Garlic Mustard: a little-known invasive species

To the most prevalent invasive plants – like the formidable common reed, buckthorn and Eurasian watermilfoil – we have to add a new one: Garlic Mustard (Alliaria petiolata). This medicinal plant was introduced into our gardens by Europeans, who used it as a condiment because of its garlic taste. Currently in Québec it’s experiencing a significant, but largely unnoticed, expansion. Thanks to its small seeds, produced in abundance, it’s making its way quietly into our maple forests. In just a few years it may come to dominate the forest floor, attacking natural environments that are rich in biodiversity.

Its secret: a short life cycle

Garlic Mustard is a biennial plant. It germinates in late April and produces a rosette of leaves that remain green even in winter. At the start of the following spring, the floral stem grows rapidly, sometimes to a length of more than one meter. In optimal conditions, a single plant can produce several floral stems containing a total of over a thousand seeds, which will be scattered along roads and footpaths.

The biodiversity of our forests under threat

This species grows in highly diversified conditions, and tolerates shade, which makes it very competitive in a forest environment. A rare phenomenon, the roots of Garlic Mustard produce toxins, glucosinolates, which affect the mycorrhiza in the soil, doing harm to all the neighboring vegetation (herbaceous plants, shrubs, and even tree seedlings). The taste of the Garlic Mustard is unpleasant to deer, and they prefer to graze on native plants, which as a result become sparse, leaving space for Garlic Mustard. Dense colonies of it are quick to form, leading to important changes in the plant composition of the invaded forests.

The fight against Garlic Mustard: prevention tastes better!

Garlic Mustard is difficult to eliminate from heavy infested locations where native vegetation is getting scarce. The seedbank can generate new seedlings for more than 10 years, even if all the flowering plants are uprooted every spring. Sustained efforts have to be made, and revegetating must be done. Fortunately, isolated plants are easily removed, preventing the expansion of the species to new sites. Here’s where everyone can make a difference! Keep an eye out: the plant is often observed in ditches, and along roads or trails.

A Garlic Mustard plant can be pulled up easily, especially in damp soil after it rains. All you need to do is grab the plant at the base and pull: out comes the root, which looks like a little carrot. Put the uprooted plants in black garbage bags and leave them in full sun for a few weeks so that they’re destroyed by the heat (bagging technique). Above all, don’t compost them, or the seeds may return to the soil. Then put the bags out with the trash. In July, when the plants start releasing their seeds, don’t go near them! If you pull them up you risk scattering the hundreds of small seeds quite a distance, swelling the future invasion.

Collective action to halt the invasion

In the United States and in Ontario, an important popular movement, The Stewardship Network, was created to check the invasion. In Québec, group action take place every year in Mount Royal Park and at the Boisé-des-Douze nature reserve in Saint-Hyacinthe. Taking part in a collective activity like these will help you familiarize yourself with the species so that you can protect your particular area. Interested parties, take note: we’re currently putting together bank of volunteers for collective action in Montérégie. To sign up, contact [email protected].

References:

• Anderson, H. 2012. Invasive Garlic Mustard (Alliaria petiolata): Best Management Practices in Ontario. Ontario Invasive Plant Council. 30p.
• Lavoie, C. 2019. 50 plantes envahissantes: protéger la nature et l’agriculture. Les publications du Québec. 415p.
• Nuzzo, V. 1999. “Invasion pattern of the herb garlic mustard (Alliaria petiolata) in high quality forests.” Biological Invasions 1: 169-179.
• Pardini, E.A., Drake, J.M., Chase, J.M. and Knight, T.M. 2009. “Complex population dynamics and control of the invasive biennial Alliaria petiolata.” Ecological Applications 19(2): 387-397.
• Rodgers, V.L., Stinson, K.A. and Finzi, A.C. 2008. “Ready or not, Garlic Mustard is moving in: Alliaria petiolata as a member of Eastern North American forests.” Bioscience 58(5): 426-436.

Useful links :

• For more information: http://www.invadingspecies.com/garlic-mustard/ 
• Garlic mustard pesto: https://www.facebook.com/naturegroupie/videos/1206954652764585/