Humulus scandens
(Japanese hop)

Humulus scandens is an annual or occasional perennial climbing or trailing vine, native to East Asia. It has been introduced to Europe and North America where it has proved to be invasive, covering open ground or low vegetation, eventually blanketing the land and vegetation, smothering small trees and having significant impacts on native plant biodiversity and ecosystem services especially in riparian areas. The pollen of H. scandens is allergenic. It is not easy to clear and considering that it may be further introduced as an ornamental, it is considered a high risk invasive species in temperate riverside habitats.

Humulus scandens is native to eastern Asia from the Russian Far East in the north, to northern Vietnam. It has been introduced as an ornamental in both Europe and North America where it is becoming an invasive alien species in several regions.

In North America, it is most abundantly established in New England, the mid-Atlantic States and some areas of the Midwest that include Ohio, Indiana, Illinois, Missouri, Iowa, Nebraska and Kansas (NatureServe, 2017). The establishment of H. scandens is more scattered in the northern Midwest (Michigan, Wisconsin, Minnesota, North and South Dakota) and the south-east (Kentucky, Arkansas, Tennessee, North and South Carolina, Georgia and Alabama). It is not yet established in the most southerly states (Florida, Louisiana, Mississippi, Oklahoma and Texas) or west of the Great Plains region. In total, H. scandens has been reported in 31 US states (NatureServe, 2017; Flora of North America Editorial Committee, 2020). In Canada, it is present in Ontario and Quebec (USDA-NRCS, 2019).

Humulus scandens was included in the EPPO Alert List in 2007 and subsequently transferred to the List of Invasive Alien Plants in 2012. In 2016, H. scandens was identified as a priority for risk assessment within the requirements of Regulation 1143/2014 (Branquart et al., 2016; Tanner et al., 2017). A subsequent pest risk analysis concluded that H. scandens carries a high phytosanitary risk to the endangered area (EPPO, 2018) and was added to the EPPO A2 List of pests recommended for regulation.

In Italy, H. scandens is included in the Lombardy region blacklist according to the Regional Act No. 10 of March 2008 (Disposizioni per la conservazione della piccola fauna e della flora spontanea) and is also included in the Piedmont region blacklist according to DGR No. 23-2975 of February 2016 (EPPO, 2018).

In the USA, H. scandens is considered a noxious weed; in Connecticut it is categorized as ‘potentially invasive, banned’ while in Massachusetts it is ‘prohibited’ (USDA-NRCS, 2019).

In both its native and introduced range, H. scandens is a plant of riversides, including loose, bare surfaces of alluvial bars formed by temporary floods (Balogh and Dancza, 2008; Fried et al., 2018), ruderal areas with no dry seasons and hydrophilous tall herb fringe communities of plains and of the montane to alpine levels.

H. scandens invades open disturbed areas such as roadsides and disturbed riverbanks, as well as open woodlands, prairies, herbaceous wet meadows and floodplain forest communities, indicating that it has some shade tolerance (NatureServe, 2017).

Genetics

The chromosome number of H. scandens is 2n = 16 = 14 + XX in female plants and 2n = 17 = 14 + XY1Y2 in male plants and sex is determined by the ratio of X chromosomes to autosomes sets (A), with an X:A ratio of 1.0 resulting in a female and a ratio of 0.5 resulting in a male, with the Y chromosomes being dispensable (Alexandrov et al., 2012). The closest relative, the common hop H. lupulus has the same sex determination system as in Japanese hop (X/A) but it differs in the chromosome number (2n = 20 in both female and male plants) and sex chromosome systems (XX/XY).

Reproductive Biology

Flowers are mainly wind pollinated but are frequently visited by honeybees (Balogh and Dancza, 2008).

In its native range in China and North and South Korea, H. scandens flowers from August to October (Park et al., 1999). In the introduced range in Europe, flowering occurs from July to September (Balogh and Dancza, 2008). In Hungary, fruits ripen from the middle of August and seeds remain viable for approximately 3 years (Krauss, 1931). Seeds germinate in early spring, from as early as February in southern France (Pinston, 2013), to mid-April in Hungary (Balogh and Dancza, 2008) which is consistent with observations from the native range with emergence from February to early May, peaking in March with 32-37 seedlings/m² (Masuda and Washitani, 1990), compared to a mean of 38 seedlings/m² in the south of France (Fried et al., 2018).

Longevity

H. scandens is a dioecious herbaceous annual or occasional perennial. 

Environmental Requirements

Humulus scandens is principally a temperate species, though the native range extends south to southern China and northern Vietnam thus indicating that it is at least tolerant to warmer, sub-tropical climates. However, no detail on temperature, rainfall and soil preferences were identified.

Zheng et al. (2004) reviewed natural enemies feeding on H. scandens in its native range, including nine fungi known to infect Humulus species, with only one, Pseudocercospora humuli, that may be specific to H. scandens. Of the 27 insects associated with Humulus spp., Epirrhoe sepergressa and Chytonix segregata were reported to have a narrow host range (Zheng et al., 2004) and H. scandens was considered as one of the two main hosts of Apolygus lucorum (Heteroptera: Miridae) (Lu et al., 2012). The granivorous beetle Amara gigantea was observed to feed particularly on H. scandens seed in Japan (Sasakawa, 2010).

A survey of insects on H. scandens in France in 2013-2014 noted Thrips urticae, Oxythrips ulmifoliorum, Dendrothrips saltator [Dendrothrips saltatrix],Paratettix meridionalis, Aphididae and Collembola spp., as well as eggs of Melanostoma sp., larvae of Altica sp., Chromatomyia horticola and Carabidae, Chrysomelidae, Cicadellidae, Theridiidae and Thripidae (EPPO, 2019). Young leaves were also observed being eaten by snails including Cernuella virgata and Oxyloma elegans. Zonate leaf spot caused by Hinomyces moricola was first reported on H. scandens in wetlands in Yeongdong, Korea (Cho et al., 2013). Initial symptoms included greyish-green to greyish-brown spots without border lines; as the lesions enlarged, they coalesced leading to leaf blight.

Cuscuta campestris is an alien plant parasite that is also frequently observed on H. scandens in riparian habitats, where it is also attacked by fungi including Oidium sp., Fusarium sp. and Cladosporium sp.. H. scandens is also a host of the aphid Phorodon humuli (EPPO, 2019).

Medicinal, pharmaceutical

• Traditional/folklore

Ornamental

• garden plant

Humulus scandens is closely related to the commercial hop (H. lupulus), but differs in chromosome number in both male and female plants and can be readily differentiated using molecular markers (Alexandrov et al., 2012).

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Physical/Mechanical Control

Humulus scandens can be hand-pulled any time of the year, with the most effective time being late spring or early summer before seed formation. If the vine has climbed a tree, below-ground removal only is required. If seeds are present during removal, avoid movement off-site unless material can be transported without spreading seed. Three years of removal is typically needed to eradicate an infestation and exhaust the seed bank (Meyers-Rice, 2014). Mowing or cutting many times a year can control new populations after 3 years, but well-established populations will only be suppressed. The use of a mower that bags cut material is recommended or the cut material should be raked and bagged after mowing. Cut material should be disposed off in a landfill or burnt to avoid spreading seeds to other areas.

Meyers-Rice (2014) recommends that if H. scandens is growing as an annual, attempts should be made to remove as much of the rootstock as possible as re-sprouts could occur from both the rootstock and the leafy portions of the plant and pulled plants should be removed or left where they cannot re-root. If the plants are growing as perennials, they should be pulled either during May-June when the rootstock is most exhausted and small, or just prior to flowering when the rootstock should be plump and robust. Burning the plants would be unwise since the fire might be carried into the crowns of trees.

Biological Control

There are no known biological control agents for H. scandens. However, the US Forest Service has been investigating natural enemies of plants of Asian origin that are invasive in the USA (EPPO, 2019) and have identified two moths (Epirrhoe sepergressa and Chytonix segregata) and one fungus (Pseudocercospora humuli) as potential natural enemies and will continue research on those species. However, in view of the commercial value of the very closely related common hop (H. lupulus) used in the manufacture of beer, it would appear very unlikely that any species-specific agent could be found and failing that, that any release would be authorized noting the potential impacts.

Chemical Control

Chemical control using herbicides has been shown to be effective at controlling infestations of H. scandens (Panke and Renz, 2013). However, any chemical application near water bodies may require specific permission from national regulatory authorities.

Meyers-Rice (2014) noted that when farmers in the USA wish to eradicate H. lupulus, they spray with glyphosate and this should also be effective against H. japonicus. If the plant is behaving as an annual, spot applications of glyphosate any time during the year (prior to flowering) should damage the plant enough so it will not be able to flower and set seed. If it is growing as a perennial, the best time to apply glyphosate would be when the rootstock is most rapidly accumulating carbohydrates (July-September), whereas applying glyphosate earlier in the year would not be effective as it would not be translocated into the rootstock.

The cost of control in France was estimated as follows: (i) manual control by hand pulling at €10.4/m², (ii) mechanical control by grinding at €1.1/m² and (iii) mechanical control by mowing the cheapest at €0.6/m² (EPPO, 2019). Based on the area (20,000-30,000 m²) where the species was recorded between 2012 and 2015 on the Gardon River and with an average cost of €6/m² for management and supplementary costs including travel and surveying time and miscellaneous and unexpected expenses, the estimated cost of managing all populations was €580,000 over 2 years.

03/01/20 Original text by:

Nick Pasiecznik, Consultant, France