Humulus japonicus: an emerging invader
Humulus japonicus (= H. scandens) (Cannabaceae) is an annual climber vine originating from East Asia. Its common name in English is ‘Japanese hop’. In Europe, it is only recorded in France, Hungary and Italy where it showed invasive behaviour in wetlands. Because distribution is still very limited, this plant can be considered a new emerging invader.
H. japonicus has stems growing up to 5-8 m, leaves are opposite, 5 to 6 cm long, palmate with 5 to 7 lobes. Petioles tend to be longer than the length of leaves. Stems and leaves have rough hooked hairs. Male and female flowers are on separate plants and bloom from mid to late summer. Male flowers are 5 mm in diameter, female inflorescence 15-20 mm, pale green.
EPPO Region: the plant is recorded in southern France, western Hungary and northern Italy (Tutin et al., 1964-1980). The plant is known to be native in the Russian Federation. It is recorded as invasive in France, Hungary and Italy.
In France, the plant was observed as naturalized in Russan in the Gard Department in September 2004 by Sarah Brunel and Jean-Marc Tison in a degraded riparian habitat near the Gard river. It is thought to have escaped from a garden and has colonized more than 500 m², covering almost 100% of the soil layer and was less abundant on another 500 m². Both female and male plants were found. The plant has thereafter been observed as naturalized in other stations along the Gard river.
North America: USA (Alabama, Delaware, District of Columbia, Georgia, Illinois, Indiana, Kentucky, Maine, Maryland, Massachusetts, Michigan, Minnesota, Missouri, Nebraska, New Hampshire, New Jersey, New York, North Carolina, North Dakota, Ohio, Pennsylvania, Rhode Island, South Carolina, South Dakota, Vermont, Virginia, West Virginia, Wisconsin).
Asia (native): China, Japan, North Korea, South Korea, Taiwan, Vietnam
Biology and ecology
H. japonicus reproduces by seeds which are dispersed by wind and water. Preferring moist soils, it can form dense stands in floodplains and along stream banks and lake shores, but can also thrive in disturbed areas such as roadsides and urban lots. It can be found in full sun or shade.
H. japonicus is a rapidly-growing vine that can form dense stands and displace native vegetation by out-competing for essential resources. This plant can be a threat to the environment when invading riparian habitats.
Plants can be hand-pulled and removed from the invaded area before seeds ripen. The herbicide glyphosate can be used on foliage before plants flower.
In the USA, this plant is prohibited in Connecticut and Massachusetts.
Tutin TG, Heywood VH, Burges NA, Moore DM, Valentine DH, Walters SM and Webb DA (1964/80) Flora Europeaea, Vol 1-5. Cambridge University Press, Cambridge.
Wisconsin Department of Agricultural Resources Website
References for geographical distribution:
Germplasm resource information network (GRIN):
USDA Plant database: http://plants.usda.gov/java/profile?symbol=HUJA