported to be invasive in natural areas in Connecticut, Delaware, Indiana, Maryland, Pennsylvania, Virginia, and the District of Columbia. Japanese hop prefers plentiful sunlight and moisture, rich exposed soil, and is most commonly found along stream banks and floodplains. Growth is less vigorous in shade and on drier soils, but it can grow in disturbed areas with fairly moist soils, including roadsides, old fields, and forest edges. In milder climates, it can survive the winter.

Ecological Threat
Japanese hop can spread to cover large areas of open ground or low vegetation including understory shrubs and small trees. Many thousands of hop plants per acre may be produced, eventually blanketing the land and vegetation. The vines grow rapidly during the summer, climbing up and over everything in their path and can form dense mats several feet deep, blocking light to plants underneath. Hop vines also twine around shrubs and trees causing them to break or fall over. It is invasive in riparian and floodplain habitats where it displaces native vegetation, prevents the emergence of new plants, and kills newly planted trees installed for streamside habitat restoration. Hop can quickly cover small trees hiding them from view, preventing mowing and obstructing herbicide applications.

Description and Biology

• Plant: herbaceous annual, twining, shallow-rooted vine that can climb to heights of ten or more feet with the help of rough-textured stems covered with short, sharp, downward pointing prickles that can be very irritating to the skin.
• Leaves: leaves are rough-textured, paired, simple, palmate (like a hand) with typically 5-7 lobes; leaf margins are toothed.
• Flowers, fruits and seeds: flowering occurs in July and August; male and female flowers are borne on separate plants; male flowers are very small, greenish yellow and occur in branched panicles; female flowers are in pale green, plump, drooping, cone-like structures with overlapping scales that become ‘hops’; hop scales and the seeds are covered with yellow glands; seeds are about 1/8 in. in diameter, roundish with a blunt tip, and light brown with darker specks; seeds mature through September.
• Spreads: by seed which begins to germinate in early spring, but new plants may continue to emerge as the season progresses if sunlight and moisture are available; seeds are dispersed by animals (including people), machinery and floodwaters.
• Look-alikes: native common hop (Humulus lupulus) looks very much like Japanese hop but it is usually 3-lobed or unlobed; native bur cucumber (Sicyos angulatus) lacks prickles, has tendrils, and the leaves have much less pronounced lobes.

Prevention and Control
Do not purchase, plant or transplant this species. Vines can be hand-pulled wearing gloves, long-sleeves and long pants and shoes to avoid injury from the prickles. Systemic herbicides are most effective (see Control Options).

Native Alternatives
Native grapes (Vitis), Virginia creeper (Parthenocissus quinquefolia) and coral honeysuckle (Lonicera sempervirens).