Alliaria petiolata

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Garlic Mustard
Alliaria petiolata marais-belloy-sur-somme 80 26042007 3.jpg
Scientific classification
Kingdom: Plantae
(unranked): Angiosperms
(unranked): Eudicots
(unranked): Rosids
Order: Brassicales
Family: Brassicaceae
Genus: Alliaria
Species: A. petiolata
Binomial name
Alliaria petiolata
(M.Bieb.) Cavara & Grande

Alliaria petiolata is a biennial flowering plant in the Mustard family, Brassicaceae. It is native to Europe, western and central Asia, and northwestern Africa, from Morocco, Iberia and the British Isles, north to northern Scandinavia,[1] and east to northern Pakistan and western China (Xinjiang).[citation needed]

In the first year of growth, plants form clumps of round shaped, slightly wrinkled leaves, that when crushed smell like garlic. The next year plants flower in spring, producing cross shaped white flowers in dense clusters. As the flowering stems bloom they elongate into a spike-like shape. When blooming 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,[2] 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. Some people give the species name Alliaria officinalis for this plant.[3]

All parts of the plant, including the roots, give off a strong odour. In 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.[4] Early European settlers brought the herb to the New World to use as a garlic type flavoring, and as a good source of vitamins A and C. The herbs medicinal purposes include use as a disinfectant, a diuretic,[5] and sometimes being used to treat gangrene and ulcers. The herb was also planted as a form of erosion control.[6]

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 southeastern Canada. 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.[7]


It is a herbaceous biennial plant growing from a deeply growing, thin, white taproot that is scented like horseradish. In the first year, plants appear as a rosette 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 grow from 30–100 cm (rarely to 130 cm) tall. The leaves are stalked, triangular to heart-shaped, 10–15 cm long (of which about half being the petiole) and 5–9 cm broad, with a coarsely toothed margin. The flowers are produced in spring and summer in button-like clusters. Each small flower has four white petals 4–8 mm long and 2–3 mm broad, arranged in a cross shape. The fruit is an erect, slender, four-sided pod 4 to 5.5 cm long,[8] called a silique, green maturing pale grey-brown, containing two rows of small shiny black seeds which are released when the pod splits open. A single plant can produce hundreds of seeds, which scatter as much as several meters from the parent plant.

Depending upon conditions, garlic mustard flowers either self-fertilize or are cross-pollinated by a variety of insects. Self-fertilized seeds are genetically identical to the parent plant, enhancing its ability to colonize an area where that genotype is suited to thrive.[9]

Close-up of Garlic Mustard flowers
Fruits and seeds

Cultivation and uses[edit]

Garlic mustard is one of the oldest discovered spices to be used in cooking in Europe. Evidence of its use has been found from archeological remains found in the Baltic, dating back to 6100-5750 BP.[10]

The chopped leaves are used for flavoring in salads and sauces such as pesto, and sometimes the flowers and fruit are included as well. These are best when young, and provide a mild flavour of both garlic and mustard. The seeds are sometimes used to season food directly in France.[5] Garlic mustard was once used medicinally[11] as a disinfectant or diuretic, and was sometimes used to heal wounds.[5]

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.[7]

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.[4]

In North America, the plant offers no known wildlife benefits and is toxic to larvae of certain butterfly species (e.g. Pieris oleracea and Pieris virginiensis) that lay eggs on the plants, as it is related to native mustards.[12] 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.[7]

As an invasive species[edit]

Garlic mustard was introduced in North America as a culinary herb in the 1860s and is an invasive species in much of North America. As of 2006, it is listed as a noxious or restricted plant in the US states of Alabama, Connecticut, Massachusetts, Minnesota, New Hampshire, Oregon, Vermont, West Virginia and Washington.,[13] and occurs in 27 midwestern and northeastern states in the US, and in Canada.[14] Like most invasive plants, once it has an introduction into a new location, it persists and spreads into undisturbed plant communities. In many areas of its introduction in Eastern North America, it has become the dominant under-story species in woodland and flood plain environments, where eradication is difficult.[15]

The insects and fungi that feed on it in its native habitat are not present in North America, increasing its seed productivity and allowing it to out-compete native plants.

Garlic mustard produces allelochemicals, mainly in the form of the compounds allyl isothiocyanate and benzyl isothiocyanate,[16] which suppress mycorrhizal fungi that most plants, including native forest trees, require for optimum growth.[17] However, allelochemicals produced by garlic mustard do not affect mycorrhizal fungi from garlic mustard's native range, indicating that this "novel weapon" in the invaded range explains garlic mustard's success in North America.[18] Additionally, because white-tailed deer rarely feed on garlic mustard, large deer populations may help to increase its population densities by consuming competing native plants. Trampling by browsing deer encourages additional seed growth by disturbing the soil. Seeds contained in the soil can germinate up to five years after being produced (and possibly more).[19] The persistence of the seed bank and suppression of mycorrhizal fungi both complicate restoration of invaded areas because long-term removal is required to deplete the seed bank and allow recovery of mycorrhizae.[20]

Garlic mustard produces a variety of secondary compounds including flavonoids, defense proteins, glycosides, and glucosinolates that reduce its palatability to herbivores.[21][22][23] Research published in 2007 shows that, in northeastern forests, garlic mustard rosettes increased the rate of native leaf litter decomposition, increasing nutrient availability and possibly creating conditions favorable to garlic mustard's own spread.[24]

Control strategies[edit]

Biological control is the method of control that is the least-damaging to ecosystems not typified by monoculture, like forested areas, while also being the most efficient in terms of costs.[25][26] Biological control is the foundation of the differentiation between native species living in complex ecological balance and non-native invasive species. It is nature's method of maintaining ecological balance.[27] For the management of some invasive plants, or in some cases when dealing with garlic mustard, herbicide application and human-managed labor such as mowing, tilling, burning, and pulling may be preferred for managing unwanted vegetation on land that is highly-disturbed by human activity, such as agricultural land. This effort is usually rendered more effective by the supplemental presence of biological control agents. For more complex ecosystems such as forests, trampling and other physical disturbance such as soil compaction, the spreading of seeds from clothing, chemical toxicity, unwanted non-targeted species damage, demanding human labor, petrochemical consumption, and other factors are eliminated or greatly reduced with effective biological control.[28] One species of weevil that targets garlic mustard, for instance, consumes the seeds.[29] Unlike with some invasive plants which are annuals, such as Microstegium vimineum (Japanese stiltgrass), the mowing of garlic mustard is less effective because it will regrow from its tap root, especially if it is mowed in its second, flowering, year — where the root has grown enough to store considerable energy.

Monophagous controllers, such as the weevil C. scrobicollis, which only feeds on garlic mustard, are usually the most ideal candidates for initial introduction to combat invasive plants, as they greatly reduce the chance that the introduced controller will itself become a pest.[30] Difficulties involved in using biological control are identifying species that are safe to introduce as well as relying on fewer controlling species being present in the non-native ecosystem. Up to 76 things feed on garlic mustard in its native environment. By contrast, nothing eats it to a significant extent in the United States where it is non-native.[31] Despite there being so many controlling agents for that plant, it is currently estimated that adequate control of garlic mustard's invasiveness in portions of the United States where it is problematic can be achieved by the introduction of just two weevils, with C. scrobicollis being the most important of the two.[32]

The example of garlic mustard shows how effective, at least in Minnesota's controlled trials and European field observations, even one monophagous biological control agent can be, while having the fewest costs.[33] Despite the demonstrated effectiveness of C. scrobicollis and, potentially, C. constrictus, the importation and release of biological control agents such as those may be stymied by heavy research and regulation requirements.[34] Those who believe the regulations are well-crafted argue they are needed to prevent the agents from becoming highly undesirable pests while critics argue that the regulations, as currently written and implemented, make it too difficult to bypass more damaging, less effective, and more costly methods of control — such as applying herbicides in forests.[35] As of May 2017, there is no legally-approved biological control agent to combat garlic mustard in the United States. Garlic mustard has been researched by the United States since the 1990s and C. scrobicollis has been studied specifically since 2002. The 2012 recommendation to release it into the US was blocked by the TAG group.[36]

Preventing seed production and depletion of the soil seed bank are key to eradicating infestations, but seeds can last as long as twelve years and just one plant can produce thousands of seeds.[37] Seeds are also easily tracked around by animals, vehicles, and people. Non-chemical non-biological control methods include removal by hand-pulling or cutting at the base, mowing, burning, or manipulation of the environment to reduce light. Pulling is more effective if the entire root is removed and desirable plants and soils are not trampled and compacted.[38] Garlic mustard can invade stable forests as well as disturbed sites. It can grow in deep shade as well as full sunlight and in a wide range of moisture levels. Therefore, management by planting or encouraging other plants to intercept light will not prevent new infestations, although it may slow them. Control is best in early spring prior to flowering because the plants are smaller which reduces soil disturbance and loss from pulling, as well as giving competing plants more of the season to expand. However, it is easy to miss the small plants, which can flower even when less than three inches in above-ground height.[39] The flowers increase visibility, especially in lower light situations. Some plants' roots will also break off, even with careful pulling technique, leaving pieces in the soil that will regrow. Root breakage is most common in soil compacted by foot traffic and in drier conditions. Mowing and cutting are also more effective prior to the plants flowering because the mowed and cut plant pieces are less likely to possess enough energy to bloom and generate viable seed. Removed plants should be bagged (and disposed of correctly) or burned, as seeds or roots may survive composting. Pulled plants can bloom and produce seed, particularly if the roots are attached, even while the plants are withering and dying.[40]

Chemical control may be achieved to some extent by foliar application with a number of herbicides, although their use is much more efficacious in highly-disturbed situations, like agricultural monocultures or urban and suburban gardens, than in complex settings, like forests and well-established meadows or prairies. Timing herbicide applications to the earliest spring may help to better protect native or desirable plants in the same locations as garlic mustard is generally active earlier than most other plants in northern temperate climates, one of the reasons it can generally outcompete native plants and displace them.[41] However, there are native and desirable plants that are active even before garlic mustard is, and/or at the same time in early spring, such as flowers from the genera Pulsatilla and Helleborus of the family Ranunculaceae. Some native and desirable plants also are evergreen and thus vulnerable to foliar and post-emergent herbicides at all times.[42] Chemical control methods that involve heavy equipment or human trampling can compact soils, affecting all plants negatively. Such methods can disturb wildlife and chemical solutions may cause chemical pollution such as tainted water through runoff. All non-biological methods of control must be repeated for 2-5 years to be effective—as most infestations occur in sites where a considerable seed bank has been established.[43] Those will continue to germinate for over a decade. Surviving roots regrow and produce new seed pods, enabling the infestation to potentially be quickly reestablished. Continual reintroduction of garlic mustard to areas where it has been eradicated is also highly likely until an effective biological control situation is established, as the long-lived seeds are produced in great quantities and are readily distributed by animals and human activity.[44]

Of the 76 natural enemies garlic mustard has in its native range, several have been tested for use as potential biological control agents. Five weevil species from the genus Ceutorhynchus and one flea beetle were selected as candidates during preliminary testing. Since that time, the United States' employees studying these candidates narrowed the list. The monophagous weevil C. scrobicollis, studied since 2002, was officially recommended for introduction into the US in 2012 but the TAG group blocked its introduction, requesting further research be conducted.[45] As of May 2017, it has not been approved for introduction and the continued research funding has not been provided.[46]


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