Alliaria petiolata - (Bieb.) Cavara & Grande
Garlic Mustard
Other Common Names: garlic mustard
Synonym(s): Alliaria officinalis Andrz. ex Bieb.
Taxonomic Status: Accepted
Related ITIS Name(s): Alliaria petiolata (Bieb.) Cavara & Grande (TSN 184481)
French Common Names: alliaire officinale
Unique Identifier: ELEMENT_GLOBAL.2.127936
Element Code: PDBRA01010
Informal Taxonomy: Plants, Vascular - Flowering Plants - Mustard Family
 
Kingdom Phylum Class Order Family Genus
Plantae Anthophyta Dicotyledoneae Capparales Brassicaceae Alliaria
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Concept Reference
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Concept Reference: Kartesz, J.T. 1994. A synonymized checklist of the vascular flora of the United States, Canada, and Greenland. 2nd edition. 2 vols. Timber Press, Portland, OR.
Concept Reference Code: B94KAR01HQUS
Name Used in Concept Reference: Alliaria petiolata
Conservation Status
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NatureServe Status

Global Status: GNR
Global Status Last Reviewed: 22Mar1994
Global Status Last Changed: 22Mar1994
Rounded Global Status: GNR - Not Yet Ranked
Nation: United States
National Status: NNA
Nation: Canada
National Status: NNA (25Oct2017)

U.S. & Canada State/Province Status
Due to latency between updates made in state, provincial or other NatureServe Network databases and when they appear on NatureServe Explorer, for state or provincial information you may wish to contact the data steward in your jurisdiction to obtain the most current data. Please refer to our Distribution Data Sources to find contact information for your jurisdiction.
United States Arkansas (SNA), Colorado (SNA), Connecticut (SNA), Delaware (SNA), District of Columbia (SNA), Georgia (SNA), Illinois (SNA), Indiana (SNA), Iowa (SNA), Kansas (SNA), Kentucky (SNA), Maine (SNA), Maryland (SNA), Massachusetts (SNR), Michigan (SNA), Minnesota (SNA), Missouri (SNA), Montana (SNA), Nebraska (SNA), New Hampshire (SNA), New Jersey (SNA), New York (SNA), North Carolina (SNA), North Dakota (SNA), Ohio (SNA), Oklahoma (SNA), Oregon (SNA), Pennsylvania (SNA), Rhode Island (SNA), South Carolina (SNA), Tennessee (SNA), Utah (SNA), Vermont (SNA), Virginia (SNA), West Virginia (SNA), Wisconsin (SNA)
Canada Alberta (SNA), British Columbia (SNA), New Brunswick (SNA), Newfoundland Island (SNA), Nova Scotia (SNA), Ontario (SNA), Prince Edward Island (SNA), Quebec (SNA)

Other Statuses

NatureServe Global Conservation Status Factors

Range Extent Comments: Alliaria is native "throughout Europe from about 68o north southwards, but less common in the extreme south" (Tutin et al. 1964), occurring from England (Martin 1982) east to Czechoslovakia (Lhotska 1975), and from Sweden and Germany south to Italy, but is noticeably absent from Iceland, the Azores, Sardinia and Spitsbergen (Tutin et al. 1964). From this native range Alliaria has spread to North Africa, India, Sri Lanka (Cavers et al. 1979), and New Zealand (Bangerter 1985), as well as Canada (Cavers et al. 1979) and the United States (Gleason and Cronquist 1991, Nuzzo 1993a).

The North American range extends from British Columbia (Cavers et al. 1979, White et al. 1993) to New England (Gleason and Cronquist 1991), and from Ontario (Cavers et al. 1979) to Tennessee (Nuzzo 1993a). Alliaria was first recorded in North America in 1868 on Long Island NY, and by 1991 had spread to 30 states and 3 provinces (Nuzzo 1993a). This plant has spread exponentially since introduction in both Illinois (Nuzzo 1992b) and North America (Nuzzo 1993a).

In the United States Alliaria is most abundant in the New England and Midwestern states, but also has populations established as far west as North Dakota and Kansas, and south to Tennessee and North Carolina. Infrequent collections from western states indicate the plant may be a sporadic rather than established component of the regional flora, and/or in the process of becoming established in Utah (1971, 1983, 1984) and eastern Colorado (1952, 1958, 1966) (dates of herbarium collections). As of 1991 Alliaria had not been recorded west of the Rocky Mountains in the United States, with the exception of an 1892 record from Idaho, and a 1959 record from Portland Oregon (population absent in 1991). Alliaria is well established in Victoria B.C. and Vancouver in western Canada (Cavers et al. 1979, White et al. 1993).

In Canada Alliaria occurs in Victoria, British Columbia, and in the St. Lawrence Valley from Point Pelee in Ontario to Quebec City in Quebec (Cavers et al. 1979). Alliaria is especially abundant in southwestern Ontario, and near Toronto and Ottawa (White et al. 1993). White et al. (1993) recorded the plant as common in deciduous woods on the Canadian Shield, although 25 years earlier Cavers et al. (1979) stated the plant was noticeably absent from the region.

Other NatureServe Conservation Status Information

Distribution
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Global Range: Alliaria is native "throughout Europe from about 68o north southwards, but less common in the extreme south" (Tutin et al. 1964), occurring from England (Martin 1982) east to Czechoslovakia (Lhotska 1975), and from Sweden and Germany south to Italy, but is noticeably absent from Iceland, the Azores, Sardinia and Spitsbergen (Tutin et al. 1964). From this native range Alliaria has spread to North Africa, India, Sri Lanka (Cavers et al. 1979), and New Zealand (Bangerter 1985), as well as Canada (Cavers et al. 1979) and the United States (Gleason and Cronquist 1991, Nuzzo 1993a).

The North American range extends from British Columbia (Cavers et al. 1979, White et al. 1993) to New England (Gleason and Cronquist 1991), and from Ontario (Cavers et al. 1979) to Tennessee (Nuzzo 1993a). Alliaria was first recorded in North America in 1868 on Long Island NY, and by 1991 had spread to 30 states and 3 provinces (Nuzzo 1993a). This plant has spread exponentially since introduction in both Illinois (Nuzzo 1992b) and North America (Nuzzo 1993a).

In the United States Alliaria is most abundant in the New England and Midwestern states, but also has populations established as far west as North Dakota and Kansas, and south to Tennessee and North Carolina. Infrequent collections from western states indicate the plant may be a sporadic rather than established component of the regional flora, and/or in the process of becoming established in Utah (1971, 1983, 1984) and eastern Colorado (1952, 1958, 1966) (dates of herbarium collections). As of 1991 Alliaria had not been recorded west of the Rocky Mountains in the United States, with the exception of an 1892 record from Idaho, and a 1959 record from Portland Oregon (population absent in 1991). Alliaria is well established in Victoria B.C. and Vancouver in western Canada (Cavers et al. 1979, White et al. 1993).

In Canada Alliaria occurs in Victoria, British Columbia, and in the St. Lawrence Valley from Point Pelee in Ontario to Quebec City in Quebec (Cavers et al. 1979). Alliaria is especially abundant in southwestern Ontario, and near Toronto and Ottawa (White et al. 1993). White et al. (1993) recorded the plant as common in deciduous woods on the Canadian Shield, although 25 years earlier Cavers et al. (1979) stated the plant was noticeably absent from the region.

U.S. States and Canadian Provinces

Due to latency between updates made in state, provincial or other NatureServe Network databases and when they appear on NatureServe Explorer, for state or provincial information you may wish to contact the data steward in your jurisdiction to obtain the most current data. Please refer to our Distribution Data Sources to find contact information for your jurisdiction.
Color legend for Distribution Map
NOTE: The distribution shown may be incomplete, particularly for some rapidly spreading exotic species.

U.S. & Canada State/Province Distribution
United States ARexotic, COexotic, CTexotic, DCexotic, DEexotic, GAexotic, IAexotic, ILexotic, INexotic, KSexotic, KYexotic, MA, MDexotic, MEexotic, MIexotic, MNexotic, MOexotic, MTexotic, NCexotic, NDexotic, NEexotic, NHexotic, NJexotic, NYexotic, OHexotic, OKexotic, ORexotic, PAexotic, RIexotic, SCexotic, TNexotic, UTexotic, VAexotic, VTexotic, WIexotic, WVexotic
Canada ABexotic, BCexotic, NBexotic, NFexotic, NSexotic, ONexotic, PEexotic, QCexotic

Range Map
No map available.


U.S. Distribution by County Help
State County Name (FIPS Code)
MO Andrew (29003), Atchison (29005), Boone (29019), Buchanan (29021), Clark (29045), Clay (29047), Cole (29051), Cooper (29053), Franklin (29071), Gentry (29075), Grundy (29079), Henry (29083), Holt (29087), Jackson (29095), Lafayette (29107), Lincoln (29113), Linn (29115), Marion (29127), Moniteau (29135), Morgan (29141), Oregon (29149), Perry (29157)*, Platte (29165), Saline (29195), St. Charles (29183), St. Louis (29189), Sullivan (29211), Taney (29213)
* Extirpated/possibly extirpated
U.S. Distribution by Watershed Help
Watershed Region Help Watershed Name (Watershed Code)
07 Lower Des Moines (07100009)+, The Sny (07110004)+, Cuivre (07110008)+, Cahokia-Joachim (07140101)+, Upper Mississippi-Cape Girardeau (07140105)+*
10 Tarkio-Wolf (10240005)+, Nodaway (10240010)+, Independence-Sugar (10240011)+, Platte (10240012)+, Upper Grand (10280101)+, Thompson (10280102)+, Lower Grand (10280103)+, South Grand (10290108)+, Lower Missouri-Crooked (10300101)+, Lower Missouri-Moreau (10300102)+, Lamine (10300103)+, Lower Missouri (10300200)+
11 Bull Shoals Lake (11010003)+, Eleven Point (11010011)+
+ Natural heritage record(s) exist for this watershed
* Extirpated/possibly extirpated
Ecology & Life History
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Diagnostic Characteristics:

Alliaria petiolata is an obligate biennial herb of the mustard family (Brassicaceae). Seedlings emerge in spring and form basal rosettes by midsummer. Immature plants overwinter as basal rosettes. In the spring of the second year the rosettes (now adult plants) produce flower stalks, set seed, and subsequently die.

Basal leaves are dark-green and kidney-shaped with scalloped edges, 6-10 cm diameter. Stem leaves are alternate, sharply-toothed, triangular or deltoid, and average 3-8 cm long and wide, gradually reducing in size towards the top of the stem. All leaves have pubescent petioles 1-5+ cm long. New leaves produce a distinct garlic odor when crushed. The fragrance fades as leaves age, and is virtually non-existent by fall.

Plants usually produce a single unbranched or few-branched flower stalk, although robust plants have been recorded with up to 12 separate flowering stalks. Flowers are produced in spring (usually May) in terminal racemes, and occasionally in short axillary racemes. Some plants produce additional axillary racemes in mid-summer. Flowers are typical of the mustard family, consisting of four white petals that narrow abruptly at the base, and 6 stamens, two short and four long. Flowers average 6-7mm in diameter, with petals 3-6mm long. Fruits are linear siliques, 2.5-6cm long and 2mm wide, held erect on short (5mm), stout, widely divergent pedicels. Individual plants produce an average of 22 siliques (range 2 to 422; Nuzzo unpublished). Siliques contain an average of 16 seeds (range 3- 28; Nuzzo unpublished), arranged alternately on both sides of a papery sinus. Seeds are black, cylindrical (3mm x 1mm) and transversely ridged, and range in weight from 1.62-2.84mg.

Adult plants range in height from 0.05m to 1.5m, and average 1.0m, at the time of flowering. As plants of all sizes are found in the same cluster, plant height is likely a response to competition rather than genetically determined.

Immature plants can be confused with other rosette forming species, especially violets (Viola sp.), white avens (Geum canadense), and Cardamine sp. Alliaria petiolata can be distinguished from these plants by the strong garlic odor in spring and summer. In fall and winter Alliaria can be distinguished by examining the root system. Alliaria has a slender, white, taproot, with a distinctive "s" curve at the top of the root, just below the root crown (Nuzzo, personal observation). Axillary buds are produced at the root crown and along the upper part of the "s".

Chromosome number of 2n=36 has been recorded for European material, and 2n=24 for North American and European material (Cavers et al. 1979).

Excellent illustrations are contained in Cavers et al. (1979). Descriptive characteristics derived from Cavers et al. (1979) and Gleason and Cronquist (1991) except where otherwise noted. There is one other species in this genus (Gleason and Cronquist 1991).

Ecology Comments:

Alliaria seeds germinate in early spring, beginning in late February or early March, and concluding by mid May in northern states and Canada (Cavers et al. 1979, Kelley et al. 1991, Roberts and Boddrell 1983). In northern Illinois, germination coincides with emergence of spring beauty (Claytonia virginica) and false mermaid weed (Floerkea proserpinacoides).

Seedling density in heavily infested forests was recorded at 5,080/m2 at the cotyledon stage, and 2,235/m2 at the 2-3 leaf stage, in Illinois (Nuzzo unpublished), and approximated at 20,000/m2 in Ohio (Trimbur 1973). Seedlings undergo high mortality, declining by 41% (Trimbur 1973) to >50% (Cavers et al. 1979) by late spring.

By June seedlings develop the characteristic rosette of first year plants. First year rosettes are sensitive to summer drought (Byers 1988) and approximately 95% die by fall (Nuzzo 1993b). By mid-fall rosettes average 4-10 cm diameter and are dark green to purplish in color (range 1-15 cm). The rosettes continue to grow in winter during snow-free periods when temperatures are above freezing (Cavers et al. 1979).

Natural mortality continues through winter: in northern Illinois rosette density in November averaged 186.4/m2 (range 50-466/m2), and declined significantly to an average of 39.9/m2 (range 4-102/m2) by the following spring (Nuzzo 1993b). Rosette density varies between sites and years; mean densities range from 30/m2 to 80/m2 (Nuzzo 1991a), and reach a high of >450 adult plants/m2 (Nuzzo 1993b). Over-winter mortality is only slightly density-dependent: 9% of the variation between fall and spring densities was due to initial density in fall (Nuzzo 1993b). Total survival rate from seedling to adult stage varies from 1% (Nuzzo 1993b) to 2-4% (Cavers et al. 1979).

Alliaria is an obligate biennial: all plants that survive the winter produce flowers, regardless of size, and subsequently die (Cavers et al. 1979, Byers and Quinn 1988, Bloom et al. 1990). Plants only 5cm tall, with 3-4 leaves, have been observed with flowers and seeds. The majority of plants are taller, averaging 0.7 to 1.0 meters when in flower. Flower stalks begin to elongate in March or April, and flowers open early April through May. This is some 6-10 weeks after new seedlings germinate; in established populations generations overlap, and two cohorts can be seen from March through July. Alliaria flowers can be self-or cross-pollinated (Cavers et al. 1979, Babonjo et al. 1990). Syrphid flies, midges and bees visit flowers and may effect pollination (Cavers et al. 1979). Whether in-bred or out- bred, Alliaria plants maintain substantial genetic variation within populations (Byers 1988).

Plants usually produce 1-2 flowering stems, although a single individual may produce up to 12 separate stems. Damage to the primary flower stem stimulates growth of additional stems (Cavers et al. 1979) from axillary buds at the stem base and along the root crown, although such damage is not a prerequisite for development of multi-stem plants (Nuzzo personal observation). Some plants continue to produce flowers through August in small axillary inflorescences. Large plants produce flowers earlier and for a longer time period, and consequently produce significantly more seeds, than plants with small rosettes (Byers and Quinn 1988).

Seeds develop in a linear silique, with siliques forming on the lower part of the inflorescence while flowers are still opening on the upper part. Seeds ripen and disperse between mid-June and late September (Cavers et al. 1979, Kelley et al. 1991). Alliaria produces an average of 16.4 (+ 3.0) seeds/silique (range 3 to 28), and 21.8 (+ 22.5) siliques/plant (range 2 to 422; Nuzzo unpublished, Cavers et al. 1979). Actual production varies significantly within and between communities, with plants in drier communities tending to produce fewer seeds than plants in mesic and wet communities (Byers 1988). Plants produce an average of 360.5 seeds, ranging from 194.3 in mesic sand forest to 608.2 in mesic floodplain forest (Nuzzo unpublished). Maximum production per plant is estimated at 7,900 seeds on a plant with 12 stems, while minimum production is 14 seeds on a plant with 2 siliques (Nuzzo unpublished). Seed production is density dependent, with plants producing fewer seeds as density increases (Trimbur 1973). However, total seed production increases with increasing density (Trimbur 1973). In Illinois, seed production within dense patches of Alliaria ranged from 3,607/m2 to >22,000/m2 (Nuzzo unpublished), while in Ohio Trimbur (1973) reported 19,060 to 38,025 seeds/m2, and in Ontario Cavers et al. (1979) estimated average production at 19,800 to 107,580 seeds/m2.

Seeds are dormant at maturity and require 50 to 100 days of cold stratification to come out of dormancy (Byers 1988, Lhotska 1975, Baskin and Baskin 1992). Dormancy period lasts eight months in southern locales (Baskin and Baskin 1992, Byers 1988) and 22 months in northern areas (Cavers et al. 1979). Alliaria seeds may break dormancy more rapidly when exposed to low temperatures that fluctuate around freezing (0.5 to 10 C, as occurs in central states such as Kentucky) than under a constant temperature regime well below freezing (as occurs in northern states and Canada). This is likely a physiological rather than genetic response, as Ontario seed germinated at 20% and 50% in 3 months when moist stratified at 5 and 2 degrees C, respectively (Cavers et al. 1979).

Unlike some forest crucifers that fail to germinate under leaf cover, Alliaria seeds germinate in both light and dark after dormancy is broken (Bloom et al. 1990, Byers 1988). Light alone will not stimulate germination during cold stratification (Byers 1988). Germination rates of 12-100% have been reported (Baskin and Baskin 1992, Byers 1988, Cavers et al. 1979), but vary greatly within and between populations and habitats (Byers 1988, Cavers et al. 1979). Interestingly, substrate affects germination rate: Baskin and Baskin (1992) reported lower germination on sand substrates than on soil, as seeds on sand failed to afterripen (possibly due to water relations at the seed:soil interface). The majority of seeds germinate as soon as dormancy is broken (Roberts and Boddrell 1983, Baskin and Baskin 1992). A small percentage of seed remains viable in the seed bank for up to four years (Roberts and Boddrell 1983, Baskin and Baskin 1992).

Byers (1988) determined that seeds were concentrated in the upper 5cm of soil, and that three of four populations maintained a seed bank after germination. The fourth population, located in a floodplain, lacked a seedbank due to flooding and scouring of the surface, but was expected to gain new seeds during flood deposition.

Alliaria spreads exclusively by seed (Cavers et al. 1979). Seeds typically fall within a few meters radius of the plant. Wind dispersal is limited, and seeds purportedly do not float well, although seeds readily attach to moist surfaces (Cavers et al. 1979). Anthropogenic distribution is the primary dispersal mechanism (Lhotska 1975, Nuzzo 1992b, 1993a). Seeds are transported by natural area visitors on boots and in pant cuffs, pockets and hair, and by roadside mowing, automobiles and trains (Nuzzo 1992b). Seeds are widely dispersed in floodwaters. Seeds may be dispersed by rodents or birds; isolated plants are frequently found at the bases of large trees in forest interiors. Seeds may possibly be distributed directly or indirectly by white- tailed deer (Odocoileus virginianus).

In southern locales Alliaria populations are even-aged, alternating annually between immature plants and adult plants (Baskin and Baskin 1992), probably due to the 8 month seed dormancy. In northern climates Alliaria populations can be even-aged in early stages of invasion, and then become multi-aged as the seed bank builds up. Dormant seeds may have different longevity rates in northern and southern locales, but no work has been conducted to test this.

Alliaria is frequently overlooked at low density levels. In many sites Alliaria can be present for a number of years before appearing to "explode" in favorable years. Once Alliaria reaches this level of infestation control is difficult to achieve. At any given site Alliaria frequency and cover fluctuate annually, reflecting the biennial nature of the plant. These annual fluctuations are deceptive, as Alliaria consistently occurs with increasing frequency through time, on average doubling in four years (Nuzzo 1992a). The greatest increases in presence occur in sites subjected to large-scale natural disturbances. One site, flooded in mid- summer, experienced a 241% increase in frequency two years later (Nuzzo 1992a). In a site hit by a severe windstorm that blew down overstory trees, Alliaria frequency increased 1000% during the same time period (Nuzzo 1992a).

Alliaria is rarely if ever browsed by deer or other large herbivores. Alliaria is only rarely observed with obvious signs of insect herbivory in the U.S., although it is a preferred host plant for Pieridae butterflies in Europe (Forsberg and Wiklund 1989, Courtney and Duggan 1983, Remorov 1987, Kuijken 1987), and is utilized by a European curculionid weevil (Ceutorhynchus constrictus; Nielsen et al. 1989). In the Netherlands Alliaria is targeted by the orange- tip butterfly Anthocharis cardamines (Pieridae) when the preferred host species Arabis glabra is unavailable (Kuijken 1987). In eastern Europe Alliaria is utilized by butterflies that feed on commercial crucifers (Remorov 1987) and thus may be a threat to commercial production of cabbage. However, macerates of Alliaria leaves sprayed on cauliflower deterred oviposition by the garden pebble moth (Jones and Finch 1987). Alliaria is a preferred host of the monophagous weevil Ceutorhynchus constrictus (Nielsen er al. 1989). The general lack of insect utilization in North America may be due to Pieridae preference for open habitat (Alliaria usually occurs in forested habitat), general lack of natural enemies in the US, and Alliaria's cool season growth habit: Plants undergo rapid growth at low temperatures in late fall or early spring when insects are not very active (Cavers personal communication 1989). Cavers observed plants in Britain and Europe with greater obvious insect damage to leaves than in Ontario, suggesting that natural enemies may have some impact on the plant (Cavers personal communication 1989). Pieridae butterflies are common in North America, and Alliaria may be used by Pieridae species when the preferred host plant is unavailable.

In Ontario an unidentified virus (or several viruses) has been observed to kill flowering plants and prevent them from ripening viable seeds (Cavers personal communication 1989). Alliaria is frequently infected with a strain of turnip mosaic virus (TuMV-Al) in both Ontario and Europe, with infected plants developing a mosaic leaf pattern (Stobbs and Van Schagen 1987). The virus does not affect total seed production or seed germination, but does reduce diameter of individual seeds and average silique length (Stobbs and Van Schagen 1987). Although closely related to TuMV-Br, a virus that infects crops Brassicaceae, the two viruses are mutually exclusive: the Alliaria virus is not transmissible to commercial Brassicaceae species, specifically rutabagas and canola, nor does TuMV-Br infect Alliaria (Stobbs and Van Schagen 1987). In Europe Alliaria is a host plant for a number of viruses, including cucumber mosaic virus (CMV) and turnip mosaic virus (TuMV), that infect commercially propagated crucifers (Polak 1985). Alliaria is host for an isolate of turnip yellow mosaic virus (TYMV-A) that induces systemic infection in broccoli, turnip, and other crucifers grown in Europe (Pelikanova et al. 1990). This was the first finding of TYMV-A virus in wild growing vegetation in the former Czechoslovakia (Pelikanova 1990).

Alliaria was historically eaten as a potherb, particularly in winter and early spring when few greens were unavailable (Georgia 1920). There is no direct evidence that Alliaria was specifically imported for garden or medicinal use, although Fernald et al. (1958) state that this "old fashioned garden plant...has spread somewhat to roadsides and borders of groves", and cite earlier authors who describe the use of Alliaria as a salad plant. Zennie and Ogzewalla (1977) promote eating Alliaria for it's high Vitamin A content (8,600 units/100g in young leaves, 19,000 in basal leaves) and Vitamin C content (190mg/100g in young leaves), both substantially higher than levels in commercially grown fruits and vegetables.

Habitat Comments:

In its native Europe Alliaria is an edge species, growing in hedges and fencerows (Fitter et al. 1974, Martin 1982) and in open woods (Wilmanns and Bogenrieder 1988). Alliaria is disturbance adapted, and is frequently a component of ruderal communities (Swies and Kucharczyk 1982), including open, highly disturbed forests (Klauck 1986).

In North America Alliaria invades wet to dry-mesic deciduous forest (Cavers et al. 1979, Nuzzo 1992a, 1993a), and also occurs in the partial shade characteristic of oak savanna, forest edges, hedgerows, shaded roadsides, and urban areas, and occasionally in full sun (Nuzzo 1991a). Alliaria is rarely found under coniferous trees in the Midwest, but has been reported from under seven species of coniferous trees in Ontario (Cavers et al. 1979). Alliaria grows on sand, loam, and clay soils, and on both limestone and sandstone substrates, but has been observed only once growing on a drained peat soil, and does not occur on muck soils. Alliaria frequently grows in well-fertilized sites (Cavers et al. 1979), and is described as a nitrophile by Passarge (1976) and Wilmanns and Bogenrieder (1988). In Europe, Alliaria increased in cover with deposition of air-borne industrial emissions, which increased soil nitrogen, nitrate, phosphorous and pH (Wilmanns et al. 1986, Wilmanns and Bogenrieder 1988).

Alliaria is common in river-associated habitat, particularly in the Northeast (Nuzzo 1993a). It may preferentially invade drier forest communities in the Midwest than it does in the northeast (Nuzzo 1993a). This is supported by the higher presence along railroads in the Midwest (Nuzzo 1993a), which are generally indicative of drier habitats. Byers and Quinn (1987) reported that Alliaria, once considered a plant of floodplains and moist woods in New Jersey, had become common in a wider range of habitats. In the Great Plains Alliaria is most frequently recorded from moist, usually riverine, habitat and waste ground (Kansas and Oklahoma), while on the eastern edge of the Rocky Mountains Alliaria has been recorded along hiking trails (Utah), and on hotel grounds and around a beaver pond (Colorado).

Economic Attributes Not yet assessed
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Management Summary Not yet assessed
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Population/Occurrence Delineation Not yet assessed
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Population/Occurrence Viability
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U.S. Invasive Species Impact Rank (I-Rank)
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Disclaimer: While I-Rank information is available over NatureServe Explorer, NatureServe is not actively developing or maintaining these data. Species with I-RANKs do not represent a random sample of species exotic in the United States; available assessments may be biased toward those species with higher-than-average impact.

I-Rank: High/Medium
Rounded I-Rank: High
I-Rank Reasons Summary: Widespread, but commonly in highly disturbed systems. Although recent evidence points to Alliaria petiolata starting to invade a greater range of geographic and ecological areas, including intact, healthy ecosystems.
Subrank I - Ecological Impact: Medium/Low
Subrank II - Current Distribution/Abundance: High
Subrank III - Trend in Distribution/Abundance: High/Medium
Subrank IV - Management Difficulty: Medium
I-Rank Review Date: 28Apr2006
Evaluator: Fellows, M.
Native anywhere in the U.S?
Native Range: Europe (Rowe and Swearingen 1997) including England, Belgium & Netherlands (Nuzzo 2000).

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Screening Questions

S-1. Established outside cultivation as a non-native? YES
Comments: (Kartesz 1999).

S-2. Present in conservation areas or other native species habitat? Yes
Comments: (Rowe and Swearingen 1997; Nuzzo 2000).

Subrank I - Ecological Impact: Medium/Low

1. Impact on Ecosystem Processes and System-wide Parameters:Low significance/Insignificant
Comments: Inferred - Element Stewardship Abstract (Nuzzo 2000) is very thorough and mentions nothing. Richter, however, (pers. comm.) suggests an alteration in fire frequency in oak woodlands. But it states "no significant changes" in Randall and Marinelli (1996).

2. Impact on Ecological Community Structure:Medium/Low significance
Comments: In areas with Alliaria petiolata cover of native herbs declined; but species richness did not change (Nuzzo 2000). Impact is greatest early in the season (Hall, pers. comm.). Community structure may undergo more profound changes over time, however: Stinson et al. (2006) speculate that garlic mustard may supress seedling regeneration of dominant canopy trees by inhibiting the mycorrhizal fungi on which these depend; this may favor weedy herbaceous plants which have less mycorrhizal dependency.

3. Impact on Ecological Community Composition:High significance
Comments: Severe threat to community (plants and animals) (Rowe and Swearingen 1997). Aggressively monopolizes light & resources (Rowe and Swearingen 1997). Deprives wildlife of native forage (Rowe and Swearingen 1997). Dominates the understory (Nuzzo 2000). No documented correlation with species richness and Alliaria petiolata presence (Nuzzo 2000). Potential to form monospecific stand (Richter, pers. comm.). A recent study by Stinson et al. (2006) indicates that garlic mustard produces a phytochemical that kills or inhibits mycorrhizal fungi on which many woody plants depend; they showed virtual elimination of the activity of native arbuscular mycorrhizal fungi and speculate that this could cause profound changes in plant species composition over time.

4. Impact on Individual Native Plant or Animal Species:High significance
Comments: Rare native insect: Pieris virginiensis (Alliaria petiolata is outcompeting it's food
source (toothworts) & eggs laid on A. petiolata do not hatch (population sink)) (Rowe and Swearingen 1997). Also affects Pieris napi oleracea and Pieris napi marginata (Nuzzo 2000).


5. Conservation Significance of the Communities and Native Species Threatened:High/Low significance
Comments: Usually found in highly disrupted sites, but may invade some high quality riverine habitats (Nuzzo 2000; Richter, pers. comm.; Hillmer, pers. comm.). But Hall (pers. comm.) and Keech (pers. comm.) stress that heavy impact is on disturbed sites, and invasions are rare onto high quality sites.

Subrank II. Current Distribution and Abundance: High

6. Current Range Size in Nation:High significance
Comments: (Kartesz 1999).

7. Proportion of Current Range Where the Species is Negatively Impacting Biodiversity:High significance
Comments: CT, IL, IN, MO, WI, MN - negative impacts native species/conservation impacts; VT invasive species 1 (Nuzzo 2000).

8. Proportion of Nation's Biogeographic Units Invaded:High/Moderate significance
Comments: Potentially in c. 35-40 ecoregions - inferred from Kartesz (1999) and TNC (2001).

9. Diversity of Habitats or Ecological Systems Invaded in Nation:Moderate significance
Comments: Moist, shaded soil of river floodplains, forests, roadsides, edges of forests, trail edges, forest openings (Rowe and Swearingen 1997). It is an edge species in both it's native and non-native range; but also found in deciduous forest, oak savanna, and rarely under conifers (Nuzzo 2000). Once it was limited to floodplain and moist woods habitats only, but now in drier habitats as well in New Jersey (Nuzzo 2000).

Subrank III. Trend in Distribution and Abundance: High/Medium

10. Current Trend in Total Range within Nation:High/Moderate significance
Comments: No known enemies in US; spread exponentially since introduction (Nuzzo 2000). Root rot can kill 80-90% of population in lab conditions (Nuzzo 2000). Starting to invade new habitats in New Jersey (Nuzzo 2000). Experts sat rapid to moderate (Hall, pers. comm.; Ricther, pers. comm.; Keech, pers. comm.; Hillmer, pers comm.).

11. Proportion of Potential Range Currently Occupied:Medium/Low significance
Comments: Expanding into new habitats and range (Nuzzo 2000).

12. Long-distance Dispersal Potential within Nation:High/Moderate significance
Comments: Plant moves several meters on its own, was dispersed by humans intentionally in ~1800s. no wind or water dispersal (Rowe and Swearingen 1997). Moves by people (adhesion onto people) and floodwaters (Nuzzo 2000).

13. Local Range Expansion or Change in Abundance:High significance
Comments: Rapidly expands where disturbance occurs, but range will decrease between disturbances; on average doubling every 4 years, tripling in 8 (Nuzzo 2000). 27 states in 1992 and 34 in 2000 (Hall, pers. comm.).

14. Inherent Ability to Invade Conservation Areas and Other Native Species Habitats:High significance
Comments: Usually invades early successional communities and disturbance adapted locales; or near the bases of trees in floodplain forest interiors (Nuzzo 2000). It is, however, shade tolerant and can invade forested areas (Nuzzo 2000). It is reported from intact healthy stands by Hall, Keech and Richter (all pers. comms).

15. Similar Habitats Invaded Elsewhere:Low significance
Comments: Also present in Canada (but edge habitats) (Kartesz 1999; Weber 2003). New Zealand (Nuzzo 2000).

16. Reproductive Characteristics:Moderate significance
Comments: Biennial, set seed by July of year following germination, seeds viable throughout summer, 1000s seeds per plant; 5+ yr seed bank; resprouts from root fragments (Rowe and Swearingen 1997). Seed production depends on density with lower density yielding a greater number of seeds per area (up to 107000/m2) but on average 136-295 seeds / plant (Nuzzo 2000). 8-22 month dormancy (Nuzzo 2000). The majority of seeds germinate after dormancy with small % remaining viable in seed bank up to 4 years (Nuzzo 2000).

Subrank IV. General Management Difficulty: Medium

17. General Management Difficulty:Moderate significance
Comments: Hand pulling in light infestions with native plants (Rowe and Swearingen 1997). Can clip close to ground if plants have flowered or if plants have immature fruit, clip but remove seed heads (Rowe and Swearingen 1997). May apply Roundup (but may negatively effect surrounding vegetation) (Rowe and Swearingen 1997). Difficult to eradicate once established - considerable expenditures required (Nuzzo 2000).

18. Minimum Time Commitment:Moderate significance
Comments: Effective management is long-term - at least 5 years (Rowe and Swearingen 1997).

19. Impacts of Management on Native Species:Medium/Low significance
Comments: Many native look-alikes (Rowe and Swearingen 1997). Non-target damage associated with herbicides.

20. Accessibility of Invaded Areas:Low significance/Insignificant
Comments: Access can be granted any time of year, problems may be associated with need to go back several years in a row.
Authors/Contributors
Help
NatureServe Conservation Status Factors Edition Date: 15May1994
NatureServe Conservation Status Factors Author: Victoria Nuzzo Native Landscapes 1947 Madron Road Rockford, IL 61107 EDITED BY: John M. Randall.

Botanical data developed by NatureServe and its network of natural heritage programs (see Local Programs), The North Carolina Botanical Garden, and other contributors and cooperators (see Sources).

References
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  • Anderson, R.C., S.S. Dhillion, and T.M. Kelley. 1996. Aspects of the ecology of an invasive plant, Garlic Mustard (ALLIARIA PETIOLATA), in central Illinois. Restoration Ecology 4(2):181-191.

  • Burke, D.J. 2008. Effects of Alliaria petiolata (Garlic Mustard; Brassicaceae) on mycorrhizal colonization and community structure in three herbaceous plants in a mixed deciduous forest. American Journal of Botany 95(11): 1416-1425.

  • Catling, P.M., G. Mitrow, and A. Ward. 2015. Major Invasive Alien Plants of Natural Habitats in Canada. 12. Garlic Mustard, Alliaria petiolata. Canadian Botanical Association Bulletin 48(2): 51-60.

  • Catling, P.M., and G. Mitrow. 2005. A prioritized list of the invasive alien plants of natural habitats in Canada. Canadian Botanical Association (CBA / ABC) Bulletin 38(4): 55-57.

  • Cruden, R.W., A.M. McClain, and G.P. Shrivastava. 1996. Pollination biology and breeding system of ALLIARIA PETIOLATA (Brassicaceae). Bulletin of the Torrey Botanical Club 123(4):273-280.

  • Douglas, G.W., G.D. Straley, D. Meidinger, and J. Pojar, eds. 1998. Illustrated Flora of British Columbia, Vol. 2, Dicotyledons (Balsaminaceae through Cucurbitaceae). B.C. Minist. Environ., Lands and Parks, and B.C. Minist. For. Res. Program. 401pp.

  • Flora of North America Editorial Committee. 2010. Flora of North America North of Mexico. Vol. 7. Magnoliophyta: Salicaceae to Brassicaceae. Oxford University Press, New York. xxii + 797 pp.

  • Kartesz, J.T. 1994. A synonymized checklist of the vascular flora of the United States, Canada, and Greenland. 2nd edition. 2 vols. Timber Press, Portland, OR.

  • Kartesz, J.T. 1996. Species distribution data at state and province level for vascular plant taxa of the United States, Canada, and Greenland (accepted records), from unpublished data files at the North Carolina Botanical Garden, December, 1996.

  • Kartesz, J.T. 1999. A synonymized checklist and atlas with biological attributes for the vascular flora of the United States, Canada, and Greenland. First edition. In: Kartesz, J.T., and C.A. Meacham. Synthesis of the North American Flora, Version 1.0. North Carolina Botanical Garden, Chapel Hill, N.C.

  • Lane, Rachel. 1999. Invasion of Alliaria petiolata and Rosa multiflora in Six Indiana State Parks. Hanover College Independent Study. 33 pp.

  • Mulligan, G.A. 2002. Weedy introduced mustards (Brassicaceae) of Canada. Canadian Field-Naturalist 116(4): 623-631.

  • Nuzzo, V. 1994. Element stewardship abstract for ALLIARIA PETIOLATA (ALLIARIA OFFICINALIS), Garlic Mustard. The Nature Conservancy, Arlington, Virginia.. 21 pp.

  • Nuzzo, V. 2000. Element Stewardship Abstract for Alliaria petiolata (Alliaria officinalis) Garlic Mustard.

  • Randall, J.M. and J. Marinelli (eds.) 1996. Invasive plants: weeds of the global garden. Brooklyn Botanic Garden, New York.

  • Rowe, P. and J. M. Swearingen. 1997. Garlic Mustard. Plant Conservation Alliance Alien Plant Working Group. Accessed 8/14/2002 & 12/16/2003.

  • Stinson, K. A., S. A. Campbell, J. R. Powell, B. E. Wolfe, R. M. Calloway, G.C. Thelen, S. G. Hallett, D. Prati, and J. N. Klironomos. 2006. Invasive plant supresses the growth of native tree seedlings by disrupting belowground mutualisms. Public Library of Science (4)5.

  • Swink, F., and G. Wilhelm. 1994. Plants of the Chicago Region. Morton Arboretum. Lisle, Illinois.

  • The Nature Conservancy. 2001. Map: TNC Ecoregions of the United States. Modification of Bailey Ecoregions. Online . Accessed May 2003.

  • Weber, E. 2003. Invasive plant species of the world: a reference guide to environmental weeds. CABI Publishing, Cambridge, Massachusetts. 548 pp.

  • Weber, W. A. and R. C. Wittmann. 1992. Catalog of The Colorado Flora: A Biodiversity Baseline. University Press of Colorado, Niwot, CO.

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