ANNAPOLIS, MD (June 4, 2007) - While it sounds like a boon to the neighborhood microbrewery, Japanese Hops is more likely
to be the bane of local land managers. This Asian vine, scientific name Humulus japonicus, is
not good for beer making, though it is reputed to have pharmaceutical properties. Originally
imported in the 19th century for use as a tonic in Asian medicine and as a climbing landscape vine,
it is still sold for these purposes today. However, as happens with many exotic plants, it
soon started escaping into the wild throughout the Eastern United States. Long present as an
occasional curiosity, in the last few years it has developed into one of the worst weeds along
the rivers and creeks in central and western Maryland. It outcompetes desirable native plants
and is particularly a problem in streamside tree planting sites, where it can overtop and smother
shrubs and young trees.
The seed of this annual vine begins to germinate in early spring, but new seedlings may
continue to emerge all summer if conditions allow. It likes plenty of sun and moist, rich
exposed soil, and is most commonly found along streambanks and floodplains, where the seeds
are brought in on floodwaters. It grows very rapidly. By the time the first hard frost of
autumn kills them, the aggressive vines can climb to eight feet in height, completely cover a
large area, and produce a bumper crop of seeds to spread the infestation the next year. Many
thousands of hop plants per acre may be produced, eventually blanketing the land and vegetation,
shading out all other plants, and leaving the soil exposed to erosion over the winter once the
hops dies off. The bare soil is then primed for germination of new hops seeds in the spring.
Japanese Hops is identifiable by the rough textured leaves and stems, the stems being
covered with short, sharp prickles that make handling the plant difficult. The tooth-edged
leaves are typically 5-lobed (sometimes 7 or 9), looking somewhat like its cousin marijuana.
The small green flowers are inconspicuous. It is sometimes confused with native bur cucumber,
though that plant has tendrils along the stem, has less deeply lobed leaves and lacks the sharp
downward-angled prickles of Japanese Hops.
Japanese Hops can be controlled by hand pulling or digging, though it is best to do this
while the plants are still small and not tangled with other vegetation. Cutting or mowing
will set the hops plants back, but they soon re-grow to their former size, so it will need to be
repeated periodically. Spray treatment with herbicides containing glyphosate work well on hops,
though triclopyr-based herbicides also work and will not kill grasses. When using herbicides,
make sure the product is registered for use in your state, and always read and follow the label
directions. Whatever treatment is used, control needs to be started early in the summer and kept
up so that the hop plants do not produce seed. Hops seeds can remain alive in the soil for
three years, so control needs to be carried out for at least that long.
For more information about other Invasive Species of Concern,
visit www.mdinvasivesp.org or call the
Maryland Department of Agriculture at 410-841-5920.
For more information on the Internet:
Riparian Forest Buffer Restoration: Maryland Stream ReLeaf. MD DNR - Forest Service (scroll to bottom of page for links to pdf and ppt)
Plant Conservation Alliance, Alien Plant Working Group
USDA Forest Service