Management of Japanese Hops
on Forest Regeneration Sites

Summary of Draft Report, December 2007

Philip Pannill, MD Dept of Natural Resources, Forest Service and
Aaron Cook, Western Maryland Resource Conservation & Development Council

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Background

Japanese Hops (Humulus japonicus) is an invasive exotic weed that has created problems in recent years on tree planting sites near waterways in Maryland and nearby States.  Little definitive information was available on the best means to address the problem, and control practices initially applied were sometimes ineffective.  In 2006 and 2007 a study was carried out in Frederick County, MD to determine more effective management practices.  This included a survey of known Hops locations within the Monocacy watershed in MD and PA, discussions with Foresters, weed control agencies and landowners who had been dealing with this species.  The project also included observations on Hops life-cycle and growth habits, and application of various manual, mechanical and chemical controls.  The following observations and recommendations are likely to be revised and refined over the next few years as more is learned about control of this alien invader.

Description

The vine was originally imported 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 escaped into the wild throughout the Eastern US.

For photographs and additional details about the plant go to the USDA Forest Service’s Invasive Plants website description at http://www.na.fs.fed.us/fhp/invasive%5Fplants/weeds/japanese-hop.pdf .

The plant is identifiable by the rough textured leaves and stems, the stems being covered with short, sharp, irritating prickles that make handling the plant difficult.  The tooth-edged leaves are typically 5 lobed, though they sometimes have 7 or 9 lobes.  The small green flowers are inconspicuous.  Hops are sometimes confused with burcucumber, though that plant has tendrils along the stem and lacks the sharp downward-angled prickles of Japanese Hops.

Seeds from this annual vine begin to germinate in early spring but new plants may continue to emerge if conditions allow.  Hops likes plenty of sun and moist, rich exposed soil, and is most commonly found along stream banks 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 ten feet in height, completely cover a large area, and will already have produced 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.

Effects on Tree Regeneration

Japanese Hops grows rapidly during the summer, forming a mat of vines up to 4 feet thick, and climbing up and over tall grasses, forbs, shrubs and trees to a height of 10 feet or more.  Hops create a dense shade that covers, smothers and twines around trees and shrubs, often breaking or pulling them down.  Attempts to mow, or even drive a vehicle through, tree plantings sites with well-developed Hops vines climbing the trees often results in the vines pulling and breaking down the trees and tree shelters.  The Hops can quickly cover small trees, hiding them from view and preventing mowing or application of non-selective herbicide.

Control Options

Prevention
Since Japanese Hops seed is sold for landscape and herbal uses, it would be valuable to discourage people from planting it, through public education and contacts with suppliers.  Education is also important in helping people identify the plant, look for it on their property and to learn how to control it before it spreads further.  Since most of the transport of Hops seed seems to be by water, it would be particularly valuable to seek out and control Hops growing along waterways.  It is also appropriate to check for Hops in areas that have flooded in the previous year since these areas are the most likely to have new Hops infestations.

Biological Control
The only insect observed feeding on Japanese Hops were adult Japanese beetles (Popillia japonica).  While commons, the beetles never did enough damage in any one area to reduce the Hops growth.  The only disease observed affecting Japanese Hops was powdery mildew (Sphaerotheca spp.), which was only seen during periods of wet weather.  While this disease certainly limited the growth during those times, when drier conditions returned the Hops recovered and grew rapidly.

There are no known pathogens that are suitable for treatment of a site infested with Japanese Hops.

Cultural Control
Japanese Hops prefers direct sunlight and does not tolerate heavy shade.  As soon as the tree canopy closes the Hops will cease to be a problem.  Practices that favor fst tree growth, early crown closure and heavy shade will help the new stand survive and outgrow the Hops.  This includes planting fast-growing species adapted to the site, effective weed control measures, closely spaced planting, and planting tree species that cast a dense shade in spring and summer.

Japanese Hops does not readily germinate in grassy areas, particularly in tall, sod-forming perennial grasses such as Tall Fescue or Reed Canary Grass.  It is much more likely to germinate and grow in an area where the soil is exposed or dominated by sparse broadleaf weed cover.  However, one germinated it will overtop and kill grass, leaving a bare area for the next year’s Hops seed to grow.  Consideration should be given to avoiding practices such as non-selective herbicide use that would reduce or remove the grass cover.  It should also be noted that these sod-forming cool-season grasses are among the worst weeds from the standpoint of tree growth, so this presents a challenge that land managers will need to address on a case-by-case basis.  Establishing or favoring other groundcover vegetation that is thick and growing in early spring could possibly reduce Hops germination and seedling survival.  Fall planted vetch, wheat, barley or cereal rye might serve this purpose. 

Japanese Hops likes to climb up and over shrubs and small trees, but is neither a high climber nor a strong climber, needing a ladder of tall weeds, shrubs and low tree branches to climb.  Where Hops may be a problem, it would be advantageous to minimize the proportion of shrubs and smaller trees that would be susceptible to Hops for many years in favor of tall-growing species.  As the trees grow taller proper pruning of lower limbs and basal sprouts should be used to interrupt the ladder available for the Hops to climb.

Use of tree shelters can assist hops control in several ways – by marking the location of the seedling, protecting from herbicide spray, reducing low branching and providing a slightly more difficult ladder to climb (though the shelter is smooth Hops can still climb a shelter via the stake or adjacent vegetation).  Try to prevent Hops vines from growing inside or overtop the shelters and depositing seed there.  Practices such as adequate site preparation, pre-emergent herbicide application or hand weeding insider the shelter, and herbicide application around the shelter can be used.  Where shelters are not used it is especially important to prevent Hops entirely or to detect and act on infestations early, before the vines climb onto the trees.

Manual Control
Japanese Hops does not develop an extensive or deep root system and is fairly easy to pull or dig early in the season, especially when the soil is moist.  This is an effective method, but care must be used to remove the root, not just break it off at the ground line.  It is important to start the hand weeding process early in the growing season (April – May) while the vines roots are small and before the vines become tangled with other vegetation, and repeat it periodically thereafter.  Manual control is the most targeted method, with the least likelihood of damage to other plants.  However, it is slow and labor-intensive, best used in fairly small, readily accessible areas.  It is important to wear gloves, long pants and long sleeves as needed to avoid skin contact with the plant, since it is very irritating.  Started early enough, and using proper precautions, this is a good method for use by homeowners, or parks with a source of volunteers.

Manual or Mechanical Cutting
Cutting the Hops vines, as close to the ground as possible is an acceptable control as long as the cutting is started early, the entire site is thoroughly cut, and the practice is repeated frequently.  Cutting can be done with hand tools, mechanized (weed-eater type) brush-cutters, lawn mowers, or tractor-mounted mowers.  Aside from the usual difficulties, common problems found with this method are that vines become tangled in the trees and shelters resulting in the equipment pulling them over, and that the vines quickly re-grow from the cut stems and from uncut vines around the trees.  This method tends to retain and promote the development of perennial grasses.  A combination of mowing between the tree rows and spraying around the trees could be one effective approach.

Herbicide Application

Pre-emergent Herbicide

The use of pre-emergent herbicides, which typically kill weed seeds as they germinate, is potentially very valuable in controlling Hops.  The advantages are that (depending on product, rate and timing) they may be used safely over and around young trees, generally cause minimal or no damage to other perennial groundcover vegetation, prevent the weed problem from occurring rather than needing to “rescue” the trees from an established Hops infestation.  When combined with post-emergent herbicides applied later in the season they may provide a longer period of control that may prevent successful re-seeding of the Hops.

Post-emergent Herbicide

The use of post-emergent herbicides, those that kill plants that are already growing, is one of the most common approaches for control.  The ideal situation would be to make application after most seeds have germinated (mid-March – mid-May) but before Hops vines are covering the trees (early June – late July, depending on tree size) or seed formation starts (August).

Tentative Recommendation for Post-Emergent Herbicide Treatment

Timing

Two treatments are recommended in order to protect trees from damage by the Hops vines and to prevent or reduce seed production.  Delaying the first treatment until the Hops are thick and tangled may mean poor coverage and need for re-treatment soon afterward.

• 1st treatment – May 15 – July 1 (or whenever Hops has reached the point where it is readily visible but before it has begun to climb trees)

• 2nd treatment – July 20 – September 15 (or whenever later emerging Hops appears or treated Hops has begun to re-grow, but before flowering can occur – any later growth should be killed by frost before setting seed)

Herbicide Products
Be sure to read the label carefully and investigate to assure that the product is suitable for your situation.

For site preparation – to control Hops in the year before planting trees – use either glyphosate or Escort XP – or use Vanquish, Garlon 3A or 2, 4-D individually or in combination.

For seedling sized trees (less than one-inch diameter) spray Hops around and close to the trees with glyphosate.  Do not spray onto the bark or leaves of the tree seedlings.  This can also be used throughout the planting site if desired.  Alternatively, along with the glyphosate application near the tree seedlings, areas between the trees can be treated by thorough and repeated mowing, or by herbicide application with Escort XP or Garlon 3A.

For areas not within the potential root zone of seedling sized trees, such as under larger trees, open areas, field edges, etc., use either glyphosate or Escort XP – or use Vanquish, Garlon 3A or 2, 4-D individually or in combination.

For stream banks and saturated wetlands use an appropriately labeled glyphosate product such as Rodeo or Accord, or use Garlon 3A.  Use a surfactant labeled for aquatic uses.

Note – A non-ionic surfactant at the rate of ¼ to ½% of the spray solution (depending on label) should be used in all spray solutions.  It is important t to thoroughly wet the leaves and stems of the Hops vines.

Summary
Based upon what we know now, Japanese Hops can be controlled, but regardless of the methods used it takes repeated effort – a one-shot treatment will usually not be sufficient.  Further work on control with pre-emergent and post –emergent herbicide is needed, as well as better understanding of the life cycle of this weed.