What is Japanese Hop?

By: Celestine Duncan, TechLine Editor

By: Celestine Duncan, TechLine Editor

Figure 1: Japanese hop distribution in the United States and Canada. States and provinces shaded in blue indicate presence of the plant (USDA Plants Database).

Figure 1: Japanese hop distribution in the United States and Canada. States and provinces shaded in blue indicate presence of the plant (USDA Plants Database).

Japanese hop (Humulus japonicas) is an annual, climbing or trailing vine that is native to eastern Asia. The invasive plant was introduced to North America in the mid-to-late 1880s as an ornamental and possibly for medicinal use. Unlike common hop (Humulus lupulus), Japanese hop is not used in beer production.

Distribution and Susceptible Habitat

Most Japanese hop infestations occur in eastern states (Figure 1); however, the plant is spreading westward and is considered problematic in some Midwestern states. Hops thrive in full or partial sunlight, and prefer riparian sites, fencerows, meadows, and roadsides. The plant will readily invade disturbed sites, but can also colonize in-tact forest edges and fields.

Japanese hop is listed as banned in Connecticut, and prohibited in Massachusetts and Minnesota (USDA Plants and state weed lists).

Identification:

Japanese hop is a member of the hemp family. The vine has opposite leaves that are generally five-lobed, but can have from three (upper leaves) to nine lobes. Leaves have toothed margins (Figure2). Stems have downward pointing hairs that give the plants a rough feel and can cause skin dermatitis.

Male and female flowers occur on separate plants (dioecious). Male flowers are very small, greenish yellow and occur in branched panicles (flower clusters); female flowers are pale green, plump, drooping, cone-like structures with overlapping scales. Small seeds are distributed by wind, water, wildlife, livestock, equipment and vehicles.

Japanese hop seed remain viable in soil for at least three years. Seedlings germinate in early spring and vines can reach lengths of 10 to 35 feet in a growing season. The plant can grow one foot or more per day under optimum growing conditions.

Figure 2: Japanese hop leaf (A); flower (B); and vine covering low shrub and utility line (C). Leaf photo by Chris Evans, Univ Ill., Bugwood; flower and vine by Leslie J. Mehrhoff, Univ Conn., Bugwood.

Figure 2: Japanese hop leaf (A); flower (B); and vine covering low shrub and utility line (C). Leaf photo by Chris Evans, Univ Ill., Bugwood; flower and vine by Leslie J. Mehrhoff, Univ Conn., Bugwood.

Impact

Japanese hop forms dense infestations that out-compete and displace native vegetation. The vine germinates in early spring and develops mats of vegetation up to four feet thick, smothering existing vegetation including understory trees and shrubs.

Prevention and Management

Preventing introduction of Japanese hop to non-infested sites is critical. Do not plant seeds, and stop movement from infested sites by removing seeds from clothing, pets, vehicles and machinery.

Hand-pulling can be used to control small infestations. Multiple hand pulling events must be conducted during the growing season for at least three years to stop seed production. Plants that are blooming and/or have viable seed should be placed in a bag, removed from site and destroyed. Wear gloves and other protective clothing when handling Japanese hop because of prickly, hooked hairs.

Mowing can reduce seed production. However, vines re-grow quickly from cut stems so frequent cutting is necessary. Do not mow after seed set to reduce seed dispersal.

Foliar application of a systemic herbicide containing glyphosate or metsulfuron can be an effective tool for controlling Japanese hop. Apply in the early summer to prevent seed production. Glyphosate is non-selective and can cause injury to non-target plants that are contacted by the spray solution. Results of field trials conducted in Maryland compare the effectiveness of several different herbicides on Japanese hop control (Pannill and Cook). Be sure the herbicide is labeled for use on the site you are treating. Always read and follow the herbicide label.

References:

Pannill and Cook. Managing Japanese Hops – What We Have Learned. Online

Minnesota Department of Agriculture. Japanese hops. Online

Missouri Department of Conservation. Japanese Hop Online

USDA, NRCS. 2019. The PLANTS Database (http://plants.usda.gov) 26 March 2019). National Plant Data Team, Greensboro, NC 27401-4901 USA.