Contact: Phil Pannill, Regional Watershed Forester, Maryland Department of Natural Resources, Forest Service | email@example.com
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 the Maryland Invasive Species Council.
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