Management
summary
A comprehensive plan for managing garlic mustard via conventional means
includes the following elements adapted from Nuzzo (1991). Because garlic
mustard is a disturbance-adapted plant, all management efforts should
strive to reduce soil and vegetation disturbance to prevent giving further
advantage to garlic mustard.
1. Where
garlic mustard is not well established, efforts should focus on
detecting and eradicating new satellite infestations before a seed
bank develops (i.e. dormant seeds in the soil). Monitoring should
focus on areas where garlic mustard seeds are likely to be dispersed
and find disturbed areas suitable for germination. Trails, parking
areas, transportation corridors and recreation sites in suitable
habitats are known sites of early infestation. (See
small infestations)
2. Once
garlic mustard has established an invasion front (several years
of flowering plants), the goal is to prevent further seed set until
the seed bank is exhausted; a period of up to five years. Depending
on the site characteristics and infestation level, pulling, cutting,
applying herbicide or repeated fire will be required. |
Pattern
of spread and management strategy
Garlic mustard spreads from established (core) infestations along
an invasion front. Satellite infestations occur when seeds are transported
to new areas. This often occurs along trails, roads or forest edges.
Top priority should be given to annual removal of all satellite infestations
to prevent further spread. |
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a. In small
infestations or sensitive plant communities, hand clip or pull seedlings,
rosettes or flowering plants prior to seed set. (See small
infestations)
b. For core
infestations (long established, dense populations) in fire-intolerant
communities, apply herbicide in spring (if few or no non-target species
will be damaged) or dormant season if non-targets are present. In fire-adapted
communities, use a mid-intensity burn as late in the spring as possible
to avoid undue harm to non-targets, followed by herbicide, cutting or
pulling of surviving individuals. (See medium to large
infestations) Back
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Detailed
management options -- Small infestations |
Where
garlic mustard is not established, efforts should focus on detecting
and eradicating infestations before a seed bank develops. Pulling
individual garlic mustard plants by hand is the simplest and most
effective approach to managing small or isolated infestations. When
pulling plants, it is important to remove the upper portion of the
roots as well as the stem, since buds in the root crown can produce
additional stems. All pulled plants should be removed from the site
as seed ripening continues even after plants are pulled. Repeatedly
hand pulling of garlic mustard is reported to be effective for control
in small areas but has limitations. Because seeds remain viable in
the soil for up to five years, it is important to pull all
garlic mustard plants in an area every year until the seed bank is
exhausted and seedlings no longer appear. This will require multiple
efforts each year as rosettes can continue to bolt and produce flowers
over an extended period (April-June). In fire-adapted communities,
regular prescribed burns may deter garlic mustard from entering by
stimulating native communities and killing early invaders. Back
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Detailed
management options -- Medium to large infestations |
In medium to
large sized infestations, a combination of hand pulling, cutting
or localized herbicide treatment is appropriate for managing garlic
mustard. First, an aggressive effort at hand pulling all
newly established or "satellite" infestations is recommended
to limit garlic mustard's spread. Next, consider managing well-establish
"core" populations with a combination of pulling, cutting
and or herbicides.
Cutting:
Cutting flowering plants at ground level by hand or with a string
trimmer or lawn mower will kill a high percentage of garlic mustard
plants. The lower the cut, the more effectively the plant will be
killed. If the plants are flowering, it is important to remove all
cut stems since seed development will continue even if the stem
is severed from the root. Because mechanical cutting does not remove
the root crown, it may be necessary to cut multiple times in a season
to prevent seeds from developing on secondary stems that sprout
from the rootstock. It will also be necessary to cut for up to five
consecutive years or until the seed bank is exhausted. Unselective
cutting with a string trimmer or lawn mower damages desirable vegetation
and may slow re-establishment of desirable vegetation or even allow
garlic mustard populations to expand. The goal is to selectively
remove garlic mustard, leaving the desired plant community intact.
Herbicides: Several herbicides have a role in garlic mustard
management. Herbicides should only be applied according to their
label directions and using the protective equipment specified. Use
of application equipment that can direct the herbicide to the target
plant and reduce overspray or drift onto non-target plants is required.
This is referred to as spot treatment.
It is very important
to limit damage to non-target vegetation. If other plants are killed,
garlic mustard will likely replace them. Indiscriminate herbicide
applications can thus increase garlic mustard populations! As with
cutting, the goal is to selectively remove garlic mustard leaving
the desired plant community.
As a cool season
herb, garlic mustard continues to grow on snow-free days when temperatures
exceed freezing. This provides an opportunity for selective treatment
of garlic mustard if applications are made when other plants have
not yet appeared (spring) or have died for the year (late fall).
Application
of 1-2% glyphosate (Roundup) provides effective control of garlic
mustard seedlings and rosettes. Note: glyphosate is a non-selective
herbicide meaning that it will kill or damage most plants it comes
into contact with (including woody plants). However, to be effective,
this herbicide must be absorbed by growing leaf tissue or bark,
i.e. the plant must be actively growing. Applications in very early
spring (March-April) can often be timed for periods when few if
any other plants beside garlic mustard are actively growing. Similarly
in late fall, applications can be made with reduced risk to many
non-target species. However, glyphosate will damage sedges and other
species that are actively growing at this time and therefore susceptible
to herbicide uptake. Always take precautions to avoid contacting
desirable plants with the herbicide. This may include the hard to
see stems of small woody shrubs and trees. Bentazon (Basagran) applied
at 8 ounces (by weight) per acre may be an acceptable substitute,
less effective on garlic mustard but with reduced risk to some non-targets
particularly annual and perennial grasses.
Prescribed
fire: Professional land managers trained in the use of prescribed
fire may also consider burning for control of medium to large infestations
of garlic mustard. The effectiveness of fire differs based on site
characteristics and management goals. Most research shows that dormant
season fires (March) are ineffective in garlic mustard control and
that growing season fires (May) suppress garlic mustard but also
adversely affect native understory forbs. Carefully timed spring
fires (after garlic mustard emergences but prior to emergence of
desirable plants) may be effective. While a single dormant season
fire (fall) might actually increase the abundance of garlic mustard
the following spring, repeated burns (fall, spring, spring; or spring,
spring, spring), have been used to maintain garlic mustard in a
reduced condition and stimulate herbaceous species richness and
cover. This fire regime did not reduce the number of woody shoots
but did decrease their height in the fire-adapted oak woodland where
it was tested. Fire accelerated loss of woody seedlings on upland
but not lowland sites. Back to top
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Detailed
management options -- Regional infestations |
Extensive work
on conventional controls has failed to yield practical methods for
control of generally infested areas on county, state or regional scales.
Natural area managers report that the above measures are generally
too labor intensive for all but the most critical of sites and overall
they are losing the battle with this invasive species. Biological
control appears to be a promising option for long-term control of
extensive infestations. |
For more information:
Nuzzo,
V.A. 1991. Experimental Control of Garlic mustard [Alliaria petiolata
(Bieb.) Cavara and Grande] in Northern Illinois Using Fire, Herbicide
and Cutting. Natural Areas Journal. 11: 158-167.
Nuzzo, V.A., 1996.
Impact of dormant season herbicide treatment on the alien herb Garlic
mustard (Alliaria petiolata (M.Bieb.) Cavara and Grande) and groundlayer
vegetation. Transactions of the Illinois State Academy of Science 89:
25-36.
Nuzzo, V.A., W. McClain,
and T. Strole. 1996. Fire Impact on Groundlayer Flora in a Sand Forest.
The American Midland Naturalist. 136: 207-221. Back
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Biological
control of garlic mustard |
Research is
currently being conducted to explore the potential for biological
control of garlic mustard. Classical biological control (also known
as importation biological control) is a technique for controlling
exotic species by introducing natural enemies of the specific target
species from the native range. Ideally, the biological control agents
will become naturalized in the new range and maintain populations
that swell and recede with the population of the target pest.
In its native
range, garlic mustard is a component of forest ecosystems but does
not form the same extensive and damaging infestations that it does
in North America. In Europe, garlic mustard is found in similar
habitats as in North America; however, populations are typically
scattered and smaller in size and are consistently attacked by a
community of plant feeding insects (herbivores) resulting in reduced
shoot number, shoot height and seed output as well as direct mortality.
This constant attack by herbivores in the native range appears to
reduce the competitive advantage that garlic mustard exhibits in
North American habitats. The long-term management goal for garlic
mustard is to identify insects from the native range that feed and
breed exclusively on garlic mustard and to release them in North
America if they are proven safe and effective.
In 1998, work
was initiated to investigate the potential for classical biological
control of garlic mustard in North America. In this process, potential
biological control agents are tested for their effectiveness at
controlling garlic mustard and for their host specificity. An ideal
control agent will cause extensive damage to garlic mustard, but
will not feed on other non-target species, even if the supply of
garlic mustard is exhausted. Additionally, the larvae of the control
agent must only develop on garlic mustard. If these conditions are
met, once a garlic mustard population is reduced, the population
of control agents will diminish as well and will not damage other
non-target species.
Importation
biological control of weeds is highly regulated in the United States.
All prospective importation projects fall under the supervision
of the USDA Technical Advisory Group (TAG) on Weed Biological Control.
This committee reviews potential importations considering specific
potential risks and benefits. Extensive testing is required by the
TAG to evaluate the host specificity of a natural enemy. Once approved
for importation by TAG a natural enemy must undergo further testing
under quarantine conditions to determine that it is the desired
species and is free of diseases or other contaminates.
From an initial
literature survey, 70 insects and 7 fungi attacking garlic mustard
in the home range were identified. Of these, 5 species of weevils
(Curculionidae) and one leaf-feeding beetle (Chrysomelidae) were
determined to have the greatest potential impact and were selected
for further testing. The leaf beetle, Phyllotreta ochripes (Curtis),
has since been rejected because it feeds too broadly. Testing is
currently focusing on five species of Ceutorhynchus weevils. Adult
C. alliariae and C. roberti feed on leaves while the larvae mine
in stems and leaf petioles. These two species exhibit very high
attack rates in the field (generally greater than 80 percent of
garlic mustard infested). Larval C. scrobicollis are root miners
while C. constrictus and C. theonae are seed feeders. Host-specificity
testing on the Ceutorhynchus weevils is promising. The USDA Technical
Advisory Group (TAG) on weed biological control may permit initial
introductions of C. scrobicollis in 2004. Other natural enemies
will likely follow as indicated by host specificity tests and USDA
TAG review.
The effort to
explore garlic mustard biological control is being conducted by
a consortium coordinated through Cornell University and involving
many state and federal partners. A similar consortium was highly
successful in implementing biological control of purple loosestrife
in North America. Michigan was a part of the purple
loosestrife consortium contributing research and education products
for national use and benefiting from the receipt, release and establishment
of the effective Galerucella natural enemies.
For
more information:
Blossey,
B., V. Nuzzo, H. Hinz, E. Gerber. 2001. Developing biological control
of Alliaria petiolata (M.Bieb.) Cavara and Grande (Garlic mustard).
Natural Areas Journal 21: 357-367. Back
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Garlic mustard management strategy in Michigan
The Michigan
Invasive Plant Council (MIPC) convened a meeting in December 2002
to outline a broad based strategy for managing garlic mustard in Michigan.
It was generally agreed that an effective strategy would seek to: 1) exclude
garlic mustard from uninfested parts of the state, 2) eradicate localized
populations in areas where garlic mustard is not widely established and
3) manage existing infestations to reduce their impact and limit spread.
To these ends, the following activities were considered high priorities:
* Document the current distribution of garlic mustard in Michigan.
* Determine the impact of garlic mustard on Michigan ecosystems.
* Educate the public about garlic mustard biology, impacts and management.
* Develop a list of non-target plants in Michigan that could potentially
be attacked by garlic mustard natural enemies and to assure their safety
in advance of biological control by pre-release host specificity testing
in Europe.
* Establish baseline information on garlic mustard in Michigan to determine
the potential for biological control.
Because conventional
controls are currently inadequate to successfully manage existing garlic
mustard populations in Michigan, meeting participants urged that a rigorous
analysis of the potential for biological control of garlic mustard should
be conducted. Based on the outcomes of these studies, a final decision
on the desirability of implementing biological control of garlic mustard
can ultimately be made.
Current
research efforts at Michigan State University are focused on establishing
baseline information on garlic mustard populations and documenting its
impacts in Michigan.
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