Background
Garlic mustard (Alliaria petiolata) is an invasive herb that has spread throughout much of the United States over the past 150 years, becoming one of the worst invaders of forests in the American Northeast and Midwest. While it is usually found in the undergrowth of disturbed woodlots and forest edges, recent findings have shown that garlic mustard has the ability to establish and spread even in pristine areas. This spread has allowed it to become the dominant plant in the undergrowth of some forests, greatly reducing the diversity of all species. Garlic mustard is one of very few non-native plants to be able to successfully invade forest understories.
Biology
Garlic mustard has a biennial life cycle, that is, it takes two years to fully mature and produce seeds. Seeds germinate in February to early March of the first year and grow into a short rosette by the middle of the summer. In the plant's second year, a stalk develops, flowers form, and the plant dies by June. Siliques, four-sided seedpods, develop in May, containing small black seeds lined up in a row. On average, a garlic mustard plant will produce 22 siliques, each of which can contain as many as 28 seeds. A particularly vigorous plant may produce as many as 7,900 seeds (Nuzzo, 1993) although the average is more likely to be in the 600 seed range. The seeds generally germinate within one to two years, but may remain viable for up to five years in the seed bank. Seed dispersal is mainly by humans or wildlife carrying the seeds.
Characteristics and Identification
Identification of first year plants can be difficult; the task is made easier by smelling the garlic odor produced when the leaves of the plant are crushed. The basal leaves of an immature plant are dark-green and kidney shaped with round teeth (scalloped) along the edges; average size of the leaves is 6 to 10 cm in diameter. The petiole, or leaf stalk, of first year plants are 1 to 5 cm long. In its second year, the alternating stem leaves become more triangular shaped, 1 to 5 cm long, and have sharper teeth, with leaves becoming gradually smaller towards the top of the stalk. Leaf stalks of mature plants are hairy. As with the younger plants, second year plants have a garlic odor when crushed but the odor is less obvious with increasing age.
Garlic mustard flowers arrive in early April and die by June. Flowers develop on an unbranched (occasionally weakly branched) stalk and have 4 small white petals arranged symmetrically. Flowers are approximately 6 to 7 mm in diameter with 3 to 6 mm petals. Individual flowers contains six stamens, two shorter and four longer. Mature flowering plants reach 3.5 feet tall, although shorter flowering specimens may be found.
Impacts
Garlic mustard is a non-native species originating from Europe and parts of Asia. It is believed that garlic mustard was introduced into North America for medicinal purposes and food. The earliest known report of it growing in the United States dates back to 1868 on Long Island, NY. It has since spread throughout the eastern United States and Canada as far west as Washington, Utah, and British Columbia.
Garlic mustard has the potential to form dense stands that choke out native plants in the understory by controlling light, water, and nutrient resources. Plants most affected by these dense stands are herbaceous species that occur in similar moist soil forest habitats and grow during the spring and early summer season. Although unsupported by the lack of long-term research into garlic mustard impacts, the plant has been circumstantially tied to decreased native herbaceous species richness in invaded forests. Researchers have found that garlic mustard is allelopathic (it releases chemicals that hinder the growth of other plant species) and has inhibited growth of both grasses and herbs in laboratory settings (Michigan State University, 2008). Some researchers also believe that these compounds may hinder the beneficial relationships some plant species have with soil fungi (Roberts and Anderson, 2001). Experimental trials have shown that removal of garlic mustard leads to increased diversity of other species, including annuals and tree seedlings (MSU, 2008).
Other aspects of the forest ecosystem may be altered due to the change in the vegetative community tied to garlic mustard invasion. While the impacts to wildlife are not completely understood, altering the plant diversity can cause a change in leaf litter availability, potentially impacting salamanders and mollusks (MSU, 2008). Insects, including some butterflies, may be affected through the lost diversity in plants and loss of suitable egg-laying substrate (MSU, 2008). Garlic mustard may also affect the tree composition by creating a selective barrier that some seedlings, such as the chestnut oak (Quercus prinus), may not be able to overcome (MSU, 2008). These changes in tree composition could have significant long-term effects.
Prevention, Control and Management
There are few effective natural enemies of garlic mustard in North America. Herbivores, or animals that eat plant material, such as deer (Odocoileus virginianus) and woodchucks (Marmota monax) only remove up to 2% of the leaf area in a stand of garlic mustard (Evans et al. 2005). This level of herbivory is ineffective in controlling reproduction or survival of garlic mustard. Although 69 herbivorous insects have been found to be associated with garlic mustard in Europe, less than a dozen have been found on North American infestations of the species (Hinz and Gerber, 1998).
Manual removal of plant has been shown to prevent the spread of garlic mustard. Pulling by hand must remove at least the upper half of the root to prevent a new stalk from forming; this is most easily accomplished in the spring when the soil is soft. Hand-pulling should be performed before seeds are formed and needs to be continued for up to five years in order to deplete any established seed bank. This method works best in smaller pockets of invasion or in areas recently invaded to help prevent the development of a seed bank.
Chemical applications can also be effective for controlling garlic mustard, particularly in areas too large for removal by hand. In dense stands where other plant species are not present, a glyphosate-based herbicide such as Roundup® can be an effective method for removal. Glyphosate herbicides are non-selective, so caution must be used when non-target species are in the area. Chemical applications are most affective during the spring (March-April) when garlic mustard is one of the few plants actively growing. Fall applications may be used; however other plant species still in their growing season may be harmed. Readers are advised to check with local regulatory agencies to determine the regulations involved with chemical treatments.
The best method for controlling garlic mustard, or any other invasive plant, is to prevent its establishment. Disturbances in the forest understory that would allow for rapid invasion should be minimized. This would include limiting foot traffic, grazing, and erosion-causing activities. Monitoring the forest understory and removing any garlic mustard plants as soon as they are introduced will help to prevent the establishment and spread of this invader.
There is some indication that garlic mustard is capable of sprouting following
fire, but several questions remain. To what extent is postfire sprouting in
garlic mustard influenced by fire severity? What, if any, physiological
conditions promote or constrain postfire root crown sprouting? To what extent are
resprouting plants successful at producing seed?
Nuzzo and others [54] reported that a fall burn in a central Illinois black oak forest
removed 79% of the litter layer, and very few adult garlic mustard plants were encountered in
these plots the following spring. Conversely, many garlic mustard plants resprouted following
a mid-spring burn at the same site that resulted in removal of only 32% of the litter layer.
Spring burn plots retained a damp 0.4- to 0.8-inch (1-2 cm) layer of litter which
seems to have protected the root crowns of top-killed plants, fostering survival via sprouting
of multiple secondary shoots from adventitious buds located just below the soil surface [54].
Hintz [30] conducted a late-March prescribed burn in a mesic upland oak-hickory
forest in northern Illinois. Garlic mustard established following the fire,
although it is unclear whether these were sprouting burned plants or new spring
seedlings. The burn was conducted near the time when seedling emergence might be expected,
leaving some question as to which life-cycle stage was observed to be "sprouting".
There is reference to "very little" garlic mustard producing seed that summer,
intimating that at least some adult plants were present both prior to and after the fire.
Luken and Shea [41] conducted a prescribed fire experiment in a northern
Kentucky mesic deciduous forest in which they showed that garlic mustard plants
could be removed by a fall burn. Yet it was also apparent from this experiment that populations
can persist following even repeated burns. Garlic mustard remained the dominant species in the
herb layer of both burned and unburned plots through 3 seasons of fall burning,
and beyond. The authors proposed 3 possible explanations. First, persistence of individual garlic
mustard plants immediately following fire may result from the patchy nature of many understory
or mixed-severity burns. Under such conditions some extant plants may escape damage, and because
of its ability to self-pollinate [3,15,17], the survival of a single plant may be sufficient to
perpetuate a population. Second, the data of Luken and Shea [41] showed that burning resulted in
higher densities of flowering stems compared with control plots. They speculated this as being
due to either resprouting or release from competition. No observations of sprouting were reported.
Third, even if all plants are killed, the existing seed bank may remain viable for several years
[7,14], requiring subsequent annual burns to completely eradicate the population.
The Research Paper by Bowles and others 2007 provides information on postfire responses of several plant species, including garlic mustard, that was not available when this species review was originally written.
Garlic mustard is an established, cool-season, monocarpic, taprooted, herbaceous biennial [6,15,26,31,49,61] or occasional winter annual [15,31,61]. The common name is derived from the scent of garlic, which is noticeably exuded by its aboveground plant parts, especially foliage [15,31,73,82,86].
Seedlings develop into rosettes 0.8-4 inches (2-10 cm) in diameter during the 1st growing season. Mature plants produce erect flowering stems up to 4.13 feet (1.25 m) high [15]. Each rosette usually produces a single flowering stem, although multiple stems from a single rosette occur occasionally [41]. Flowers are borne in racemes, with fully expanded corollas 0.12-0.48 inches (3-12 mm) across [6,15,17,26,27,61,68,73,74,82,86]. Average plant biomass is quite variable within a habitat, between habitats, or between generations within the same habitat, and is strongly influenced by light. Plants grown under higher irradiance levels typically produce greater biomass per plant [3].
Seeds are produced in pods (siliques) up to 6 inches (15 cm) in length [15,27,31,74,82]. Fully developed siliques typically contain 12-19 seeds, and the number of siliques per plant can vary greatly from 1 to more than 200 [74]. Seeds are oblong to nearly cylindrical [15,61] and about 0.12 inch (3 mm) long [27,31,61].
Fire Adaptations: Although garlic mustard plants are readily top-killed when exposed to fire, they may ultimately survive by sprouting from the root crown [54]. Ecological conditions that permit sprouting are not well understood and it is unclear to what extent resprouted plants are capable of producing viable seed (see Fire Effects section of this summary).
At the population level, garlic mustard may be adapted to perpetuate itself in mixed-severity or low-severity surface FIRE REGIMES, although this has not been quantified. Even though individual plants may be killed by fire, postfire conditions may be favorable for rapid population expansion because of increases in the area of disturbed habitat and, depending on the extant community, temporary reductions in interspecific competition. Additionally, garlic mustard seed banks may facilitate rapid recolonization of disturbed areas [14]. For example, 3 consecutive years of prescribed burning in a central Illinois black oak forest, which were described as "hot and fast" with flame lengths to 4 feet (1.2 m), failed to eradicate garlic mustard populations. This was attributable, in part, to the protection afforded a small number of plants by refugia such as the lee of a downed log or an area of damp litter [54]. The ability of individual plants to escape mortality will depend upon fire severity and the heterogeneity of the fire landscape.
Fuels: Although it has been demonstrated that fire can top-kill garlic mustard (see IMMEDIATE FIRE EFFECT ON PLANT), it is also apparent that garlic mustard plants can be difficult to ignite. Nuzzo [49] noted that low fuel loads, coupled with abundant green garlic mustard plants, "which on occasion literally extinguished fires", made it difficult to achieve prescribed fire objectives.
FIRE REGIMES: Garlic mustard may be found within understory surface, stand-replacement, mixed-severity fire, and nonFIRE REGIMES [13]. Because garlic mustard has become established only relatively recently in most areas in North America, and because natural FIRE REGIMES have been substantially altered in many of these areas, predicting the response of garlic mustard to any particular fire regime is speculative. In some areas colonized by garlic mustard, estimated mean fire return intervals may be longer than the time in which garlic mustard has been present. As natural areas and preserve managers reintroduce fire into locations where natural and anthropogenic fire has been suppressed in recent times, the response of this and many other species may become better understood. Those who intend to reintroduce fire where it has been absent for a substantial period are encouraged to plan and implement research and monitoring programs and share their findings.
Fire return intervals of some of the plant communities in which garlic mustard occurs are summarized below. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find FIRE REGIMES".
Community or Ecosystem Dominant Species Fire Return Interval Range (years) maple-beech-birch Acer-Fagus-Betula > 1000 silver maple-American elm A. saccharinum-Ulmus americana sugar maple A. saccharinum > 1000 sugar maple-basswood A. saccharinum-Tilia americana > 1000 [83] bluestem prairie Andropogon gerardii var. gerardii-Schizachyrium scoparium 37,58] sugarberry-America elm-green ash Celtis laevigata-U. americana- Fraxinus pennsylvanica beech-sugar maple Fagus spp.-A. saccharum > 1000 black ash Fraxinus nigra 83] tamarack Larix laricina 35-200 [58] yellow-poplar Liriodendron tulipifera eastern white pine-northern red oak-red maple Pinus strobus-Quercus rubra-A. rubrum 35-200 Virginia pine-oak P. virginiana-Quercus spp. 10 to sycamore-sweetgum-American elm Platanus occidentalis-Liquidambar styraciflua-U. americana 83] eastern cottonwood Populus deltoides 58] aspen-birch P. tremuloides-Betula papyrifera 35-200 [21,83] black cherry-sugar maple Prunus serotina-A. saccharum > 1000 oak-hickory Quercus-Carya spp. northeastern oak-pine Quercus-Pinus spp. 10 to southeastern oak-pine Quercus-Pinus spp. white oak-black oak-northern red oak Q. alba-Q. velutina-Q. rubra northern pin oak Q. ellipsoidalis bur oak Q. macrocarpa 83] oak savanna Q. macrocarpa/Andropogon gerardii-Schizachyrium scoparium 2-14 [58,83] chestnut oak Q. prinus 3-8 northern red oak Q. rubra 10 to post oak-blackjack oak Q. stellata-Q. marilandica black oak Q. velutina 83] elm-ash-cottonwood Ulmus-Fraxinus-Populus spp. 21,83]Garlic mustard has a wide tolerance of environmental conditions for growth and reproduction, including moisture regimes ranging from periodically flooded areas to dry sand forest [15,42], light environments ranging from open fields to shaded forest interior [12,14], and a range of various soil characteristics including texture [14,15,57], nutrient level [14], organic matter content [14,15], and pH [4,14]. It is apparently not found on acid soils in Indiana, Kentucky, Massachusetts, or the Canadian Shield region [15], and is absent from undrained peat and muck soils [49].
Garlic mustard may be less competitive in areas with low soil pH, as evidenced by an experiment demonstrating a significant positive correlation (r = 0.98; p < 0.001) between plant dry weight and soil pH. This has been hypothesized as a contributing factor in the limited colonization of garlic mustard in the southern third of Illinois, where soils are more acidic than in the more heavily colonized central and northern sections of the state [4]. Inhibition of garlic mustard by acidic soils may explain its apparent absence from conifer-dominated communities [66].
Garlic mustard appears to favor shaded sites [50], and is often found in dense groups of nearly pure stands, sometimes covering large areas, particularly under moist shaded conditions such as mature eastern deciduous woodlands. In drier or more open areas plants increase allocation to fruit production, perhaps in response to observed declines in seed weight, seed germination, and seedling survivorship [14,46]. While biomass production may be greatest under full sun [15], and garlic mustard plants can also be found under dense shade, they are most commonly found in woodland understories with partial shade and are probably less invasive under extreme conditions of light or shade [49]. Nuzzo [50] describes typical habitat in Illinois as mesic upland or floodplain forest, usually shaded, and often associated with some type of disturbance. Despite its apparent affinity for moist shaded environments, garlic mustard is not tolerant of growing season inundation, which may limit its ability to invade wetland communities [49].
Most populations of garlic mustard appear to be connected to some form of disturbance [14,49]. Garlic mustard is often associated with anthropogenic disturbance such as trails, roads, or railroads [49,50], and less commonly, in farm fields and gardens [50]. Garlic mustard is sometimes linked to naturally disturbed habitats such as floodplains and riverbanks, where the combination of flooding as a dispersal agent and moist, shaded conditions may promote invasion [46]. Garlic mustard was invasive in relatively undisturbed woodlands in central Illinois. Establishment was thought to occur where small-scale anthropogenic and natural disturbance removed competing vegetation, such as areas browsed by white-tailed deer [3].
Experiments examining mechanisms that link disturbance and garlic mustard occurrence and spread are scarce. One study showed that disturbance of soil in a young hardwood forest in northern Kentucky resulted in lowered garlic mustard densities compared to undisturbed plots [39]. An experiment in a southwestern Ohio deciduous forest examined the effects of small-scale litter disturbance on garlic mustard invasiveness. There were no differences (p = 0.7184) in garlic mustard germination, rosette survival, growth, or reproduction among total litter removal, partial litter removal, and control treatments, indicating forest floor disturbance alone may not be a prerequisite for invasion [46].
More research is needed to help understand factors that influence garlic mustard invasiveness and habitat invasibility, particularly for the role of disturbance. In particular, questions involving which life history traits are affected by disturbance seem most appropriate. Experiments that separate disturbance-mediated dispersal from other interactions between disturbance and garlic mustard invasiveness might provide important insights leading to more effective management prescriptions.
Impacts: The control of garlic mustard may be desirable to undisturbed deciduous forests of the eastern and midwestern United States and southern Ontario [3,15,17,49,55]. In forested natural areas, garlic mustard has the potential to dominate the herb layer [41,52,56,91]. Invasion of mature eastern deciduous forests by garlic mustard is notable because these habitats were thought to be relatively resistant to nonindigenous plant invasion, particularly by herbaceous species [43,45,55,56]. From the results of a greenhouse study examining the competitive potential of garlic mustard, Meekins and McCarthy [45] postulated that competition for light within dense garlic mustard stands might inhibit oak regeneration in the understory of eastern deciduous woodlands. However, this same study failed to show greater levels of interspecific competition among garlic mustard, jewelweed, and box elder, 2 potential understory associates.
McCarthy [43] demonstrated removal of garlic mustard from a deciduous forest understory resulted in increased richness and abundance of understory species, especially annuals and woody perennials. Garlic mustard may be particularly detrimental to native spring ephemerals in eastern deciduous forest understories [15]. McCarthy [43] failed to demonstrate a link between the magnitude of garlic mustard infestation and native species diversity. Removal experiments, while providing some insight into possible effects of nonindigenous plant invaders, may be inherently limited in their ability to reflect impacts of invasives on preinvasion communities [84]. Limited and conflicting evidence surrounding the assumption that garlic mustard infestation necessarily results in reduced richness and cover of native herbaceous species points out the critical need for more research in this area.
The allelopathic potential of garlic mustard has received some study, with mixed results. McCarthy and Hanson [44] found little evidence of allelopathic effects of garlic mustard on several plant species studied. They attributed the success of garlic mustard invasiveness strictly to its competitive abilities. Other evidence indicates at least the possibility for allelopathic interference between garlic mustard and neighboring herbaceous plants, as well as the possibility for toxicity against mycorrhizal fungi [35,80]. Roberts and Anderson [64] found a significant negative correlation (r2 = 0.29; P< 0.05) between garlic mustard density in the field and the mycorrhizal inoculum potential of the soil. McCarthy [43] found garlic mustard inhibited establishment of seedlings of other species, yet no quantitative relationship could be discerned between garlic mustard biomass and native species diversity. This finding suggests that the mere presence of garlic mustard depresses native diversity, perhaps an allelopathic effect. Further research is needed to a) determine what mechanisms, if any, are responsible for garlic mustard allelopathy, and b) sort out the relative effects of allelopathy vs. resource competition in interactions between garlic mustard and native plants.
Control: The biology of garlic mustard presents significant challenges to its control because it simultaneously possesses characteristics of native forest herbs such as shade tolerance and relatively large seeds, as well as characteristics often ascribed to weeds such as xenogamy and autogamy, and high seed production and germination under a range of environmental conditions. It is also not impacted by its native herbivores and parasites [3,5,17,44]. While garlic mustard invades relatively undisturbed woodlands, invasion may be expedited by natural and anthropogenic disturbance that removes competing native vegetation. Once garlic mustard becomes established, further dispersal and perpetuation within a particular habitat may require little to no further disturbance [46,55].
Deciduous forest fragments that are isolated in an otherwise predominantly agricultural landscape may be more resistant to garlic mustard invasion, due to limited seed sources and inhibitive dispersal distances [12]. However, in areas with large populations of white-tailed deer, even these insular forest remnants may become colonized by garlic mustard.
As with most invasive plants, deterrence is the most effective strategy against garlic mustard. This includes annual monitoring and removal of all invading plants prior to seed production. Garlic mustard is prolific partly because of its ability to self-pollinate. A single individual can produce large numbers of genetically similar but interfertile progeny, which in turn may colonize even small, local microsite disturbances, leading to a potential garlic mustard outbreak. Allaying invasion may require reducing habitat perturbation in susceptible areas and promoting the health of native plant communities [3].
Garlic mustard population densities may oscillate widely from year to year [56]. Its biennial nature and its seed banking propensity can lead to occasions in which dense stands of garlic mustard appear where none were apparent the year before, and then seemingly disappear the following year only to reappear yet again in subsequent seasons. Further, in years where rosettes are apparently sparse and may evade detection, those monitoring such sites may easily but falsely conclude that garlic mustard is absent. In previously infested areas or areas of suspected susceptibility, careful annual monitoring may be the only way to ensure that garlic mustard is indeed absent from the site.
Once garlic mustard appears within an area, management activities should focus on preventing seed production. While most seeds of garlic mustard tend to germinate during the 1st or 2nd spring following their production, a small number of seeds remain within the seed bank and may germinate over the next several years. Because garlic mustard seed banks may remain viable for up to 6 years, long-term control for a particular stand requires vigilant attention for several consecutive seasons [3,7,14,49]. Even after successful management leads to the apparent absence of garlic mustard, continued periodic monitoring is prudent. A method for destroying seeds of garlic mustard in the soil that would not harm seeds of other species has not been determined [7].
Because of the biennial life-history strategy of garlic mustard, eradication treatments conducted during spring, after seedlings have germinated and before adults can produce viable seed, have the advantage of affecting 2 generations simultaneously [49]. Ideally, this maximizes the kill of new germinants and seedlings, as well as prevents seed production in adults. Since natural mortality is greatest at the seedling stage garlic mustard may be most vulnerable to control efforts during this time [20]. One potential downside to this strategy is that delaying treatment too late into spring risks unwanted effects on native spring emergents.
An alternative approach is to delay management activities until after the 1st growing season to take advantage of significant natural mortality of rosettes. First year garlic mustard mortality at a site in northern Illinois was estimated at greater than 95% between April and November [51]. This strategy may be especially prudent when the control method requires intensive labor, such as cutting or hand-pulling plants, if minimizing quantities of applied chemicals is desired, or simply if costs of more intensive management activities are prohibitive.
Control of garlic mustard has been tested using several different methods. Since a single control method is rarely 100% effective, a combination of more than 1 may often be a useful strategy. Regardless of methodology, treatments for eradication of garlic mustard must be applied over the entire area of infestation to prevent seed production.
Manual or Mechanical Removal: Pulling entire plants may be an effective method for control of garlic mustard. Care should be taken to remove as much of the root system as possible, to reduce resprouting potential. Pulling can cause soil disturbance and redistribute seeds stored within the upper soil horizons. This problem may be mitigated by thoroughly tamping disturbed soil after pulling. Generally speaking, cutting results in fewer disturbances than pulling. However, pulling may be done at any time during the plant lifecycle, while cutting must be performed during the 2nd growing season while the flowering stem is elongating. Due to the labor-intensive nature of cutting and pulling plants, these practices may only be practical in small or lightly infested areas, especially where burning or herbicide application is inadvisable [49,56]. Hand removal may be most useful for preventing establishment of new garlic mustard colonies in previously uninfested areas [43].
Control may be accomplished by cutting flowering stems, i.e. using sickles, clippers, or string trimmers, prior to seed production and dissemination. Cutting as close to ground level as possible appears to be most effective. Nuzzo [49] found that cutting at ground level killed 99% of plants and resulted in virtually no seed production, while cutting at 4 inches (10 cm) resulted in 71% mortality and 98% lower total seed production. Mortality was 6% in control plants during the 3-month study period. Cutting plants prior to full flowering or the onset of seed development may result in production of additional flowering stems from buds located on the root crown [56]. However, waiting until after plants have finished flowering risks dissemination of viable seed. Cut or pulled plant material should consequently be removed from the site and destroyed whenever possible to minimize the risk of inadvertently distributing viable seed [56,70].
Mowing may be similar in effect to cutting, but with more possible negative consequences. Mowing of flowering plants may result in regrowth of new flowering shoots, although this response reportedly diminishes as the season progresses [15]. While mowing may be convenient in large, relatively open areas of infestation such as roadsides, this practice may be more problematic than cutting, as described above. Mowing may promote seed dispersal and is more likely to be indiscriminate regarding which plant species are destroyed. Mowing equipment may also create more disturbed habitat that is likely to be recolonized by garlic mustard [56].
Prescribed Fire: In areas with a fire-tolerant native flora, frequent prescribed burning may deter garlic mustard invasion by both directly killing invading plants, and perhaps in some areas by enhancing growth of native herbaceous competitors and thereby reducing habitat for garlic mustard colonization [49,88]. For more information about using prescribed fire as a management tool to control garlic mustard, see the Fire Management Considerations section of this summary.
Chemical Control: Chemical control of invasive plants such as garlic mustard can be effective, particularly against large areas of infestation or dense monotypic colonies, and especially when considered within the context of an integrated management plan [47,49]. This report briefly examines the effectiveness of selected chemicals for controlling garlic mustard, some issues involved in the timing of application, and potential effects on native plant communities. Use of herbicides in natural areas should be cautiously considered, and appropriate education and training should be sought before proceeding. Particular caution should be exercised with the use of Bentazon or Acifluorfen. Bentazon is very soluble in water and does not bind to soil well, leading to potential groundwater contamination problems. Acifluorfen is toxic to fish, is moderately persistent in soil and kills native grasses and herbs, and can cause serious eye injury [79]. For further information regarding the use of herbicides in natural areas for control of this and other invasive plant species, see the Weed Control Methods Handbook [76].
The effectiveness of 2,4-D against garlic mustard is questionable [56]. Use of 2,4-D in mixtures with other chemicals may improve its effectiveness, but scant evidence is available [15,56].
Application of 1% and 2% glyphosate during the dormant season significantly (p < 0.05) reduced adult garlic mustard cover and density in mesic and dry-mesic upland forest and mesic floodplain forest in northern Illinois, but also damaged other species that were green at the time, especially sedges and white avens [53]. Treatment with foliar-applied glyphosate also significantly (p < 0.05) reduced adult densities of garlic mustard, regardless of spring or fall application, in a northern Illinois oak woodland. Seedling frequency in these same plots was significantly (p < 0.001) reduced by spring application [49].
Dormant-season application of bentazon was less effective at controlling garlic mustard in northern Illinois mesic deciduous forest, but showed none of the nontarget kill associated with glyphosate. At these same sites, application of acifluorfen during dormant season was highly effective againstgarlic mustard, but also killed most native herbaceous vegetation, which was mainly dormant at the time of application[53].
Use of systemic, nonselective herbicides during the growing season may not be practical in some areas due to deleterious effects on native ground-layer competitors. In these cases, dormant season application may be preferable in order to maintain viable populations of native competitors [49]. Nuzzo [49] found no difference in effect between single herbicide application and twice applied treatment to the same generation of plants (spring and fall of the same year, fall and the following spring, or 2 consecutive springs). It was suggested that management efforts focus on single applications to successive generations of plants. Fall herbicide application may be a prudent option when risk of negatively affecting native spring-emergent herbs exists. Higher garlic mustard rosette densities in fall may require higher volumes of applied herbicide to be effective [51].
Mid-summer application of bentazon reduced garlic mustard cover by 94-96% in previously dense stands of garlic mustard rosettes in northern Illinois. Similar applications of acifluoren were less effective, but still significantly reduced garlic mustard cover by 30-46%. Mortality in control plots over the same period was 15%, and not statistically significant. Chemical control activities conducted during the growing season, as above, might be justified when target species densities overwhelm the native flora [52].
Biological Control: Biological control methods for garlic mustard are not yet developed, but investigations are under way. Several insects that are associated with garlic mustard in its native European habitats are being tested to examine their potential effectiveness as control agents [56]. Fungal pathogens may also have some potential use against garlic mustard. For instance, garlic mustard has shown susceptibility to a fusarium root rot (Fusarium solani) [16].
Use of garlic mustard as a forage species by white-tailed deer is unclear [15,56]. White-tailed deer may avoid grazing garlic mustard in favor of native herbaceous plants, although this has not been empirically tested [3,49,56]. It is likely that white-tailed deer graze a variety of understory herb species in areas typically susceptible to garlic mustard invasion, and can have a dramatic negative impact on some native herb populations [1]. Deer grazing of native herbaceous plants may enhance garlic mustard at the expense of native species by providing small-scale soil disturbance and by reducing interspecific competition. White-tailed deer may provide small-scale disturbances suitable for garlic mustard colonization within forested areas by trampling and exposing soil. In addition, selective herbivory may enhance garlic mustard at the expense of the preferred native species [3,49,56].
Garlic mustard may be deleterious to some species of butterfly. Adults of several butterfly species lay eggs on garlic mustard instead of their native plant hosts. Because larval development on garlic mustard is often fatally inhibited, this can result in garlic mustard acting as a population sink for these butterfly species, a particularly perilous problem for rare species such as the West Virginia white butterfly (Pieris virginiensis) [10,56,59].
Pollination: Garlic mustard is capable of self-pollinization, as well as cross-fertilization [3,15,17]: both seem equivalent in effectiveness. Self-pollination often takes place before flowers open [3], although variation in this ability may exist between populations [3,17]. Cross-pollination has been observed to occur via generalist insect pollinators, providing a high likelihood of pollination wherever garlic mustard occurs [3,15,17].
Seed production: Because a large percentage of flowers typically set fruit, and most ovules develop seeds, garlic mustard is a prodigious seed producer [17]. Seed production varies between and within sites and between years, but under shaded, moist (apparently favorable) conditions, dense stands may produce > 100,000 seeds/m2 [14,15]. Seed production in Ohio ranged from 165 to 868 seeds/plant, depending on habitat and population density [74]. The number of seeds per silique in a southern Ontario study varied from 6 to 22 with an average of 16. The number of siliques varied greatly, from 1 or 2 on small plants to up to 150 per plant [15]. Seed production in several states was:
Estimated Seed Production (seeds/m2) Location 15,000 Central Illinois [3] 19,060 - 38,025 Ohio [74] 19,800 - 107,580 Southern Ontario [15] 30,689 - 45,018 New Jersey [14] 10,000 Northern Illinois [49]
Seed dispersal: In forested areas, garlic mustard is typically 1st seen along trails and streams, and can quickly spread via seeds throughout the forest within a few generations [7]. Seeds generally fall within a few meters of the plant [50,74], and may be ballistically dispelled from siliques [49]. Wind dispersal is doubtful. Seeds stick together when damp and adhere readily to small soil clusters [15]. Seed dispersal rates may accelerate along river corridors [46,50], although there are conflicting reports regarding the ability of seeds to float [15,74]. Humans may also spread seeds. Garlic mustard often invades natural areas along roads and trails, purportedly via seed transport on muddy boots or pant cuffs. Seed dispersal may also be facilitated by roadside mowing, as well as on mud-encrusted automobile tires [50]. Animals, especially white-tailed deer, may promote seed dispersal and spread of garlic mustard. Deer are thought to provide an important seed dispersal vector over short distances by transporting seeds in their fur, although this has not been tested as of this writing [3,15]. Foraging deer may create microsite disturbances favorable to garlic mustard dispersal by mixing mineral soil and garlic mustard seeds [49].
Germination: Seeds of garlic mustard require cold stratification before they can germinate, with 1 season's overwintering usually sufficient to break dormancy at most North American locations [7]. An additional year of dormancy was reportedly required prior to germination in southern Ontario [15], and this lengthier dormancy period may be required in other northern locations [55,70]. Germination often occurs in early spring and can occur at temperatures approaching 32 degrees Fahrenheit (0 °C) [7,63]. Low-temperature germination is ecologically important because garlic mustard seedlings incur a competitive advantage by being the 1st germinants of the season [7,45].
Seed banking: Garlic mustard produces small but potentially important seed banks. Seed viability has been shown to drop off substantially after the 1st growing season following stratification, indicating seed banks of garlic mustard are relatively short lived [7,63]. In a study of garlic mustard seed biology, roughly 88% of seeds that germinated did so during the 1st spring following production [7]. In a study comparing garlic mustard populations from contrasting habitats in New Jersey, 3 out of 4 populations were found to maintain a seed bank. The 4th population was located in a seasonal floodplain where flooding actions were thought to either remove the seedbank or produce a patchy distribution that was difficult to sample [14].
A small percentage of seeds may remain viable for 4-6 years [7,15,63]. Because garlic mustard is a prodigious seed producer, elimination of a single season's crop may not suffice to eradicate the species from an area because germination and survival of only a few individuals in subsequent years may quickly lead to repopulation at or near previous levels [7].
Seedling establishment/growth: Garlic mustard seedlings emerge in early spring, just before or simultaneous with germination of native spring ephemerals [49]. They establish during periods of relatively high light availability in the forest understory prior to canopy leaf-out, typically with reduced interspecific competition and drought potential [7,15,45]. Greatest mortality rates occur in spring during the seedling stage [15]. Seedling mortality can vary substantially, often depending on moisture availability [14]. Initial seedling density may be very high (20,000 seedlings/m 2) [49,74]. In reports where natural spring seedling densities were approximately 3,100 to 5,600/m2, only about 1% to 16% survived to produce flowers the following year [14,15]. Two consecutive cohorts retained similar numbers of mature flowering plants during their 2nd spring, despite having initial seedling densities differing by more than 100% [3].
Asexual regeneration: Garlic mustard spreads exclusively by seeds, with no reports of vegetative reproduction [15,74].
Garlic mustard occurs in communities that represent a wide range of successional stages, from prairie openings to understories of mature, shade-tolerant eastern hardwood forests. While garlic mustard colonizes a variety of sites, it is often mentioned with particular concern to invasiveness in the herb layer of mature eastern deciduous forests, since these communities were thought to be somewhat resistant to invasion by nonindigenous plants (see Impacts and Control). In some areas of eastern deciduous forest, dense garlic mustard stands may inhibit recruitment of woody seedlings, perhaps altering successional trajectories [45].
The ability of garlic mustard to invade and compete in habitats with light environments ranging from partial to deep shade may be due to its ability to acclimate to variation in irradiance [2,15]. Despite substantial plasticity in photosynthetic response to variation in irradiance, photosynthetic rates of garlic mustard at high light levels lag behind those of species typically found in unshaded environments, inhibiting the competitiveness of garlic mustard under these conditions [20]. Nevertheless, the ability of the species to acclimate to a wide range of light environments almost certainly contributes to its ubiquitous and invasive nature [2].
Garlic mustard is often mentioned in association with oak savannah communities which, when viewed from the context of fire as the determinant of successional trajectory, represent a transitional state between grassland and forest. For example, garlic mustard was present mainly in areas of lower ambient light levels in a northern Illinois oak savanna remnant, invading where reduced fire frequency resulted in increased tree canopy cover [11]. Because the presence of garlic mustard may inhibit the ability of a forest understory to carry surface fire [49], invasion of garlic mustard could potentially accelerate succession in these oak savannas by further suppressing fire.
Alliaria petiolata, or garlic mustard, is a biennial flowering plant in the mustard family (Brassicaceae). It is native to Europe, western and central Asia, north-western Africa, Morocco, Iberia and the British Isles, north to northern Scandinavia,[2] and east to northern Pakistan and Xinjiang in western China.[1]
In the first year of growth, plants form clumps of round, slightly wrinkled leaves, that when crushed smell like garlic. The plants flower in spring of the next year, producing cross-shaped white flowers in dense clusters. As the flowering stems bloom they elongate into a spike-like shape. When flowering is complete, plants produce upright fruits that release seeds in mid-summer. Plants are often found growing along the margins of hedges, giving rise to the old British folk name of jack-by-the-hedge. Other common names include: garlic mustard,[3] garlic root, hedge garlic, sauce-alone, jack-in-the-bush, penny hedge and poor man's mustard. The genus name Alliaria, "resembling Allium", refers to the garlic-like odour of the crushed foliage. All parts of the plant, including the roots, have this smell.
It is an herbaceous biennial plant growing from a deeply growing, thin, whitish taproot scented like horseradish. In their first years, plants are rosettes of green leaves close to the ground; these rosettes remain green through the winter and develop into mature flowering plants the following spring. Second-year plants often grow from 30–100 cm (12–39 in) tall, rarely to 130 cm (51 in) tall. The leaves are stalked, triangular through heart shaped, 10–15 cm (3.9–5.9 in) long (of which about half being the petiole) and 5–9 cm (2.0–3.5 in) broad, with coarsely toothed margins. The flowers are produced in spring and summer in small clusters. Each small flower has four white petals 4–8 mm (0.2–0.3 in) long and 2–3 mm (0.08–0.12 in) broad, arranged in a cross shape. The fruit is an erect, slender, four-sided capsule 4–5.5 cm (1.6–2.2 in) long,[4] called a silique, green maturing to pale grey brown, containing two rows of small shiny black seeds which are released when a silique splits open. A single plant can produce hundreds of seeds, which often scatter several meters from the parent plant.
Depending upon conditions, garlic mustard flowers either self-fertilize or are cross-pollinated by a variety of insects.
Sixty-nine insect herbivores and seven fungi are associated with garlic mustard in Europe. The most important groups of natural enemies associated with garlic mustard were weevils (particularly the genus Ceutorhynchus), leaf beetles, butterflies, and moths, including the larvae of some moth species such as the garden carpet moth.[5] The small white flowers have a rather unpleasant aroma which attracts midges and hoverflies, although the flowers usually pollinate themselves. In June the pale green caterpillar of the orange tip butterfly (Anthocharis cardamines) can be found feeding on the long green seed-pods from which it can hardly be distinguished.[6]
Garlic mustard is one of the oldest spices used in Europe. Phytoliths in pottery of the Ertebølle and Funnelneck-Beaker culture in north-eastern Germany and Denmark, dating to 4100–3750 BCE,[7] indicate its use in that era.
In the 17th-century Britain, it was recommended as a flavouring for salt fish. It can also be made into a sauce for eating with roast lamb or salad.[6] Early European settlers brought the herb to the New World to use as a garlic-type flavouring. Its traditional medicinal purposes include use as a diuretic.[8] The herb was also planted as a form of erosion control.[9]
Today, the chopped leaves are used for flavouring in salads and sauces such as pesto, and sometimes the flowers and fruit are included as well. The leaves, best when young, taste of both garlic and mustard. The seeds are sometimes used in France to season food.[8] Garlic mustard was once used medicinally[10] as a disinfectant or diuretic, and was sometimes used to treat wounds.[8]
Young first-year garlic mustard plants contain up to 100ppm cyanide, a level which is toxic to many vertebrates.[11] Once the plant is chopped up the cyanide gas is eliminated.[12]
Garlic mustard was introduced to North America by European settlers in the 1800s for culinary and medicinal purposes.[13] The species was recorded as being in Long Island in 1868.[14] It has since spread all over North America, apart from the far south of the US and some prairie states and Canadian provinces.[15] It is toxic or unpalatable to many native herbivores, as well as to some native Lepidoptera.[16][17][18][19]
The plant is classified as an invasive species in North America. Since being brought to the United States by settlers, it has naturalized and expanded its range to include most of the Northeast and Midwest, as well as south-eastern Canada.[20] It is one of the few invasive herbaceous species able to dominate the understory of North American forests and has thus reduced the biodiversity of many areas.[5]
Of the many natural enemies it has in its native range, several have been tested for use as biological control agents. Five weevil species from the genus Ceutorhynchus and one flea beetle were selected as candidates for preliminary testing in the 1990s. Since that time, those studying the candidates have narrowed the list to two or three weevils. Despite the demonstrated effectiveness of C. scrobicollis and C. constrictus in field testing, the importation and release of biological control agents such as those has been repeatedly blocked by the USDA's TAG (Technical Advisory Group). In particular, C. scrobicollis, which is monophagous and has been specifically studied since 2002, continues to be blocked, despite researchers' many petitions for approval.[21][22] It is currently estimated that adequate control of garlic mustard can be achieved by the introduction of just two weevils, with C. scrobicollis being the most important of the two.[23] None of the roughly 76 species that control this plant in its native range has been approved for introduction as of 2018 and federal agencies continue to use more traditional forms of control, such as chemical herbicides.[24]
In North America, the plant offers no known wildlife benefits and is toxic to larvae of certain rarer butterfly species (e.g. Pieris oleracea and Pieris virginiensis) that lay eggs on the plants, as it is related to native mustards but creates chemicals that they are not adapted to.[25][26] Native species, including two stem-mining weevils, a stem-mining fly, a leaf-mining fly, a scale insect, two fungi, and aphids (taxonomic identification for all species is pending) were found attacking garlic mustard in North America. However, their attacks were of little consequence to plant performance or reproduction of garlic mustard.[5]
Alliaria petiolata, or garlic mustard, is a biennial flowering plant in the mustard family (Brassicaceae). It is native to Europe, western and central Asia, north-western Africa, Morocco, Iberia and the British Isles, north to northern Scandinavia, and east to northern Pakistan and Xinjiang in western China.
In the first year of growth, plants form clumps of round, slightly wrinkled leaves, that when crushed smell like garlic. The plants flower in spring of the next year, producing cross-shaped white flowers in dense clusters. As the flowering stems bloom they elongate into a spike-like shape. When flowering is complete, plants produce upright fruits that release seeds in mid-summer. Plants are often found growing along the margins of hedges, giving rise to the old British folk name of jack-by-the-hedge. Other common names include: garlic mustard, garlic root, hedge garlic, sauce-alone, jack-in-the-bush, penny hedge and poor man's mustard. The genus name Alliaria, "resembling Allium", refers to the garlic-like odour of the crushed foliage. All parts of the plant, including the roots, have this smell.
_Alliaria_petiolata_-_flowers.jpg)
