DNR: Japanese Hops Impressive but Undesirable Invasive Plant | Environment
The plant Japanese hops might sound like another great variety for brewing beer or adding as a new ornamental to quickly spruce up the garden, state invasive plant experts say. But don’t be fooled.
"While Japanese hops is impressive looking, it’s not at all desirable for brewing or landscaping, and it’s especially not desirable for our forests or stream-banks," says Kelly Kearns, a conservation biologist with the Department of Natural Resources and an invasive plant expert.
Not to be confused with its more "brewable" relatives, Japanese hops cannot be used for home brewing because it lacks the oily resins that give hops their unique flavor and aroma, Kearns says. And while its vine-like growth appeals to many people as an ornamental plant, this species’ uncontrollable nature and irritating hairs make it a highly invasive plant across the Eastern United States, and Wisconsin’s forests are severely threatened by its introduction.
"This aggressive vine climbs over vegetation and forms thick monocultures of tangled vines up to several feet deep," Kearns says. "They twist up and topple trees, crowd-out desirable species, and inhibit forest regeneration."
The first record of Japanese hops (Humulus japonicus) in Wisconsin was from Crawford County, but Kearns says the species is rapidly spreading across the Wisconsin Driftless area with heavy infestations in Grant, Crawford, Vernon and Lafayette counties. New reports also are being received in neighboring counties as well as in other parts of the state.
Kearns notes that some Japanese hops plants are reported to grow over 35 feet in one year! Plants reproduce by seeds, which mature and disperse in early fall, providing a seed bank for germination the following spring. Seeds can remain viable in the soil for over three years.
"This plant loves rich soils and sunlight, making river corridors a favorite habitat to invade," Kearns says. "When populations go to seed, floating seeds are carried down-stream colonizing new areas."
Typically considered an annual, Kearns says the vines grow rapidly in the summer warmth and are killed off by winter chills, with new plants emerging from the previous year’s seed dispersal. However, there are several sites in Grant County where land managers now suspect this species is overwintering. Infestations can spread as far as people, water and animals travel.
State working to control invasive
Efforts are underway across the state to control the species. Under the Invasive Species Rule –NR 40, Wis. Adm. Code – Japanese Hops is prohibited through most of the state and listed as restricted in counties where populations are abundant --Crawford, Grant and Vernon counties. In these counties, natural resource managers, landowners and concerned citizens are fighting to keep this plant out of their forests and streams.
"Hand-pulling is effective, especially in smaller populations. For large infestations, continual mowing or cutting prevents seed production," Kearns says. "Plants grow rapidly, so frequent monitoring and re-cutting is needed."
Kearns also says that spraying leaves with a systemic herbicide, such as glyphosate, is also effective in spring. "There is still hope for controlling Japanese Hops," Kearns said. "Because it reproduces mainly by seed, a diligent regime of controlling the emerging seed bank will eventually reduce populations."
Be Alert! Report Japanese Hops
Visitors to river corridors should pay special attention for this species. Anglers, canoeists and recreationists are important "eyes on the water" and can make important contributions in early detection of this species. The following are some identification tips but Kearns stresses that people should exercise caution when working with this plant because it can be very irritating to the skin.
- Vibrant green leaves have five to nine lobes and are very rough to the touch.
- Distinguishing this species from common hops (Humulus lupulus) can be tricky at first, but there are a few defining characteristics. First is the petiole length. The petiole or leaf stem is much longer in proportion to the leaf size than the native hops. Often, the petiole length is as long as or longer than the leaf itself. Second, climbing stems have rough downward-facing barbs or hairs on the stem. Run your fingers down the vine and then carefully try back up against the prickly hairs.
- This aggressive vine climbs up anything it can twist around. The plant lacks tendrils and climbs structures by twisting. Most hop vines climb clockwise, but Japanese hops are said to twist counter-clockwise.
People who find this species are asked to report it. Collecting a specimen or taking detailed photographs of the petiole length and other diagnostic features is extremely important for confirmation. Submit a report online or send the DNR an email at firstname.lastname@example.org.