Humulus japonicus, an emerging invader in Hungary
Humulus japonicus (Cannabaceae, EPPO Alert List) is a fast growing annual climber vine originating from East Asia. In North America and Europe it has been planted over a century as an ornamental and its occurrence in the wild is thought to be from garden escapes. In Hungary and Italy, the plant is naturalized and is becoming invasive. The plant is casual in Austria, the Czech Republic, southern France, Germany, Switzerland, United Kindgom, and Ukraine. In Hungary, the first record in the wild of the species dates from 1894. It is noteworthy that the earlier occurrences of the plant are only confirmed by recent records in a few cases.
The main pathway of entry for this species is voluntary introduction as an ornamental plant. The variegated form of the species H. japonicus var. foliis variegatis is very popular as an ornamental plant. H. japonicus was at first considered disappointing by gardeners because of its uncertain germination in greenhouses or pots. Gardeners then rapidly discovered that seeds should be sown directly into the fields. Additionally, some biotechnological projects might consider the plant as a new genetic source for H. lupulus breeding, cultivated for producing beer. Korean investigations also indicated that H. japonicus could be a good fodder crop for ruminants, and in China, experiments have highlighted that the plant acts as a botanical insecticide.
In a sunlight area, single H. japonicus plant can cover up to 50 m². Flowers are wind pollinated and flowering time is July to September. Fruits ripen from mid-August and disappear from the seed bank within three years.
In Hungary, the plant occurs in moderately continental, wet habitats characterized by being over-fertilized and rich in nitrogen. H. japonicus is a strong competitor and uses its vining strategy to produce a dense, heavy canopy, even in vigorous tall herbaceous communities. H. japonicus out competes alien invasive species such as Helianthus tuberosus (Asteraceae, EPPO List of IAP) and Impatiens glandulifera (Balsaminaceae, EPPO List of IAP). The information on pathogens of H. japonicus is limited. Natural infection of the plant has been observed by Hop latent virus (Carlavirus) and Humulus japonicus virus (Ilarvirus).
In terms of impacts, the species could be a reservoir of the viruses that may affect H. lupulus. H. japonicus is a major weed in orchards in Korea. It also impacts human health by provoking pollen allergy and dermatitis. Both in its native range (Japan) and exotic range (the USA), H. japonicus is considered to be a serious weed in riparian areas and floodplains rich in nitrogen where it decreases the diversity of plant communities. Although H. japonicus is present in an increasing number of localities in Hungary (44 invaded sites reported as to 2007), it is rarely considered a threat as it is confused with the indigenous H. lupulus.
The species has occurred as a casual alien species in Europe since the late 19th century, only naturalized in a few places, and has not proved to be invasive outside Hungary. This species is a good candidate for national measures such as a Code of Conduct on Invasive Alien Plants and Horticulture, as well as for surveillance and early warning systems.
Balogh L, Dancza I (2008) Humulus japonicus, an emerging invader in Hungary. In Tokarska-Guzik B, Brock JH, Brundu G, Child L, Daelher CC ; Pyšek P (eds) Plant Invasions: Human perceptions, ecological impacts and management. Backhuys Publishers, Leiden, The Netherlands. Pp 73-91.