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Garlic Mustard

Alliaria petiolata

Summary 6

Alliaria petiolata, or garlic mustard, is a biennial flowering plant in the mustard family (Brassicaceae). It is native to Europe, western and central Asia, north-western Africa, Morocco, Iberia, and the British Isles, north to northern Scandinavia, and east to northern Pakistan and Xinjiang in western China.

Short Description 7

An invasive herb in the mustard family with triangular/heart-shaped coarsely toothed leaves. In their first year of life, they grow close to the ground but can later grow up to 0.6 to 1 m (2-3 ft) with four-petal white flowers in spring. The leaves and stems smell like garlic when crushed. Later in the season, the fruits are long, skinny seed pods.

Source: EwA Invasive Pocket Fieldguide | © Earthwise Aware

Description 6

It is an herbaceous biennial plant growing from a deeply growing, thin, whitish taproot scented like horseradish. In their first years, plants are rosettes of green leaves close to the ground; these rosettes remain green through the winter and develop into mature flowering plants the following spring. Second-year plants often grow from 30–100 cm (12–39 in) tall, rarely to 130 cm (51 in) tall. The leaves are stalked, triangular through heart shaped, 10–15 cm (3.9–5.9 in) long (of which about half being the petiole) and 5–9 cm (2.0–3.5 in) broad, with coarsely toothed margins. The flowers are produced in spring and summer in small clusters. Each small flower has four white petals 4–8 mm (0.2–0.3 in) long and 2–3 mm (0.08–0.12 in) broad, arranged in a cross shape. The fruit is an erect, slender, four-sided capsule 4–5.5 cm (1.6–2.2 in) long, called a silique, green maturing to pale grey brown, containing two rows of small shiny black seeds which are released when a silique splits open. A single plant can produce hundreds of seeds, which often scatter several meters from the parent plant.

Depending upon conditions, garlic mustard flowers either self-fertilize or are cross-pollinated by a variety of insects.

Sixty-nine insect herbivores and seven fungi are associated with garlic mustard in Europe. The most important groups of natural enemies associated with garlic mustard were weevils (particularly the genus Ceutorhynchus), leaf beetles, butterflies, and moths, including the larvae of some moth species such as the garden carpet moth. 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.

Invasiveness 6

Garlic mustard was introduced to North America by European settlers in the 1800s for culinary and medicinal purposes. The species was recorded as being in Long Island in 1868. It has since spread all over North America, apart from the far south of the US and some prairie states and Canadian provinces. It is toxic or unpalatable to many native herbivores, as well as to some native Lepidoptera.

The plant is classified as an invasive species in North America. Since being brought to the United States by settlers, it has naturalized and expanded its range to include most of the Northeast and Midwest, as well as south-eastern Canada. 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.

Of the many natural enemies it has in its native range, several have been tested for use as biological control agents. Five weevil species from the genus Ceutorhynchus and one flea beetle were selected as candidates for preliminary testing in the 1990s. Since that time, those studying the candidates have narrowed the list to two or three weevils. Despite the demonstrated effectiveness of C. scrobicollis and C. constrictus in field testing, the importation and release of biological control agents such as those has been repeatedly blocked by the USDA's TAG (Technical Advisory Group). In particular, C. scrobicollis, which is monophagous and has been specifically studied since 2002, continues to be blocked, despite researchers' many petitions for approval. It is currently estimated that adequate control of garlic mustard can be achieved by the introduction of just two weevils, with C. scrobicollis being the most important of the two. None of the roughly 76 species that control this plant in its native range has been approved for introduction as of 2018 and federal agencies continue to use more traditional forms of control, such as chemical herbicides.

In North America, the plant offers no known wildlife benefits and is toxic to larvae of certain rarer butterfly species (e.g. Pieris oleracea and Pieris virginiensis) that lay eggs on the plants, as it is related to native mustards but creates chemicals that they are not adapted to. 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.

Toxicity 6

Young first-year garlic mustard plants contain up to 100ppm cyanide, a level which is toxic to many vertebrates. Once the plant is chopped up the cyanide gas is eliminated.

⭕ Control Methods (EwA Content) 8

Garlic mustard, unlike many invasive plants, can be manually controlled by pulling because its taproot doesn't regenerate from fragments (a). However, the plant is adapted to soil disturbances, so it is better managed when soil and vegetation disturbance is minimized to prevent giving it advantages (b) and when control measures are sustained for a minimum of five years to ensure depletion of the seed bank (c). If not, soil disturbance may trigger seed germination and boost garlic mustard densities (c). For better results, start the treatment early in the garlic mustard cycle and be as selective as possible to preserve as many nontarget plants as possible and consequently prevent reinfestation by garlic mustard (a).

In non-established areas, prioritize detecting and eradicating new satellite infestations before a seed bank forms. Monitor sites prone to seed dispersal, like trails, parking lots, transportation corridors, and recreation areas in suitable habitats, recognized as early infestation sites. Once garlic mustard establishes a dense population, prevent further seed set until the seed bank is depleted, which takes up to 10 years. Combine methods such as pulling, cutting, herbicide application, or repeated fire as needed based on site characteristics and infestation level (b).

After clearing garlic mustard plants from an area, promptly replant with a cover crop or apply a layer of leaves or mulch, at least 5 cm (2 in) thick, to minimize or prevent garlic mustard seed germination (c).

Regardless of the treatment chosen, eradication might be unlikely depending on the population's characteristics. However, repeating control treatment over the years can reduce and contain the dispersal of large garlic mustard populations (h, I).

Mechanical Treatment

For small infestations or sensitive communities, manually pull plants before they release their mature seeds (a, b). Ensure the removal of the upper part of the roots and the stem, as buds in the root crown can generate additional stems (b). Avoid hand-pulling rosettes, as they often break at the root; consider using a trowel for effective removal by digging them up (d). Due to seeds remaining viable in the soil for up to ten years, it is crucial to pull all garlic mustard plants in an area annually until the seed bank is depleted and seedlings cease to appear. Remove all pulled garlic mustard plants from the site as seed ripening continues post-pulling (b).

Medium to large populations can also be controlled by cutting. Cut flowering garlic mustard plants at ground level, whether by hand, with a string trimmer, or using a lawn mower. A lower cut effectively kills a high percentage of the plant (b). The best time for cutting is when plants are in full bloom. If cut early in the flowering phase, they may retain enough resources to generate additional flower stems from buds or crown roots (f). If extra flowers emerge, consistent mowing can prevent seed production. Repeat mowing as needed within the season due to varying flowering times (c). It is necessary to continuously cut for several consecutive years until the seed bank is exhausted. Ideally, cut the garlic mustard selectively; the less disturbance there is to the desirable plant community, the faster and more efficient garlic mustard control will be (b). If possible, integrate cutting with prescribed fire or application of herbicides (b).

If the goal is simply to impede the advancement of the invasion, consistently clip the flower heads to hinder seed formation. However, performing this action repeatedly throughout the growing season is critical to counteract the emergence of fresh flowers. (c).

Prescribed-burning Treatment

The effectiveness of fire varies depending on site characteristics and management goals. In fire-adapted communities, scheduled prescribed burns can discourage garlic mustard entry by promoting native communities and eliminating early invaders. Late spring burning may impact native understory forbs, but well-timed spring fires, post-garlic mustard emergence, and pre-desirable plant emergence can be effective. Follow up with herbicide, cutting, or pulling for any surviving individuals as needed. A single late fall burning may increase garlic mustard abundance the following spring, but repeated burns (fall, spring, spring; or spring, spring, spring) can sustain garlic mustard in a reduced state while enhancing herbaceous species richness and cover (b). The fire must be of high intensity since low-intensity fire can increase population size as it does not kill the root crown and removes the litter layer, increasing seedling survival (e).

Chemical Treatment⚠

Mechanical treatments work well for garlic mustard management in general (b, f). In the case of widespread established populations, herbicides could be the best option, as manual approaches like hand-pulling and cutting are labor-intensive and demand a substantial workforce, leading to occasional economic impracticality. (g, h). Moreover, the feasibility of using mechanical methods depends on the remoteness of the location from roads and the timing of the management efforts. Implementing mechanical techniques may become impractical due to the necessity of removing cut plant parts from the area accumulating numerous bags of vegetative material. If the area is inaccessible by vehicle, individuals would need to transport all this material over significant distances. (g).

If feasible, you can also manage well-established populations by combining pulling, cutting, and herbicides. Multiple herbicides can be used to manage garlic mustard. It is critical to strictly adhere to the label directions when applying herbicides (b).

Treating the rosette stage with foliar spot application is a suitable approach at any time during the basal year when the aboveground temperature is above freezing (a). Spray until the leaves are covered, ensuring the herbicide does not drip off the leaves (c). Applying in late fall or early spring avoids overspray on non-target species. Preemergent herbicides are unsuitable due to their long germination window; they lose efficacy quickly (a).

Certain herbicides can be applied using wick or wiper applicators directly to the leaves, serving as an alternative to foliar spray if there are concerns about spray drift. Check the product label for information (c).

A water-soluble mix of triclopyr plus 2,4-D amine is selectively effective for treating garlic mustard, and it does not harm grasses or most other monocotyledons. While useful for most broadleaved plants, it is ineffective against woody species (a).

A 1-2% glyphosate concentration is also efficient in managing garlic mustard seedlings and rosettes, but its effectiveness relies on active plant growth (absorption by growing leaf tissue or bark). It's essential to be aware that glyphosate has the potential to harm sedges and other species actively growing during this period. (b).

Consider using a nonselective herbicide mix when targeting grasses, woody species, and garlic mustard on the same site. A combination of glyphosate plus triclopyr in a 2:1 ratio (more concentrated than required for garlic mustard alone) proves effective against a wide range of species, making it suitable for treating various invasive species simultaneously (a).

Another option is to use the herbicide bentazon (28 g /8 oz per acre). It may be less effective on garlic mustard but reduces risk to some non-target species, especially annual and perennial grasses (b).

PS: Check the following websites for examples of timing and treatment schedules:

https://extension.psu.edu/garlic-mustard, https://www.ontarioinvasiveplants.ca/wp-content/uploads/2016/07/OIPC_BMP_GarlicMustard.pdf

Disposal

Bag and remove pulled plants from the site to address potentially viable seeds, even in early pulling treatments, since plants might have viable seeds (a). Seal the bags securely and place them in direct sunlight for approximately a week; burn dried plants or send them to the landfill; do not compost them (c).

⚠ Disclaimers

Disclaimer #1: Harmless chemicals do not exist. Any chemical use is likely to harm non-target species and affect the soil. However, in some circumstances, chemicals might be the only way to succeed in removing an invasive species permanently. Researching and understanding the latest scientific literature before using a specific chemical or a mix of several is critical to reducing negative impact.

Disclaimer #2: Independently, several European countries recently announced future bans or massive restrictions on the use of glyphosate (e.g., Austria, Germany, France). The EU, at large, is expected to decide on the renewal of the approval of glyphosate Ultimo 2023 (More here).

Sources

[a] Garlic Mustard; 2020. Available in: https://extension.psu.edu/garlic-mustard
[b] Garlic Mustard: Management options. Available in: https://www.canr.msu.edu/ipm/Invasive_species/garlic_mustard/management_options
[c] Garlic Mustard (Alliaria petiolata); 2012. Available in: https://www.ontarioinvasiveplants.ca/wp-content/uploads/2016/07/OIPC_BMP_GarlicMustard.pdf
[d] Garlic Mustard; 2011. Available in: https://www.maine.gov/dacf/php/gotpests/weeds/factsheets/garlic-mustard-wash.pdf
[e] Nuzzo, V.A., McClain, W. and Strole, T., 1996. Fire Impact on Groundlayer Flora in a Sand Forest 1990-1994. American Midland Naturalist, pp.207-221.
[f] Pardini, E.A., Teller, B.J. and Knight, T.M., 2008. Consequences of density dependence for management of a stage-structured invasive plant (Alliaria petiolata). The American Midland Naturalist, 160(2), pp.310-322.
[g] Chapman, J.I., Cantino, P.D. and McCarthy, B.C., 2012. Seed production in garlic mustard (Alliaria petiolata) is prevented by some methods of manual removal. Natural Areas Journal, 32(3), pp.305-315.
[h] Carlson, A.M. and Gorchov, D.L., 2004. Effects of herbicide on the invasive biennial Alliaria petiolata (garlic mustard) and initial responses of native plants in a southwestern Ohio forest. Restoration Ecology, 12(4), pp.559-567.
[i] Corbin, J.D., Wolford, M., Zimmerman, C.L. and Quirion, B., 2017. Assessing feasibility in invasive plant management: a retrospective analysis of garlic mustard (Alliaria petiolata) control. Restoration Ecology, 25, pp.S170-S177.

Sources and Credits

  1. (c) Joe MacIndewar, some rights reserved (CC BY-NC), uploaded by Joe MacIndewar, https://www.inaturalist.org/observations/113666694
  2. (c) Joe MacIndewar, some rights reserved (CC BY-NC), uploaded by Joe MacIndewar
  3. (c) Claire O'Neill, some rights reserved (CC BY-NC-ND), https://www.inaturalist.org/observations/116902871
  4. (c) Claire O'Neill, some rights reserved (CC BY-NC), uploaded by Claire O'Neill, https://www.inaturalist.org/observations/114879775
  5. (c) Claire O'Neill, some rights reserved (CC BY-NC), uploaded by Claire O'Neill, https://www.inaturalist.org/observations/133958916
  6. Adapted by Claire O'Neill from a work by (c) Wikipedia, some rights reserved (CC BY-SA), https://en.wikipedia.org/wiki/Alliaria_petiolata
  7. (c) Claire O'Neill, some rights reserved (CC BY-SA)
  8. Adapted by Esther Meirelles from a work by (c) Claire O'Neill, some rights reserved (CC BY-SA)

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