Alliaria petiolata (garlic mustard)
- Summary of Invasiveness
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Plant Type
- Distribution Table
- History of Introduction and Spread
- Risk of Introduction
- Habitat List
- Hosts/Species Affected
- Growth Stages
- Biology and Ecology
- Air Temperature
- Rainfall Regime
- Soil Tolerances
- Natural enemies
- Notes on Natural Enemies
- Means of Movement and Dispersal
- Plant Trade
- Impact Summary
- Environmental Impact
- Impact: Biodiversity
- Threatened Species
- Social Impact
- Risk and Impact Factors
- Uses List
- Similarities to Other Species/Conditions
- Prevention and Control
- Links to Websites
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Alliaria petiolata (Bieb.) Cavara & Grande
Preferred Common Name
- garlic mustard
Other Scientific Names
- Alliaria alliaria Huth.
- Alliaria officinalis Andrz. ex M. Bieb.
- Arabis petiolata M. Bieb.
- Erysimum alliaria L.
- Sisymbrium alliaria Scop.
- Sisymbrium officinalis DC.
International Common Names
- English: garlic-root; garlicwort; hedge-garlic; Jack-by-the-hedge; Jack-in-the-bush; mustard-root; poor-man's-mustard; sauce-alone
- Spanish: Ajo mostaza; Hierba del ajo
- French: Alliaire officinale
- Portuguese: erva-alheira
Local Common Names
- Germany: Gemeine Knoblauchsrauke
- Italy: Alliaria; Erba alliaria
- Netherlands: Look-zonder-look
- Sweden: Loektrav
- ALAPE (Alliaria petiolata)
Summary of InvasivenessTop of page
A. petiolata has spread throughout much of the north-eastern and mid-western USA and Canada after its introduction from Eurasia. The species invades forested communities and edge habitats. The plant has no known natural enemies in North America, has a broad ecological amplitude with considerable plasticity, is self-fertile, maintains a seed-bank, and is quite difficult to eradicate once established. There are risks of further introduction to similar climatic zones in other continents.
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Plantae
- Phylum: Spermatophyta
- Subphylum: Angiospermae
- Class: Dicotyledonae
- Order: Capparidales
- Family: Brassicaceae
- Genus: Alliaria
- Species: Alliaria petiolata
Notes on Taxonomy and NomenclatureTop of page
Alliaria petiolata (Bieb.) Cavara & Grande is a member of the Brassicaceae. It is native to Eurasia (Gleason and Cronquist, 1991) but has more recently spread throughout North America. Garlic mustard is the most widely accepted international common name.
DescriptionTop of page
A. petiolata is generally considered to be a biennial throughout most of its native and naturalized range; however, it may act as a short-lived perennial in Canada (Cavers et al., 1979). Cotyledon leaves average 6 mm in length, with the first true leaves 1–5 cm in diameter and coarsely toothed. A rosette is formed during the first growing season which persists through the winter with reniform-shaped leaves 2–12 cm diameter that remain green throughout the dormant season. During the second growing season, the plant matures in early spring and produces a bolting flowering stem up to 1.5 m. The plant is simple or little-branched, and is generally glabrous with a few simple hairs. The lower leaves are often reniform; however, the remainder are deltoid, 3–6 cm long and wide, acute, and coarsely toothed. Leaves emit a distinct odour of garlic when crushed (hence common name). The flowers have 4 white petals; they are typically born in button-like clusters at the end of each stem. Fruits are linear, elongated, nearly cylindrical siliques (pods divided into two carpels by a thin division), and bear numerous black seeds. Detailed descriptions and drawings can be found in Cavers et al. (1979).
Plant TypeTop of page Biennial
DistributionTop of page
A. petiolata is a Eurasian native, with a very wide native range extending from China, through central Asia, to western Europe and North Africa. The species has become widely naturalized throughout North America and elsewhere.
Distribution TableTop of page
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|Afghanistan||Present||Native||Not invasive||USDA-ARS, 2003|
|Armenia||Present||Native||Not invasive||USDA-ARS, 2003|
|Azerbaijan||Present||Native||Not invasive||USDA-ARS, 2003|
|China||Present||Present based on regional distribution.|
|-Tibet||Present||Native||Not invasive||Flora of China Editorial Committee, 2003; USDA-ARS, 2003|
|-Xinjiang||Present||Native||Not invasive||Flora of China Editorial Committee, 2003; USDA-ARS, 2003|
|Georgia (Republic of)||Present||Native||Not invasive||USDA-ARS, 2003|
|India||Present||Native||Not invasive||USDA-ARS, 2003|
|Kazakhstan||Present||Native||Flora of China Editorial Committee, 2003|
|Kyrgyzstan||Present||Native||Flora of China Editorial Committee, 2003|
|Nepal||Present||Native||Not invasive||USDA-ARS, 2003|
|Pakistan||Present||Native||Not invasive||USDA-ARS, 2003|
|Morocco||Present||Native||Not invasive||USDA-ARS, 2003|
|Tunisia||Present||Native||Not invasive||USDA-ARS, 2003|
|Canada||Present||Present based on regional distribution.|
|-British Columbia||Present||Introduced||Nuzzo, 2003|
|-Ontario||Widespread||Introduced||1890||Invasive||Cavers et al., 1979|
|USA||Present||Present based on regional distribution.|
|-Georgia||Present||Introduced||USDA-NRCS, 2002; Zomlefer et al., 2010|
|-New Hampshire||Present||Introduced||USDA-NRCS, 2002|
|-New Jersey||Present||Introduced||Invasive||USDA-NRCS, 2002|
|-New York||Present||Introduced||1868||Invasive||USDA-NRCS, 2002|
|-North Carolina||Present||Introduced||Invasive||USDA-NRCS, 2002|
|-North Dakota||Present||Introduced||USDA-NRCS, 2002|
|-South Carolina||Present||Introduced||USDA-NRCS, 2002|
|-South Dakota||Present||Introduced||Nuzzo, 2003|
|-West Virginia||Present||Introduced||Invasive||USDA-NRCS, 2002|
|Albania||Present||Native||Royal Botanic Garden Edinburgh, 2003; USDA-ARS, 2003|
|Austria||Present||Native||Royal Botanic Garden Edinburgh, 2003|
|Belgium||Present||Native||Royal Botanic Garden Edinburgh, 2003|
|Bulgaria||Present||Native||Royal Botanic Garden Edinburgh, 2003; USDA-ARS, 2003|
|Czech Republic||Present||Native||Royal Botanic Garden Edinburgh, 2003|
|Denmark||Present||Native||Royal Botanic Garden Edinburgh, 2003|
|Finland||Present||Native||Royal Botanic Garden Edinburgh, 2003|
|France||Present||Native||Royal Botanic Garden Edinburgh, 2003; USDA-ARS, 2003|
|-Corsica||Present||Native||Royal Botanic Garden Edinburgh, 2003|
|Germany||Present||Native||Royal Botanic Garden Edinburgh, 2003|
|Greece||Present||Native||Royal Botanic Garden Edinburgh, 2003; USDA-ARS, 2003|
|Hungary||Present||Native||Royal Botanic Garden Edinburgh, 2003|
|Ireland||Present||Native||Royal Botanic Garden Edinburgh, 2003|
|Italy||Present||Native||Royal Botanic Garden Edinburgh, 2003; USDA-ARS, 2003|
|Netherlands||Present||Native||Royal Botanic Garden Edinburgh, 2003|
|Norway||Present||Native||Royal Botanic Garden Edinburgh, 2003|
|Poland||Present||Native||Royal Botanic Garden Edinburgh, 2003|
|Portugal||Present||Native||Royal Botanic Garden Edinburgh, 2003; USDA-ARS, 2003|
|Romania||Present||Native||Royal Botanic Garden Edinburgh, 2003|
|Russian Federation||Present||Present based on regional distribution.|
|-Central Russia||Present||Native||Royal Botanic Garden Edinburgh, 2003|
|-Eastern Siberia||Present||Native||Flora of China Editorial Committee, 2003|
|-Northern Russia||Present||Native||Royal Botanic Garden Edinburgh, 2003|
|-Southern Russia||Present||Native||Royal Botanic Garden Edinburgh, 2003|
|-Western Siberia||Present||Native||Flora of China Editorial Committee, 2003|
|Serbia||Present||Native||Royal Botanic Garden Edinburgh, 2003; USDA-ARS, 2003|
|Slovakia||Present||Native||Royal Botanic Garden Edinburgh, 2003|
|Spain||Present||Native||Royal Botanic Garden Edinburgh, 2003; USDA-ARS, 2003|
|Sweden||Present||Native||Royal Botanic Garden Edinburgh, 2003|
|Switzerland||Present||Native||Royal Botanic Garden Edinburgh, 2003|
|UK||Present||Native||Royal Botanic Garden Edinburgh, 2003|
|Yugoslavia (former)||Present||Native||Royal Botanic Garden Edinburgh, 2003; USDA-ARS, 2003|
|Australia||Present||Present based on regional distribution.|
|-New South Wales||Present||Introduced||Royal Botanic Gardens Sydney, 2003|
|-Victoria||Present||Introduced||Royal Botanic Gardens Sydney, 2003|
History of Introduction and SpreadTop of page
In its native habitat, A. petiolata generally grows as a solitary plant of low abundance. However, throughout much of eastern North America where introduced, A. petiolata spreads invasively and forms dense populations threatening native species. The species was first recognized as a problem by Cavers et al. (1979) in Ontario and not considered a threat in the USA until the late 1980s. By 1991, most Midwestern states recognized the species as a major concern. Nuzzo (1993) provides a comprehensive review of the distribution and spread of the species throughout the USA, using herbarium specimens and collection records. A. petiolata was first recorded in North America in 1868 on Long Island, New York (Nuzzo, 1993). The species was probably introduced by early colonists who valued it for culinary and medicinal purposes (Grieve, 1959). Its spread was gradual for the next 20 years, but reached north to Canada and west to Ohio, Iowa, and Idaho by 1890. The species entered into an exponential growth pattern of spread in the mid 1900s and is now known from most US states (USDA-ARS, 2002) and Canadian provinces (Rollins, 1993). Welk et al. (2002) using GIS models have shown that A. petiolata distribution in North America will ultimately mimic the widespread native Eurasian range based on climatic similarity. As with many invasive species, anthropogenic disturbance appears to be an important component of the species' spread.
Risk of IntroductionTop of page
Further spread throughout eastern North America is virtually guaranteed. The ecological amplitude of this species and its phenotypic plasticity will permit it to thrive in a variety of forested habitats. High elevation, desert, and warm habitats found in much of southern and western USA, however, are unlikely to see invasion. Thus, A. petiolata has probably reached many of the areas that it is capable of growing in North America and will now expand rapidly within those habitats. However, its continued available from seed companies and it uses mean that further introduction to other similar habitats in other continents cannot be discounted.
HabitatTop of page In its native environment, the plant often grows singly in hedges and fencerows, open woods, and disturbed areas (Nuzzo, 2003); rarely considered an understorey 'dominant' and is a frequent component of ruderal vegetation communities. However, its habit within invaded areas of North America is completely different. There, it usually grows in moist, rich, forest environments and in dense patches. The species has broad ecological amplitude; it thrives in areas as diverse as mesic riparian areas to relatively xeric upland hardwood forests, where it performs best in partial shade and is less successful in full shade or full sun (Dhillion and Anderson, 1999; Meekins and McCarthy, 2001). Roads and river banks have been recorded as the primary collection sites of the species, but forested riparian areas, upland forests, and urban areas are also important (Nuzzo, 1993). Shaded, moist, rich sites appear to harbour the greatest densities.
Habitat ListTop of page
|Terrestrial – Managed||Managed forests, plantations and orchards||Present, no further details||Harmful (pest or invasive)|
|Disturbed areas||Present, no further details||Harmful (pest or invasive)|
|Rail / roadsides||Present, no further details||Harmful (pest or invasive)|
|Urban / peri-urban areas||Present, no further details||Harmful (pest or invasive)|
|Terrestrial ‑ Natural / Semi-natural||Natural forests||Present, no further details||Harmful (pest or invasive)|
|Riverbanks||Present, no further details||Harmful (pest or invasive)|
Hosts/Species AffectedTop of page
A. petiolata is not normally a weed of croplands. A. petiolata is most often found in field margins and forests, often surrounding croplands. It is known as a strong competitor and has been demonstrated in the USA to out-compete other native herbs and woody plants (Meekins and McCarthy, 1999) and ultimately alter native community composition (McCarthy, 1997).
Growth StagesTop of page Pre-emergence, Seedling stage, Vegetative growing stage
Biology and EcologyTop of page
The chromosome number of A. petiolata is generally considered to be 2n=42 (Gleason and Cronquist, 1991) in North America and Europe. However, the sporophytic chromosome number has also been reported as 14 in India (Naqshi and Javeid, 1976) and 36 in the Netherlands (Gadella and Kliphuios, 1976). Based on this information, A. petiolata is hypothesized to be a hexaploid species that is based on a base chromosome number of n=7 (Al-Shehbaz, 1988). In addition, A. petiolata is known to be highly plastic in different habitats showing broad variation in patterns of resource allocation (Byers and Quinn, 1998; Susko and Lovett-Doust, 1998, 1999, 2000a). The results of analysis of within- and among-population genetic variability were that 61% of the total variation occurred among populations, with much less (16.3%) between North American and Eurasian populations and within populations (22.1%); however, North American and Eurasian populations were found to be significantly different (Meekins et al., 2001).
Physiology and Phenology
A. petiolata generally acts as a biennial in North America (Cavers et al., 1979), but may occasionally respond as a winter annual in parts of its native range (Grime et al., 1988). Propagation is entirely by seed. Seeds of A. petiolata are dormant at maturity (mid-summer) and require cold stratification to come out of dormancy. Viability at maturity is often 100% (Anderson et al., 1996). In northern Europe and Canada, A. petiolata seeds are commonly reported to show an 18 month dormancy period (Cavers et al., 1979), whereas in the south of its range, only 6 months may be required (Lhotska, 1975; Baskin and Baskin, 1992). Optimal seed germination temperatures were observed at 16/6°C, and declined with progressively higher temperatures (Baskin and Baskin, 1992). Germination may occur in either the light or dark. Germination is nearly complete at the end of the first germination season (>95%); however, studies in unheated greenhouses and field experiments have shown that seeds may still germinate up to 4 years later (Baskin and Baskin, 1992; Anderson et al., 1996), so the formation of a seed-bank is possible. Following germination in the spring, A. petiolata forms a slender taproot, sometimes branched.
The photosynthetic rate of first-year plants has been examined by Dhillion and Anderson (1999). They found that the species seemed more characteristic of a shade-adapted species as compared to sun-adapted species, which may explain why it performs well in partially shaded forest habitats. During the second growing season, A. petiolata matures in early spring and produces a bolting flowering stem up to 1.5 m. The success of A. petiolata in invaded areas in North America may be due to its achieving maximum photosynthetic rates before the active growth of many native ground layer species when irradiance reaching the ground layer is high when the temperature and moisture conditions are favourable for the species (Myers and Anderson, 2003).
Flower buds are usually seen in early April when the plants begin to bolt (Anderson et al., 1996). The maximum number of buds is in mid April which often coincides with the date of first flowering. By early June, flower buds are usually absent, but occasional flowers may be borne in the leaf axils, though these have not been reported to ever bear fruit in the field (Anderson et al., 1996). Fruits may mature and dehisce seeds as early as mid-July. Anderson et al. (1996) show a bimodal pattern of seed production with peaks in mid-August and mid-September. In the USA, seed rain was approximately 15,000 seeds/m² in Illinois field populations (Anderson et al., 1996), but considerably greater seed production, up to 38,000 seed/m² has been observed in Ohio (Trimbur, 1973). Seed production is often 100 times greater than observed seedling densities.
The species may have allelopathic properties and thus may interfere with woodland, plantation, or adjacent cropland species. The allelopathic potential of A. petiolata is difficult to confirm. There has been a long interest in the chemical products from this plant (Herissey and Boivin, 1927; Cole, 1975). Several phytotoxic hydrolysis products of glucosinolates have been isolated from A. petiolata (Vaughn and Berhow, 1999), especially root tissues, but their residence time in the environment is likely to be short. There is little population level variability of these products (Cipollini, 2002). McCarthy and Hanson (1998) found that water extracts from stem and root tissue had no direct effect on seed germination or seedling growth of four target assay species. Alternatively, Roberts and Anderson (2001) proposed that allelopathic effects may be indirect and act on mycorrhizal fungi, thus reducing the competitive ability of neighbouring plants. Alternatively, the production of these various chemicals may be solely related to plant-insect interactions. There appears to be a negative effect of these secondary compounds on egg-laying and development of butterflies in the genus Pieris (Haribal and Renwick, 1998, 2001; Renwick and Lopez, 1999; Haribal et al., 2001; Renwick et al., 2001).
The floral characteristics of A. petiolata are consistent with a generalist pollinator syndrome. Cavers et al. (1979) observed syrphid flies, midges, and bees (Halictidae and Adrenidae) visiting flowers of A. petiolata in Ontario, Canada. Anderson et al. (1996) observed a similar suite of pollinators in Illinois, USA. Cruden et al. (1996) found that the primary pollinators were short-tongued bees and flies, and the nectar contains 51% fructose, 44% glucose, and little sucrose; a composition typical for short-tongue bee pollination. Anderson et al. (1996) report the pollen:ovule ratio to be 1455:1, which tends to be characteristic of species with facultative xenogamous (out-crossing) breeding systems, plants of late successional stage, and habitats where pollination is unpredictable. Indeed, many of these conditions are reflected by A. petiolata. However, Clapham et al. (1962) state that the flowers are visited by various small insects but are automatically self-pollinated. The species appears to be self-fertile (autogamous) and is fully capable of self-pollination and fertilization, often before anthesis. Out-crossing (xenogamy) produces similar levels of fruit and seed relative to those with autogamy (Anderson et al., 1996). The breeding system could best be described as facultative xenogamy. Propagation is entirely by seed and the seed rain can vary from 10,000 to 40,000 seeds per m², and seed are released from slender siliques over a period of time.
The large-scale distribution of A. petiolata in its native and introduced ranges seems to be controlled by climatic requirements. The species requires a moderately high amount of moisture and cold winters of sufficient duration to ensure stratification and breaking of seed dormancy. The plant is characteristic of natural and anthropogenically disturbed areas. On a smaller scale, the relationship between the environment and invasive potential at the stand level has been examined (Meekins and McCarthy, 2001). Seeds sown into lowland habitats yielded many more and larger plants than those sown into upland habitats. Likewise, edge habitats generally produced larger more fecund individuals than forest interior plots which is consistent with allocation data of Byers and Quinn (1998). Thus, increased light and soil moisture are conditions at a local level that promote A. petiolata invasion, survival, and growth. Smith et al. (2003) found that plants growing in upland environments had relatively low reproductive outputs and that density had little influence on reproduction. Thus, invasion can occur in upland habitats, but the rate of spread and species displacement is likely to be slower. Meekins and McCarthy (2002) conducted a detailed demographic study of plants growing in low and high density populations. Survival to flowering was greatest in low density populations, but a relatively high intrinsic rate of reproduction often led to rapid expansion of the population.
There has been considerable study at the individual plant level on how biomass allocation patterns are affected by the environment. Meekins and McCarthy (2000) conducted a factorial experiment in a common garden to examine the relationship between density, nutrients, and water to growth and allocation patterns of A. petiolata, finding that both rosette and mature plant growth were increased at low densities, increased nutrients, and greater irradiance. Irradiance accounted for the greatest proportion of variation explained in the experiment. Likewise, site to site variation has been documented to affect patterns of seed maturation and abortion (Susko and Lovett-Doust, 1998) and population-level patterns of variation in seed mass were great both within and among populations (Susko and Lovett-Doust, 2000a). Susko and Lovett-Doust (1999, 2000b) also document strong patterns of within-plant resource allocation patterns and reproductive potential as it relates to source-sink issues.
In Europe, it is said to grow best on base-rich soils (Clapham et al., 1962). In Ontario it is found on a wide range of soil types from clays to sand and gravelly-loams, often associated with well fertilized sites (Cavers et al., 1979). Very dense populations have been observed in the Midwest USA on calcium-rich soils.
A. petiolata has no known strong species associations.
Air TemperatureTop of page
|Parameter||Lower limit||Upper limit|
|Absolute minimum temperature (ºC)||-40|
|Mean annual temperature (ºC)||4||19|
|Mean maximum temperature of hottest month (ºC)||9||22|
|Mean minimum temperature of coldest month (ºC)||-1||15|
RainfallTop of page
|Parameter||Lower limit||Upper limit||Description|
|Mean annual rainfall||635||1380||mm; lower/upper limits|
Rainfall RegimeTop of page Uniform
Soil TolerancesTop of page
Natural enemiesTop of page
|Natural enemy||Type||Life stages||Specificity||References||Biological control in||Biological control on|
Notes on Natural EnemiesTop of page
A. petiolata is a preferred food plant in Europe for the green-veined white butterfly (Pieris napi L.) (Lees and Archer, 1974). Cavers et al. (1979) observed European cabbage butterflies (P. rapae L.) ovipositing on plants of A. petiolata in Ontario, Canada, but little to no damage was ever observed. Likewise, the native American butterfly (P. napi oleracea) uses A. petiolata as its primary host, but the larvae rarely survive and their resultant effect is negligible (Renwick et al., 2001), thus the plant is protected from this potential herbivore. Leaf hoppers and flea beetles have been observed on A. petiolata, but again, no obvious damage has occurred.
Means of Movement and DispersalTop of page
Natural Dispersal (Non-Biotic)
Natural propagation is completely by seed. Dispersal is by gravity and likely to be not much more than 1-2 m. Longer distance dispersal is probably through water as the seeds float and remain viable, particularly in riparian areas.
Vector Transmission (Biotic)
Livestock and other animals may transport seeds stuck in mud attached to their hooves. Seed is likely to be transported by humans, potentially great distances, through mud on the soles of shoes, evident from many woodland hiking trails where A. petiolata lines the entrance and edges of the trailways. Seeds are also likely to be trapped in mud on vehicle tyres and dispersed throughout forest environments.
There is no empirical evidence to support accidental introduction. However, given that the species supports a seed-bank and often grows in moist areas, mud on the boots of humans or hooves of animals would most likely introduce this species to new forest patches, especially in heavily dissected landscapes.
The original introduction of A. petiolata to North America was most likely deliberate, though there is no direct evidence to support this. The species was historically eaten as a potherb, particularly in winter and early spring before other greens were available (Georgia, 1920). The first collected specimen in North America was on Long Island, New York in 1868 (Nuzzo, 1993), an area settled heavily during that time by European immigrants. A. petiolata in New York, USA was found to be genetically most similar to populations from Scotland, UK (Meekins et al., 2001). A. petiolata has begun to appear as an ingredient in certain 'gourmet' recipes (Nuzzo, 2003) and is available from at least one seed company in the USA. The editors of Hortideas newsletter have recommended that the herb not be planted (Anon., 1999).
Plant TradeTop of page
|Plant parts not known to carry the pest in trade/transport|
|Fruits (inc. pods)|
|Growing medium accompanying plants|
|Stems (above ground)/Shoots/Trunks/Branches|
|True seeds (inc. grain)|
Impact SummaryTop of page
|Fisheries / aquaculture||None|
ImpactTop of page
There has been no calculated economic impact of this species. However, its ability to serve as the host plant for many crop-related diseases (primarily viruses) may make its economic impact greater than suspected. A. petiolata is the chief host of the Alliaria mosaic virus (AIMV) which can attack cultivated plants such as petunia (Papa et al., 1973). It is host to several other important crop diseases including Cucumber mosaic virus, a strain of Cabbage black ringspot virus and Turnip mosaic virus (Brcak and Polak, 1963; Horvath et al., 1975; Lisa and Lovisolo, 1976).
Environmental ImpactTop of page The environmental impact of the species has not been explicitly investigated; however, given the dense nature under which the species often grows it may be useful in controlling soil erosion, especially in seasonally flooded riparian areas where it is often abundant. It may also be useful from a ecosystem or productivity standpoint in that the species often occupies anthropogenically disturbed areas or low diversity upland forests where few species are able to grow with the same yield.
Impact: BiodiversityTop of page Its ability to displace native plant species in high-quality natural areas has a clear effect on biodiversity, but the valuation of this is difficult. A. petiolata may threaten some butterfly species, as adults of several butterfly species native to the USA (Pieris napi oleracea, P. napi marginata, P. virginiensis) lay eggs on A. petiolata but many or all of the larvae die before completing development (Bowden, 1971), thus, A. petiolata serves as a population sink for these species. This is of particular concern with the rare West Virginia white butterfly (Pieris virginiensis) which lays eggs on A. petiolata in the absence of the related native host plant, Dentaria diphylla (syn. Cardamine diphylla) (Porter, 1994), which is often crowded out following A. petiolata invasion. A. petiolata appears to alter habitat suitability for native birds, mammals, and amphibians, and may affect populations of these species (Nuzzo, 2003). However, no studies have been conducted of the interaction between A. petiolata and these native animals. McCarthy (1997) documented significant changes in plant community composition in a floodplain forest in Maryland, USA, though diversity was not altered appreciably. The species invades many habitats with such voracity that biodiversity effects are inevitable, if not well studied to date.
Threatened SpeciesTop of page
Social ImpactTop of page
There has been no direct assessment of the social impact of A. petiolata; however, areas of high infection and spread lead to such degradation of natural areas that the wildflower flora may be greatly reduced, certainly diminishing the aesthetic quality of the habitat for naturalists and hikers.
Risk and Impact FactorsTop of page Invasiveness
- Proved invasive outside its native range
- Highly adaptable to different environments
- Tolerates, or benefits from, cultivation, browsing pressure, mutilation, fire etc
- Highly mobile locally
- Has high reproductive potential
- Has propagules that can remain viable for more than one year
- Damaged ecosystem services
- Ecosystem change/ habitat alteration
- Negatively impacts animal health
- Negatively impacts tourism
- Reduced amenity values
- Reduced native biodiversity
- Competition - smothering
- Pest and disease transmission
- Highly likely to be transported internationally deliberately
- Difficult/costly to control
UsesTop of page The leaves of the plant just prior to flowering have a higher vitamin C content than oranges and more vitamin A than spinach (Zennie and Ogzewalla, 1977) and the species has considerable nutritional value when used in salads. Grieve (1959) reported that rural people often used the plant in the preparation of sauces, hence the common name 'sauce alone', and noted that the plant also had traditional medicinal uses. The leaves can be used as a sudorific and deobstruent when taken internally, or as an external treatment for gangrene and ulcers. Leaf juices taken alone or boiled in a syrup with honey were used in treating dropsy.
Uses ListTop of page
Human food and beverage
- Spices and culinary herbs
Similarities to Other Species/ConditionsTop of page At maturity, A. petiolata is unlikely to be confused with other species. There are only two species in the genus (Gleason and Cronquist, 1991) and the genus is distinct from most other mustards, especially in its characteristic garlic odour. Immature plants could be visually confused with other rosette-forming species such as violets (Viola spp.), avens (Geum spp.) or Cardamine spp., although again, the diagnostic garlic odour will likely prevent misidentification.
Prevention and ControlTop of page
The goal of any A. petiolata management programme must be to prevent seed production (Nuzzo, 2003). Because of the presence of a seed bank, whatever control method is employed, it must be continued for a period of no less than 3 years in order to eradicate the species locally (Nuzzo, 1991). A. petiolata is not effectively controlled by grazing, and is under little to no vertebrate herbivore pressure in North America. Cavers et al. (1979) noted no grazing from white-tailed deer, and only occasional consumption by cows in Ontario, Canada, resulting in an unpleasant taste to the milk.
Hand pulling is an effective strategy for control, particularly with small or newly formed populations. When the plants have bolted in the second growing season, the stem may be easily grasped and pulled; roots will usually come out intact along with the stem. Because the fruit is photosynthetic, if fruit development has started, plants should not be left on site or hung from neighbouring vegetation as the fruits are well known to continue to develop and dehisce even while lying on the ground, thus plants should be bagged and taken off site. Given the presence of a seed bank, repeated visits to a habitat over a number of years will be required to eradicate the plant. Likewise, cutting may provide good control, but cutting at ground level is important as plants that have been mown (or 'string-trimmed') often respond by sending up new flowering shoots from the root crown. Nuzzo (1991) found that plants cut at ground level had 99% mortality and no seed production, whereas plants cut at a height of 10 cm had 71% mortality and seed production reduced by 98%. The use of cutting must be weighed against various factors, for example, certain other species that may be growing in association with A. petiolata such as native Trillium spp. are severely damaged by cutting (Nuzzo, 2003).
Foliar application of herbicides can be used to control A. petiolata where mechanical methods are impractical due to population size. Glyphosate, triclopyr, and mecoprop have all been used effectively (Nuzzo, 2003) to control A. petiolata, and because these herbicides are not target specific, they should be applied to A. petiolata during the dormant season where the plant is in the rosette stage and native vegetation has not yet emerged. However, biologically active temperatures are also usually required for certain herbicides (e.g. glyphosates) to be effective. Thus, the window of time for application may be narrow. Acifluoren, bentazon and 2,4-D are not recommended for control of A. petiolata (Nuzzo, 2003).
There are currently no known biological control programmes in use to control A. petiolata. However, Blossey et al. (2001) indicate that they are investigating a variety of species for possible use as biological control agents. Although A. petiolata is under virtually no herbivore pressure in North American habitats, over 70 species of insect herbivores and seven fungi are associated with this plant in Europe. Many are not sufficiently host specific to use for control; five monophagous weevils and one oligophagous flea beetle are being further investigated in an effort to develop a concerted suite of attack agents for the seeds, stems, and roots (Blossey et al., 2001).
Nuzzo (1991) suggests that a suite of cutting, chemical, and fire control methods can be adopted for eradication as long as they are applied sequentially for 3 years or more to exhaust the seed bank. The use of fire as a control method for A. petiolata has been well studied, but the results are somewhat conflicting. Nuzzo (1991) found that fire reduced populations of A. petiolata and that the effect was related to fire intensity; moderate intensity fires were effective whereas low intensity fires had virtually no effect. As many prescribed fires fall into this latter category, the efficacy of fire alone to control A. petiolata is questionable, but it may be used effectively in combination with other methods. Nuzzo et al. (1996) found that A. petiolata was maintained, but in a reduced condition, in forests burned repeatedly for 5 years. However, Luken and Shea (2000) found that moderate intensity dormant season fires did nothing to reduce A. petiolata abundance, and in many plots the species actually increased in abundance relative to control plots. The use of fire as a management tool should be integrated with other management objectives given the manifold effects it has on a habitat.
ReferencesTop of page
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