Garlic mustard (Alliaria petiolata, Brassicaceae) is considered one of the most harmful invasive plant species in North America. This introduced shade-tolerant herb thrives in deciduous woodlands where it out-competes and displaces native plant species, reducing biodiversity and destabilizing ecosystems. High population densities, high fecundity, and the production of allelopathic and anti-herbivory chemicals are known mechanisms for competition (Nuzzo 1999; Stinson et al. 2006; Cipollini and Gruber 2007). Additionally, garlic mustard has escaped the insect herbivores of its native range (Blossey et al. 2001). Its competitive advantages, ability to displace native species, and freedom from any serious herbivores have allowed it to invade and dominate forests across the continent.
As a biennial plant, garlic mustard requires two years to complete its life cycle. After germination the first year is spent as a small rosette. It can continue to grow throughout the winter on any snow-free day above freezing (Landis and Evans 2008). In the spring of the following year it grows rapidly. Mature garlic mustard is a tall, slender plant with broad, triangular leaves and small, white flowers with four petals. Upon fertilization, the flowers grow into thin, erect siliques (seed pods). Once mature the seeds are ejected from the pods.
Garlic mustard was introduced to the United States in the 1860s by European immigrants who may have used it for cooking (Gross 2006). When crushed the leaves give off a pungent garlic odor, and this once made the plant valuable as a seasoning. Since its introduction, garlic mustard has spread to over 30 U.S. states and four Canadian provinces (Landis and Evans 2008).
Garlic mustard is a highly successful invader. In relatively undisturbed forests, invasion fronts can advance at an average rate of around five meters per year (Nuzzo 1999). Population densities can reach as high as 274 plants per square meter, and stands can produce approximately 500 to 700 seeds per square meter (Nuzzo 1999). Flooding and human activity both disperse the seeds and create disturbed areas for this ruderal species to take hold (Nuzzo 1999). Plant cover and density increase rapidly following such disturbances, and once established it becomes a permanent part of the forest (Nuzzo 1999).
Garlic mustard succeeds in displacing native plants not only through high density and fecundity, but also through a variety of allelopathic chemicals. Direct allelopathic effects are achieved through the release of various phytotoxic chemicals including allyl isothiocyanate and benzyl isothiocyanate (Vaughn and Berhow 1999). Garlic mustard also releases antifungal chemicals that disrupt the mutualisms many trees have with mycorrhizal fungi. As a result, the seedlings of native canopy trees experience inhibited growth (Stinson et al. 2006).
Garlic mustard is also very resistant and tolerant to herbivory. In North America it has escaped from the specialist herbivores of its native range, leaving it vulnerable only to generalist herbivores (Blossey et al. 2001). It is capable of resisting generalist herbivores through chemical defenses, including the production of cyanide at levels considered toxic to many insects and vertebrates (Cipollini and Gruner 2007). Even when attacked, it can tolerate a large amount of damage. Experiments using simulated herbivory found that even after 75% leaf removal, siliques retained 81% of the biomass of controls (Bossdorf et al. 2004). Only plants that had their shoots cut off at the base experienced notably higher mortality and lower seed production (Rebek and O’Neil 2005). In the field, there is frequent browsing by North American herbivores but it is rarely this extensive (Evans and Landis 2007).
With no natural controls on garlic mustard in North America, it has spread rapidly and dominated every habitat it has invaded. Artificial controls such as pulling, burning, and herbicide application have made a limited impact. There is, however, hope that biological control agents such as specialist insect herbivores from the plant’s native Eurasian range may be effective. If ultimately released into the wild, these insects may be the best long-term solution to controlling this devastating weed.
Blossey, B., V. Nuzzo, H. Hinz, and E. Gerber. 2001. Developing Biological Control of Alliaria petiolata (M. Bieb.) Cavara and Grande (Garlic Mustard). Natural Areas Journal 21(4):357-367.
Bossdorf, O., S. Schröder, D. Prati, and H. Auge. 2004. Palatability and tolerance to simulated herbivory in native and introduced populations of Alliaria petiolata (Brassicaceae). American Journal of Botany 91(6):856-862.
Cipollini, D. and B. Gruner. 2007. Cyanide in the chemical arsenal of garlic mustard, Alliaria petiolata. Journal of Chemical Ecology 33:85-94.
Evans, J.A. and D.A. Landis. 2007. Pre-release monitoring of Alliaria petiolata (garlic mustard) invasions and the impacts of extant natural enemies in southern Michigan forests. Biological Control 42:300–307.
Gross, L. 2006. How an Aggressive Weedy Invader Displaces Native Trees. PLoS Biology 4(5):e173.
Landis, D. and J. Evans. 2005. About Garlic Mustard. MSU IPM Program. Michigan State University, East Lansing, MI.
Nuzzo, V. 1999. Invasion pattern of the herb garlic mustard (Alliaria petiolata) in high quality forests. Biological Invasions 1:169-179.
Rebek, K.A. and R.J. O’Neil. 2005. Impact of simulated herbivory on Alliaria petiolata survival, growth, and reproduction. Biological Control 34(3):283-289.
Stinson, K.A., S.A. Campbell, J.R. Powell, B.E. Wolfe, R.M. Callaway, G.C. Thelen, S.G. Hallett, D. Prati, and J.N. Klironomos. 2006. Invasive Plant Suppresses the Growth of Native Tree Seedlings by Disrupting Belowground Mutualisms. PLoS Biology 4(5): e140.
Vaughn, S.F. and M.A. Berhow. 1999. Allelochemicals isolated from tissues of the invasive weed garlic mustard (Alliaria petiolata). Journal of Chemical Ecology 25: 2495–2504.