Biology and Management of Alliaria petiolata (Garlic Mustard) in
Woodland Communities of North America
Introduction
Alliaria
petiolata is a non-indigenous herbaceous plant species
that is invading several types of woodland ecosystems within the continental
United States and Canada. A. petiolata invasion
displaces native plant populations (especially spring ephemerals) and limits
the vegetative resources of native flora available to wildlife (George, 2000). A. petiolata has been found to be
readily invasive in both low quality and high quality woodland ecosystems,
making it a critical conservation problem.
Due to the invasiveness of this species and the ill effects that it
currently has on woodland ecosystems within the United States and Canada, it is
imperative that A. petiolata be managed
to lessen these negative effects. Three techniques for management of A. petiolata, including weeding,
prescribed burning, and herbicide treatment, will be discussed following a
background on the history, geography, habitat needs, and biology of the herb.
History
and Geography
Alliaria
petiolata is a biennial herb native across many parts of
Europe and Asia. The herb was brought to North America by European settlers in
the mid 1800’s as a culinary and medicinal herb (Nuzzo, 1991). As a culinary
herb, A. petiolata has been
prepared as an accompaniment to meat dishes and eaten raw in salads in both
Europe and North America (Guest, 1997). As a medicinal herb, A. petiolata has been used internally as
a cure for dropsy and coughs, while being applied as a compress externally for
sores and gangrene (Guest, 1997).
Within the United States and Canada, the range
of A. petiolata is expanding, with
existing populations from Ontario through the northeastern United States west
into Minnesota and south to Missouri and Virginia (Guest, 1997). It has also
been documented from parts of the Pacific Northwest and Utah (Guest, 1997). The first recorded presence
of the herb in the United States dates back to 1868 in Long Island, New York
(Nuzzo, 1991). By 1950, A. petiolata was
found in at least 13 other states. In
1990, the herb was reported present within 29 states, predominantly in the
Midwest and Northeast regions of the United States but also including Utah,
Oregon, and the District of Columbia (Nuzzo, 1991). In Canada, the herb was
first documented in Toronto in 1879, found in two provinces by 1950, and in
1990 was found in southern Ontario, the St. Lawrence Valley in Quebec and
southern Vancouver, British Columbia (Nuzzo, 1991).
Habitat
In a typical North American woodland ecosystem
there are three significant layers of vegetation: the overstory, the understory
and the groundlayer. The overstory is made up of trees greater than 10.5 meters
in height. The understory is made up of
trees and shrubs ranging from approximately 2 meters to 10.5 meters. The
groundlayer consists of herbaceous and woody plant species that range from
ground level up to approximately 2 meters (The Kestrel Group, 1999). Within
North American woodland communities, A.
petiolata is found in the groundlayer vegetation layer.
In North America, A. petiolata is quickly creating a niche for itself in a variety of
woodland ecosystems and has become a dominating element in many natural areas,
displacing native woodland species. In
woodland communities, A. petiolata is
most frequently found growing under a partial shade canopy (Nuzzo, 1991).
However, it is important to note that A.
petiolata plants are also frequently found growing in sites that have light
penetration ranging from full sun to full shade (Nuzzo, 1991). The plant may
have greater growth under full sun conditions, but will be less invasive when
either full sun or full shade conditions exist (Cavers et al. 1979). In terms of soil, A. petiolata has been found growing in a variety of soil substrates
ranging from clay to gravel, but the herb has not been documented in peat soils
or soils with high acid content (Cavers et al. 1979). A. petiolata is most invasive within disturbed woodland
communities, and rapid invasion and dominance of the herb can occur within such
sites (Nuzzo, 1991). A. petiolata has
been observed to invade and dominate the herbaceous ground layer within 10-20
years in disturbed and degraded woodland communities (George, 2000). Due to the tolerance of many growing conditions,
and the plant’s ability to invade and dominate a site rapidly, A. petiolata has the possibility of
making a severe impact on the long-term natural history of woodland ecosystems
throughout North America (George, 2000).
The presence of other exotic species impacts the
structure of the woodland community and can thus have implications for A. petiolata’s success at germination
and development. For example, as non-native earthworm populations increase in
many woodland communities, the duff layer on the forest floor is reduced or
completely depleted. This unnatural condition provides an optimal habitat for A. petiolata as the species prefers a
highly mineral soil with little leaf cover to shade it during germination
(Gillette, 2000). If non-native earthworms were not changing soil and surface
structure in this way, A. petiolata may
have reduced success at germination and dominance of the groundlayer (Gillette,
2000).
It is important to note that within its native
range, A. petiolata has been found to
occur at much smaller densities in more isolated populations than in invaded
ecosystems of North America (Reston Association, 2000). Also, throughout the herb’s native range
there are over 30 different insect species that have a direct impact on the
plant and associated plant functions through stem, leaf and seed damage (Reston
Association).
Biology
Alliaria
petiolata is a cool-season biennial herbaceous plant
within the Brassicaceae family. Typical of the Brassicaceae family, flowers are
small and white with four petals per flower. Flowers are generally grouped in
small clusters at the terminal end of the main stem. Flowers of A. petiolata are self-compatible and
thus capable of self-pollination, but they are also adapted to
cross-pollination by insects including syrphid flies, midges, and bees (Nuzzo,
1991). An implication as a self
pollinating species includes the possibility of a single plant being capable of
pollinating an entire site (Nuzzo, 1991).
As
a biennial herb, A. petiolata has a
distinct growth development pattern. In the first year a basal rosette
develops. During this developmental
stage, the plant can be easily overlooked as the leaves lie close to the forest
floor and may be covered with leaf litter. In the second year a flowering stalk
is produced and the plant flowers, sets seed and dies. Biennial plants
generally are prolific seed producers due to the fact that all of the plant’s
reproductive energy is used to produce seed. As a flowering adult plant A. petiolata produces an average of 350
seeds (Guest, 1991) in its one flowering season. Seeds are held in erect
siliques and ballistically dispelled for a distance of up to several meters
(Nuzzo, 1991). Seeds are also dispersed with the aid of a variety of mobile
elements within the woodland community. Current observation has concluded that
seeds are typically dispersed via deer, rodent, and human movement (Gillette,
2000). Deer carry the seed for long distances on their hooves depositing the
seed on and along trails, while rodents such as raccoon and squirrel have been
observed depositing seed under roosting trees (Glass, 2000). Human dispersal of
A. petiolata can occur over great
distances if seed is attached to hikers’ clothing and footwear, or on tire
treads of bicycles, off-road vehicles, and on-road vehicles such as cars and
motorcycles. Human movement posses the largest threat to long range spread of
the herb in the shortest period of time (Gillette, 2000).
Water
movement through the ground plane of woodland communities, particularly in the
floodplain woodland communities, may also disperse seeds. Seed dispersal has also been observed to
follow storm water flow paths (Glass, 2000). However, seeds are not typically
wind dispersed (Nuzzo, 1991).
Seed dormancy lasts approximately 20 months
from the time when the seeds are dispelled from the parent plant to when the
seeds germinate (Baskin, 1992). Cold stratification is required for seed
germination to occur, and, once dormancy has broken, seeds are capable of
germinating at low temperatures (Baskin 1992) thus allowing early spring
germination of A. petiolata seeds. This
factor is an import element in the invasiveness of the species since A.
petiolata can
then out-compete native species for early spring light before the forest
canopy develops (George, 2000).
Typically
seed germination occurs in the early spring, often after the first or second
warm spring rain (Nuzzo, 1991). A second germination may occur in autumn,
though in much smaller quantities (George, 2000). Most germination occurs within the two years following dormancy
break (Nuzzo, 1991), but seeds may remain viable in the soil for up to four
years (Baskin, 1992) with decreasing germination success rates as the seed ages.
Management
Practices
A.
petiolata has been known as an invasive species and
managed as such since the early 1980’s (Callahan, 2000). Twenty years of management of A. petiolata in woodland situations has
resulted in the development and refinement of a few fairly effective management
techniques: prescribed burning, weeding/ cutting, and herbicide treatment. Each
one of these techniques has associated limitations, including environmental
concerns or repercussions.
Information about management practices has been
contributed by a variety of sources including Cary George of the Minneapolis
Park and Recreation Board, Larry Gillette of Hennepin Parks, and Steve Glass of
the Wisconsin Arboretum at Madison.
Due to the longitivity of seeds within the
seedbank and the possibility of a single plant being capable of producing seed
in sufficient quantity to start an invasive colony, persistence and
thoroughness are key to proper management of the herb in woodlands throughout
North America.
Prescribed
Burning
Prescribed burning is most often used in large
areas when time is limited and labor intensive techniques such as hand pulling
are too resource intensive for practical use (Gillette, 2000). Prescribed
burning is used as a tool for general woodland ecosystem management in some
areas and, thus, control of A. petiolata
can be a beneficial result of a larger scale management plan (Gillette, 2000).
What is necessary for prescribed burning to work as a management technique for A. petiolata is consistency. Burning
must occur in infested areas once a year for a minimum of 3-4 years in order to
exhaust the seed bank. Burning must also occur prior to the surviving second
year plants’ seed development, so that the seed bank is not replenished (Glass,
2000).
Experiments have shown that fires need to be of mid-intensity
(flame length up to 15 cm) to effectively destroy A. petiolata (Nuzzo, 1991).
Low-intensity fires (flame length up to 3 cm) have been proven to be
ineffective in destroying A. petiolata
(Nuzzo, 1991). Fires are conducted in early spring when A. petiolata first appears or in autumn when native plants have
gone into dormancy but A. petiolata
is still green. Burning in the spring risks negatively affecting early emerging
spring ephemeral species but results in
better density reduction of A. petiolata
in comparison to autumn burning (Nuzzo, 1991). Autumn burns reduce the
possibility of damage to native plants since burning can be done after native
plants have gone into dormancy. However, autumn is usually too late to
effectively reduce A. petiolata since
seed has already set and replenished the seed bank. The viability of seeds of A. petiolata in the seedbank that remain
after exposure to fire is unknown.
Fuel for a mid-intensity fire is essential. If
fuels are not sufficient for a mid-intensity fire throughout the infested area,
survival of individual plants and groups of plants is likely to result in a
continuation of seed production from surviving plants and ultimately a
continuation of the problem. Furthermore, fuel availability for a minimum of
3-4 prescribed burns in consecutive years can pose an even larger problem. A growing concern about the reduction of
leaf litter by non-indigenous earthworm populations has been speculated to
limit the potential to sustain prescribed fire (Gillette, 2000).
With this in mind, it is of essential importance
that, if prescribed burning is being used as the sole management technique for
control of A. petiolata, fuel must be
sufficient for a minimum of 3-4 years of annual mid-intensity burning. It is
also important to note that some land managers believe that the seedbank can
persist much longer than research data has shown, up to 10 years (Glass,
2000). Management practices must
incorporate this time period if necessary.
Weeding/Cutting
Weeding and cutting is most effective in small
areas where sufficient labor is available for this time intensive, but highly
effective, control method (Gillette, 2000).
It is most typically used when volunteer labor forces are available, or
in cultivated landscapes where weeding is a regular part of the landscape
management. There are three commonly used techniques that fall under the
weeding and cutting management method. With all of the techniques, plants must
be removed or cut back before seeds have set so as not to repopulate the area
with a new generation of seed
(Glass, 2000).
The
first technique commonly used under the weeding and cutting is to remove A. petiolata plants in their entirety.
(Gillette, 2000). Since new shoots can develop from root fragments remaining in
the soil, it is important to remove as much of the root system as possible
(George, 2000). Removal of the entire plant is the most time consuming
technique since each plant must be hand dug. It is also the most invasive
technique in regards to soil and adjacent plant disturbance.
The second technique involves cutting the plants
at the base of the stems at ground level. This technique involves some adjacent
plant disturbance as tools used for cutting the stem at the base may also cut
non-target plant species tissues (Nuzzo, 1991). As well, soil disturbance or
soil compaction may occur as laborers kneel on the forest floor to reach the
base of the plant and tools inadvertently penetrate the soil surface. Cutting
at the base is generally only a temporary remedy as new shoots may develop from
the base of the plant, and the plant may restore itself to reproductive
capacity. Although this technique may
not result in complete annihilation of A.
petiolata populations in infested areas, it is less time consuming than
weeding, and can be used in situations where time or labor is limited.
The third technique is to cut off only the
flowering section of the plant. This technique is the most time efficient and
may have the least impact on soil structure and neighboring plant health.
However, it is not as effective in reducing flowering plant populations (Nuzzo,
1991) due to possible regeneration of reproductive capacities. As with cutting
at the base, this method is useful in situations where time or labor is limited
but some management of A. petiolata
is desired on a site.
With all three weeding and cutting techniques,
disturbance of soil and neighboring groundlayer plants will occur as laborers
walk through the infested area to carry out their duties. Also, as laborers
move through the woodland to remove or cut back A. petiolata they may also be effectively transporting any seed
from the soil surface to their next destination (Glass, 2000), aiding in the plant’s dispersal. Therefore, it is
important that laborers remove clothing and footwear before entering a
non-infested site. Lastly, weeding and
cutting techniques, as with burning, must be performed on an annual basis for a
minimum of 3-4 years to effectively remove existing populations of A. petiolata.
Chemical
application
Herbicide treatment is an effective management strategy for both small and large populations of A. petiolata. The most common herbicide used is a 2-3% solution of glyphosate (George, 2000), which can be found under various trade names. This herbicide is non-specific and will destroy any plant tissue that comes in contact with it. This non-specificity limits the use of the herbicide as a management method because it should only be applied at times of the year when native plant’s foliage is dormant and therefore unaffected (Glass, 2000). The result is that herbicide treatment typically occurs in early spring or autumn. Spot treatment, however, can occur all throughout the growing season if necessary (George, 2000). Additionally, the herbicide should be applied to individual plants or groupings of target plants manually to insure that application to non-target species is avoided (George, 2000). Thus, application can require extensive amounts of time and labor if treatment involves large areas.
Herbicide treatment can be very effective as a management technique if care is taken to treat all A. petiolata plants of an infestation. Problems can arise when foliage of A. petiolata plants is covered by leaf litter on the forest floor in early spring and autumn resulting in missed plants.
Missed plants can potentially parent an entire population and ultimately generate a reinfestation of the area (Baskin, 1992).
Conclusion
A.
petiolata is a highly invasive plant within woodland
communities of North America due to many factors. These factors include its
adaptability to a wide variety of growing conditions, its prolific seed
production, its early germination capabilities, and its successful seed
distribution via mobile elements within the environment. Interactions of other
exotic species with the environment may also be impacting the success of A. petiolata as an invasive species, as
with non-native earthworm populations and their impact on soil structure.
Currently,
three management techniques are commonly in use for control of A. petiolata. These techniques include
prescribed burning, weeding/cutting, and chemical application. Each technique
has positive and negative attributes as well as a tendency to work more
effectively within certain parameters. Prescribed burning is most effective in
large areas, but can be difficult to pursue if there are insufficient
quantities of leaf litter. Weeding and cutting works well when a large labor
force is available, but can disturb soil and plants as laborers move through
the woodland. Chemical application is very effective if applied correctly and
can be carried out by a single person
over a substantial area. Still, if chemical application is carried out
incorrectly, or during the growing season, non-target plants are at risk of
being damaged. Even after the seedbank has been exhausted from use of one or
more of these management techniques, annual surveying for new invasions must be
done, and management must be incorporated for eradication of those developments
before seeds set.
In the future, use of biological control may
open up possibilities for more extensive and long-term management options.
Natural enemies of A. petiolata, including several species of
Ceutorhynchus weevils, are currently being researched at Cornell University in
Ithaca, New York (Blossey, 2000).
Within the next ten years it is possible that biological control methods
will be available for use in managing A.
petiolata (Gillette, 2000). Until then resource managers and concerned
citizens managing A. petiolata populations
on public and private lands can benefit from the use of one of, or a
combination of, the management techniques mentioned herein.
References
Baskin J. M., and C.C. Baskin. 1992. Seed
germination biology of the weedy biennial Alliaria
petiolata. Natural Areas Journal 12:191-197.
Blossey, B. November 5, 2000. Assistant
Professor and Director, Biological Control of Non-Indigenous Plant Species
Program at Cornell University. Ithaca, New York. Personal communication.
Bureau of Endangered Resources and Wisconsin Department of Natural Resources. 1997. Wisconsin manual of control recommendations for ecologically invasive plants. Bureau of Endangered Resources and Wisconsin Department of Natural Resources pages 27-29.
Callahan, R. 2000. Midwest battling exotic plant. Chicago Tribune May 8, section 1 page 8.
Cavers, P.B., M.I. Heagy, and R.F. Kokron. 1979.
The biology of Canadian weeds. 35. Alliaria
petiolata (M Bieb.) Cavara and Grande. Canadian Journal of Plant Science
59:217-229.
George, C. October 26, 2000. Head Gardener of Eloise Butler Wildflower
Garden and Bird Sanctuary of the Minneapolis Park and Recreation Board.
Personal communication.
Gillette, L. October 24, 2000. Hennepin Parks (Minnesota) Natural Resources Division. Personal communication.
Glass, S. October 25, 2000. Madison Arboretum
(Wisconsin). Personal communication.
Guest, M. 1997. Coming to grips with garlic
mustard. The American Gardener May/June 1997 20-21.
Meekins, J.F. and B.C. McCarthy. 1999.
Competitive ability of Alliaria petiolata
(garlic mustard, brassicaceae), an invasive, nonindigenous forest herb.
International Journal of Plant Sciences 160:743
Nuzzo, V.A. 1991. Experimental control of garlic
mustard [Alliaria petiolata (Bieb.)
Cavara & Grande] in northern Illinois using fire, herbicide, and cutting.
Natural Areas Journal 11:158-167.
Reston Association.1998. Reston Association Homepage. www.reston.org/BeautifyingProtectingReston/garlicmust.htm.
The Kestrel Design Group, Inc. 2000. Theodore Wirth
Park and Minnehaha Creek Corridor Land Cover Classification and Management
Plan.
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