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Humulus japonicus (Cannabaceae)

Japanese hop

 

Humulus japonicus was added to the EPPO Alert List in 2007 and transferred to the List of Invasive Alien Plants in 2012.

 

Why
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.

Leslie J. Mehrhoff
University of Connecticut, Bugwood.org

Humulus japonicus

Geographical distribution
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. Its presence was reported for the first time in 2008 in Serbia.
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.

Morphology
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.

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.

Impacts
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. Studies have shown that the pollen of H. japonicus is allergenic, and this species is considered as one of the important causes of pollen allergy in Korea and China.

Leslie J. Mehrhoff
University of Connecticut, Bugwood.org

Humulus japonicus

Control
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.

Regulatory status
In the USA, this plant is prohibited in Connecticut and Massachusetts.

Sources
Park J W, Ko S H, Kim C W, Jeoung B J & Hong C S (1999) Identification and characterization of the major allergen of the Humulus japonicus pollen - Clinical and Experimental Allergy 29, 1080-1086.
Savic D, Anackov G & Boza P (2008) New chorological data for flora of the Pannonian region of Serbia - Central European Journal of Biology 3, 461-470.
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.
INTERNET
Wisconsin Department of Agricultural Resources Website. http://www.dnr.state.wi.us/invasives/fact/japanhops.htm
References for geographical distribution: Germplasm resource information network (GRIN):
http://www.ars-grin.gov/cgi-bin/npgs/html/taxon.pl?19414
USDA Plant database: http://plants.usda.gov/java/profile?symbol=HUJA

 

EPPO RS 2006/174
Entry date 2007-10 / 2012-05

 

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