Japanese hops (Humulus japonicus)

Berries and leaves of Amur cork tree

 
 

Japanese hops (or hop) is an herbaceous (non-woody), annual vine that can grow up to 35 feet long. It can grow abundantly along the edges of waterways and cover other vegetation. While it is related to the common hops used to brew beer, Japanese hops is not used for brewing and may carry a mildew that can infect common hops. Japanese hops has limited distribution in Minnesota and is Minnesota Department of Agriculture Prohibited Noxious Weed on the Eradicate List.

Caution: Japanese hops leaves and stems have bristly, hooked hairs that can cause skin irritation and blisters.

Description

Appearance

Japanese hops is an herbaceous, annual vine that can grow up to 35 feet long.

Leaves and Stem

Japanese hops leaves have long leaf stalks and come off the stem in pairs opposite one another. The leaves are somewhat hand-shaped and most have five to seven lobes, but some leaves may have three or nine lobes. Leaves have serrated edges. Japanese hops leaves and stems have bristly, hooked hairs that can cause skin irritation. Japanese hops vines climb and twine over other plants, but it does not have any long, thin tendrils that curl around others plants or objects as some other vines do.

Flowers

Japanese hops has male and female flowers on separate plants. Flowers can vary in color from pale green to white to reddish. The male flowers are drooping and arranged in branched clusters. The female flowers look more cone-like with bracts with hairs and have the "hops" look that people may be familiar with from common hops. Japanese hops flowers in July and August.

Seeds

The female flowers produce seeds that mature in September.

Roots

Japanese hops has fibrous roots. Roots can resprout if the above ground part of the plant is pulled or cut, but the roots are not removed.

Biology

Japanese hops is an annual vine meaning it germinates, grows, flowers, sets seed, and dies all in one year. It has male and female flowers on separate pants. The female plants produce seeds that are easily spread by water. Japanese hops prefers habitat with full sunlight and moist soils such as along the banks of waterways.

Origin and Spread

Japanese hops is native to eastern Asia. It was introduced to North America in the 1880s as an ornamental plant. It has since spread from where it was planted to natural areas such as along rivers and lakes. While Japanese hops is related to common hops, it is not use for beer brewing as it does not contain the same desirable oils that common hops have. Japanese hops is found in the northeastern United States. There are limited amounts of Japanese hops in Minnesota, with populations mainly known in southeastern Minnesota. Due to its impacts and its limited distribution in the state, it is a Minnesota Department of Agriculture Prohibited Noxious Weed on the Eradicate List with the goal to treat populations that are found to prevent the plant from spreading to new areas.

Refer to EDDMapS Distribution Maps for current distribution.

Don't be fooled by these look-alikes

  • Common hops, Humulus lupulus (native) (native lookalike on page 58 in Minnesota Department of Transportation Noxious Weed Booklet) – Most common hops leaves have only three lobes, although occasionally leaves will have five lobes while most Japanese hops leaves have five or seven lobes (although occasionally three or nine).
  • Wild cucumber, Echinocystis lobata (native) and bur cucumber, Sicyos angulatus (native) – Wild cucumber and bur cucumber are native vines that can be very noticeable and abundant in some years. They have bright, light green leaves that grab people's attention. They have white flowers, spiny fruits, and their leaves have a star or maple-leaf shape. Wild cucumber and bur cucumber both have long, thin, green tendrils that tightly curl around other plants or structures they encounter while Japanese hops does not have tendrils.
  • Wild grape, Vitis riparia (native) – Wild grape leaves come off the stem one at time while Japanese hops leaves come off the stem in pairs opposite on another.
  • Virginia creeper, Parthenocissus quinquefolia (native) and woodbine, Parthenocissus vitacea (native) – Their leaves are made up of five leaflets that are arranged in a circle while Japanese hops leaves are not divided into leaflets.
 
Regulatory Classification

This species is a Minnesota Department of Agriculture Prohibited Noxious Weed on the Eradicate List meaning that the above and below ground parts of the plant must be destroyed. Additionally no transportation, propagation, or sale is allowed.

Threat to Minnesota
  • Japanese hops forms dense cover and overtops other plants, sometimes forming mats four feet thick. This can negatively impact native plants and reduce tree regeneration. Habitats along waterways are most at risk as Japanese hops spreads readily by water and does well in edge habitats by water. Potential impacts to streambank erosion are not well studied.
  • Japanese hops can carry a species of powdery mildew. Researchers are studying the genetic diversity of the mildew carried by Japanese hops and how that may affect commercial common hops crops grown for brewing beer.
  • Japanese hops leaves and stems have bristly, hooked hairs that can cause skin irritation and blisters in humans.
What you should do

One way that invasive plant seeds and fragments can spread is in soil. Sometimes plants are planted purposefully. You can prevent the spread of invasive plants.

PlayCleanGo: Stop Invasive Species in Your Tracks

  • REMOVE plants, animals & mud from boots, gear, pets & vehicle.
  • CLEAN your gear before entering & leaving the recreation site.
  • STAY on designated roads & trails.
  • PLANT non-invasive species.
Native Substitutes
  • Common hops (Humulus lupulus) - Non-native cultivars are also sold, so first determine if your purpose is native plant restoration, ornamental use, or hops for beer production. Then ask the seller about the plant so you know what it is before you purchase it and if it is right for your purpose.
  • Virgin's bower (Clematis virginiana)
  • Wild grape (Vitis riparia)
 
Control Methods

Caution: Japanese hops leaves and stems have bristly, hooked hairs that can cause skin irritation and blisters so be sure to use protective clothing to cover yourself and prevent injury.

Mechanical control can be done by digging or pulling the plant by hand or with equipment such as a shovel. Pull seedlings and small plants in May and June before they become large and have stems that are more entangled with other plants and have more extensive root systems. Try to remove as much of the root as possible, as plants can resprout from remaining roots. Follow Minnesota Department of Agriculture noxious weed disposal guidance for the pulled plants.

Mowing frequently from May to July may reduce flowering and seed set. Seed generally forms in August so do not mow after that point.

Herbicide control can be done using systemic herbicides which are taken up by plants and move within the plant, which can kill leaves, stems, and roots. Apply herbicide containing glyphosate to the leaves of Japanese hops. May through August are the most effective times for herbicide application. Since Japanese hops is an annual plant that will die after it set seeds, the goal is to prevent plants from flowering and setting seed. In some sites, pre-emergent herbicides applied in March or April before plants germinate may be appropriate. Consult with the Minnesota Department of Agriculture if you think this may be useful for your site.

Reporting

Report new occurrences by submitting a report through EDDMapS Midwest, emailing Arrest The Pest, calling Arrest the Pest (1-888-545-6684), or contacting your local county agricultural inspector.

Resources