Garlic Mustard (Alliaria petiolata) was introduced to the United States in 1868 by European immigrants that wanted to bring a taste of home with them to their new country. As the name implies, the foliage of this plant smells and tastes of mild garlic, and it is a member of the mustard family (Brassicaceae / Cruciferae). Historically, it was used as a potherb and could be found in gardens and around home sites for culinary and medicinal use. Though uncommon, and highly discouraged, to cultivate this problematic plant in our current age, it is still good for spicing up dishes and making salads, pesto, and sauces.
Life Cycle: Garlic mustard is a biennial herbaceous plant, meaning that it has a two-year life cycle and is non-woody. Seeds generally germinate in the spring and form basal rosettes, which are small, low-lying clusters of leaves. It remains in the rosette stage for the first year of growth, until the following spring. During the second year of growth, the rosette will enter the bolting phase and produce a stalk that can rapidly reach up to 3.5 feet. As bolting occurs, the plant will also begin to flower. As flowering tapers off, seed production will begin with the formation of long, slender seed pods called siliques. Once the seeds are fully formed, the siliques will burst open, disseminating the seed into the immediate area. This generally happens in May to early June in southern Illinois. Seed is the only means of reproduction for this species.
Identification:
- Height: Life stage dependent. Rosettes can be 1-6 inches tall. Flowering and fruiting plants can reach up to 3.5 feet.
- Leaves: Kidney shaped as a rosette, moving to triangular or heart-shaped with maturity, both with coarsely toothed edges. Second year plants will be of alternate arrangement.
- Flowers: Clustered at the end of stems. Small and white, with four petals in the shape of a cross.
- Fruit: Slender, green pods called siliques that are generally 1-3 inches in length. Similar to pea pods.
- Seeds: Small, round and black.
- Roots: Taproot has a characteristic “S” shape just below the base of the stem.
- Scent: Use “crush and sniff” test on leaves to detect mild garlic odor.
- Look-alikes: Native violets are very similar to garlic mustard rosettes. The easiest way to differentiate garlic mustard from other species is the “crush and sniff” test mentioned above.
Garlic mustard rosette. (Photo by Nathan Speagle)
Garlic mustard flowers. Note each flower has four petals. (Photo by Nick Seaton)
Invasive Advantages: As with most invasive species, garlic mustard is not impacted by natural predators or diseases outside of its native range. However, three factors make this species especially problematic in the forests of North America, high fecundity (reproduction), seed dispersal, and allelopathy. This species can produce up to 8,000 seeds per plant, and the seeds can remain viable in the seedbank for 7 years. Once established, several years of control measures are necessary to stop seed production and deplete the seedbank. The abundance of seed is further complicated by how easily it is transported. In southern Illinois, garlic mustard is most commonly found in floodplain and riparian areas since the tiny seeds are easily transported by water. Another commonplace environment in the region for this species is roadsides, as mowers, road graders, automobiles and other motorized equipment are effective transporters. Wildlife, hikers, and pets are contributors to the spread of this species.
In terms of seed, garlic mustard certainly has mobility and strength in numbers, but it also possesses a novel weapon that gives future generations a leg up on the competition. Allelopathy is the production of chemical compounds used to inhibit the growth of competing vegetation. Garlic mustard uses allelopathy to disrupt and destroy mycorrhizal communities, which are fungal symbionts in the soil that are important to native plant nutrition and soil health.
Habitat and Ecological Impacts: This species has a wide tolerance range. Garlic mustard can be found in moist bottomland and dry upland forests, on forest edges, floodplains, riparian areas, along roadsides, and in disturbed areas. It thrives in calcareous soils but does not do well in acidic soils. It prefers light shade to partial sun but can persist in dense shade and full sun conditions.
Garlic mustard can quickly establish and displace native plants species in a variety of habitats. Due to invasive advantages, it can form dense communities that inhibit the regeneration of grasses, forbs and trees. Losses of native species richness and abundance due to garlic mustard impairs wildlife habitat and can alter long-term forest regeneration and composition.
A volunteer hand pulls large second-year plants nearing seed maturity. (Photo by Nathan Speagle)
Control: Garlic mustard is most effectively managed through combination of mechanical, cultural, and chemical control methods.
- Mechanical treatments involve hand-pulling this species, which can be performed at all stages of the life cycle but is most effective on second year plants that have bolted. All plant materials should be bagged and disposed of as viable seed can still be produced after removal. However, this is the most labor-intensive technique and may not be effective use of time on large infestations. Additionally, hand pulling should cease when siliques on second year plants reach maturity and begin to drop seed.
- Cultural treatments generally involve the use of prescribed fire. Using fire can reduce the number of rosettes but is not an effective stand-alone means of control for this species. Fire should always be used in conjunction with mechanical or chemical treatments.
- Chemical treatments involve using foliar applications of herbicide to garlic mustard before it begins to produce seed. Glyphosate and triclopyr are both effective in treating this species. Chemical treatments are most effective when plants are still in the basal rosette stage of their lifecycle, as this completely disrupts seed production. Utilizing chemical treatments once bolting and flowering have occurred still kills the plants and can greatly diminish seed production, but some viable seed may still be produced.
Second year plants in an unmanaged infestation in Jackson County. (Photo by Nathan Speagle)