• GENERAL BOTANICAL CHARACTERISTICS
• RAUNKIAER LIFE FORM
• REGENERATION PROCESSES
• SITE CHARACTERISTICS
• SUCCESSIONAL STATUS
• SEASONAL DEVELOPMENT

GENERAL BOTANICAL CHARACTERISTICS:

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 [6,15,17,26,27,61,68,73,74,81,85]. 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 [15,27,31,74,81]. 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].

RAUNKIAER [62] LIFE FORM:

REGENERATION PROCESSES:

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 as follows:

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 [56,70]. 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/m²) [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].

SITE CHARACTERISTICS:

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 that 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.

SUCCESSIONAL STATUS:

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.

SEASONAL DEVELOPMENT:

Seedlings develop into rosettes early during the 1st growing season. In central Illinois, garlic mustard leaf area increased into early June, then declined slightly or remained nearly constant until the following spring [3]. Rosettes overwinter and may retain considerable green leaf tissue. They are capable of winter growth during periods of above-freezing temperatures and no snow cover [15,74], although some leaf tissue may be destroyed by extended periods of subfreezing temperatures [3]. Anderson and others [3] observed that new leaf growth was initiated when temperatures warmed to greater than 37-41 degrees Fahrenheit (3-5 °C) for several hours per day. Substantial winter mortality is possible. Nearly 80% mortality was recorded from November 1989 to May 1990 in a northern Illinois dry-mesic upland forest [51]. Fall and winter growth varies by season and location, but by late winter rosettes have leaf primordia surrounding 1 or more terminal flowerbuds [74].

Garlic mustard plants that survive winter undergo rapid bolt growth early in the 2nd spring (March in many areas) [14,15,74]. Reinitiation of growth during this period includes increases in leaf production and elongation of flowering stems, and all surviving plants bolt [15,74]. In central Illinois, plants bolted in March, growing quickly to a maximum leaf area by late April or early May. Stem growth was also rapid during this period, elongating at an average rate of 0.76 inch/day (1.9 cm/day) between April 18 and May 13. Following this period of rapid growth, leaf area quickly declined. Nearly all 2-year-old plants were devoid of green leaves by early July [3]. The fact that bolting occurs very early in spring may contribute to garlic mustard's competitive ability by limiting light and space resources that might otherwise be available to native herbs [45].

Flowering occurs in spring of the 2nd season, usually from early April to early June, depending upon location and season [3,15]. In central Illinois lower flowerbuds appeared in early April, and reached the maximum number per plant about 2 weeks later. Flowering, and the presence of green fruit, peaked around mid-May. Fruit dehiscence began in late May or early June, and by late June no green fruits remained [3]. Damage to terminal flowerbuds can initiate flower production via axillary buds, sometimes occurring well into summer, but these late-season blooms rarely produce viable fruit [3,74].

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