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Invasive Species - Garlic Mustard - Alliaria petiolata [Bieb]
Cavara & Grande
Mustard family: Brassicaceae
Garlic mustard poses a
severe threat to native plants and animals in forest
communities. Once introduced to an area, garlic mustard out-competes native
plants by aggressively monopolizing light, moisture, nutrients,
soil and space.
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Native Origin: Europe
Description: Garlic mustard is a cool season biennial herb in the mustard family (Brassicaceae) with stalked, triangular to heart-shaped, coarsely toothed leaves that give off an odor of garlic when crushed. First-year plants appear as a rosette of green leaves close to the ground. Rosettes remain green through the winter
and develop into mature flowering plants the following spring.
Flowering plants of garlic mustard reach from 2 to 3-1/2 feet in
height and produce buttonlike clusters of small white
flowers, each with four petals in the shape of a cross. Beginning in May
(in the mid-Atlantic Coast Plain region), seeds are produced in
erect, slender pods and become shiny black when mature. By late
June, when most garlic mustard plants have died, they can be
recognized only by the erect stalks of dry, pale brown seedpods
that remain, and may hold viable seed, through the summer.
Photo: David Cappaert, Michigan State University,
Bugwood.org
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Habitat: Garlic mustard frequently
occurs in moist, shaded soil of river floodplains,
forests, and roadsides, edges of woods and trails edges and
forest openings. Disturbed areas are most susceptible to
rapid invasion and dominance. Though invasive under a
wide range of light and soil conditions, garlic mustard
is associated with calcareous soils and does not
tolerate high acidity. Growing season inundation may
limit invasion of garlic mustard to some extent.
Distribution:
Garlic mustard is located
from eastern Canada, south to Virginia and as far west as Kansas
and Nebraska. See shaded areas on the distribution map.
Ecological Impacts:
Garlic mustard poses a
severe threat to native plants and animals in forest
communities. Once introduced to an area, garlic mustard out-competes native
plants by aggressively monopolizing light, moisture, nutrients,
soil and space.
Control and Management:
Mechanical-
Hand removal of entire
root system of plant is practical for light
infestations. For larger infestations cut stems at ground level
or within several inches of the ground, to prevent seed production.
Chemical- Herbicide (e.g., Roundup)
may be applied for very heavy infestations. Fire can be
used but can encourage germination of stored
seeds and promote growth of emerging garlic mustard seedlings.
Biocontrol- Five weevils and one
flea beetle feed on garlic mustard
Photo: Leslie J. Mehrhoff,
University of Connecticut, Bugwood.org
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BACKGROUND
Garlic mustard was first
recorded in the United States
about 1868, from Long Island,
New York. It was likely
introduced by settlers for food
or medicinal purposes.
BIOLOGY & SPREAD
After spending the first half of
its two-year life cycle as a
rosette of leaves, garlic
mustard plants develop rapidly
the following spring into mature
plants that flower, produce seed
and die by late June. In the
mid-Atlantic Coastal Plain
region, seeds are produced in
erect, slender, four-sided pods,
called siliques, beginning in
May. Siliques become tan and
papery as they mature and
contain shiny black seeds in a
row. By late June, most of the
leaves have faded away and
garlic mustard plants can be
recognized only by the dead and
dying stalks of dry, pale brown
seedpods that may remain and
hold viable seed throughout the
summer.
A single plant can produce
thousands of seeds, which
scatter as much as several
meters from the parent plant.
Depending upon conditions,
garlic mustard flowers either
self-fertilize or are
cross-pollinated by a variety of
insects. Self-fertilized seed is
genetically identical to the
parent plant, enhancing its
ability to colonize an area.
Although water may transport
seeds of garlic mustard, they do
not float well and are probably
not carried far by wind. Long
distance dispersal is most
likely aided by human activities
and wildlife. Additionally,
because white-tailed deer prefer
native plants to garlic mustard,
large deer populations may help
to expand it by removing
competing native plants and
exposing the soil and seedbed
through trampling.
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MANAGEMENT OPTIONS
Because the seeds of garlic can remain viable in the
soil for five years or more, effective management
requires a long term commitment. The goal is to prevent
seed production until the stored seed is exhausted. Hand
removal of plants is possible for light infestations and
when desirable native species co-occur. Care must be
taken to remove the plant with its entire root system
because new plants can sprout from root fragments. This
is best achieved when the soil is moist, by grasping low
and firmly on the plant and tugging gently until the
main root loosens from the soil and the entire plant
pulls out. Pulled plants should be removed from site if
at all possible, especially if flowers are present.
For larger infestations of garlic mustard, or when
hand-pulling is not practical, flowering stems can be
cut at ground level or within several inches of the
ground, to prevent seed production. If stems are cut too
high, the plant may produce additional flowers at leaf
axils. Once seedpods are present, but before the seeds
have matured or scattered, the stalks can be clipped,
bagged and removed from the site to help prevent
continued buildup of seed stores. This can be done
through much of the summer.
For very heavy infestations, where the risk to desirable
plant species is minimal, application of the systemic
herbicide glyphosate (e.g., Roundup®) is also effective.
Herbicide may be applied at any time of year, including
winter (to kill overwintering rosettes), as long as the
temperature is above 50 degrees F. and rain is not
expected for about 8 hours. Extreme care must be taken
not to get glyphosate on desirable plants as the product
is non-selective and will kill almost any plant it
contacts. Spray shields may be used to better direct
herbicide and limit non-intentional drift.
Fire has been used to control garlic mustard in some
large natural settings but, because burning opens the
understory, it can encourage germination of stored seeds
and promote growth of emerging garlic mustard seedlings.
For this reason, burns must be conducted for three to
five consecutive years. Regardless of the control method
employed, annual monitoring is necessary for a period of
at least five years to ensure that seed stores of garlic
mustard have been exhausted.
Researchers are investigating potential biological
control agents for garlic mustard which may greatly
improve the control of this insidious weed.
Prevention and Control.
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Flowering plants of garlic mustard reach from 2 to 3-½
feet in height and produce buttonlike clusters of
small white flowers, each with four petals in the shape
of a cross.
Photo: Chris Evans, Bugwood.org
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