Water chestnut (Trapa natans)
Photo credit: Anna/Flickr Creative Commons

Japanese Hop

(Humulus japonicus)

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If you believe you have found this species anywhere in Pennsylvania, please report your findings to iMapInvasives by submitting an observation record.

Species at a Glance

Japanese hop is a fast-growing, herbaceous, annual climbing vine and member of the hemp family. It can climb to heights of 3 m (10 ft) or more with the help of many small hooked prickles that cover the stem and can cause irritation to bare skin.

Identification

Leaves: Opposite, palmate leaves are approximately 5-10 cm (2-3.9 in) in length and have rough, serrated edges. They are divided into 5-9 lobes with downward pointing prickles and down-curved bracts at their base.

 

Flowers: Because they lack petals and are green in color, the flowers can be inconspicuous. They bloom in clusters about 5 cm (2 in) in length in early to mid-summer.

 

Fruits/Seeds: Green hops produced by female plants contain oval, yellowish-brown seeds, which can remain viable for up to three years.

 

Stems/Roots: Rough stems, which are covered in tiny hooked hairs, can reach 2.5-11 m (8-35 ft) in length and help the plant climb.

Similar Species

May be confused with common hop (Humulus lupulus), which typically has three-lobed leaves and the upper leaves occasionally lack lobes altogether. The leaf stems of common hop are also shorter than the leaves. The best way to distinguish Japanese hop from other species is the down-curved bracts and sharp prickles.

Habitat

Japanese hop grows dense, nearly continuous stands in floodplain areas along river and stream banks, roadsides, open fields, and woodlands; or wherever the soil is moist. It can grow in full sun or shade in sandy, clay, acidic, or neutral soil.

Spread

Numerous small seeds disperse by wind and water movement, allowing Japanese hop to spread to new locations. Seeds can remain viable for up to three years. It can also reproduce vegetatively, starting entirely new populations off of small plant fragments.

Distribution

Native to Japan and eastern China, Japanese hop was introduced to the United States as an ornamental garden plant and is now growing throughout the Eastern and Midwestern United States. It can be found in all Mid-Atlantic states.

 

Note: Distribution data for this species may have changed since the publication of the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016), the source of information for this description.

Environmental Impacts

Japanese hop can form dense, almost solid stands that outcompete native vegetation. It can be removed by hand-pulling before the seeds ripen (August through September), but protection is needed for the skin because irritation and blistering can occur from the hooked hairs covering the vines.

Video
Note

Information for this species profile comes from the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016).