by Adriana Lopez-Villalobos and Amelie Mahrt-Smith
Next time you are out for a walk around the neighbourhood, keep your eyes out for this weed in lawns, empty lots, and along trails. Alliaria petiolata, commonly known as garlic mustard, is in bloom through spring and early summer in Ontario. This widely distributed herb is a member of the Brassicaceae (Mustard) family alongside many well-known, if not well-liked cruciferous vegetables: broccoli, kale, brussels sprouts, turnip, horseradish, and wasabi, to name a few [3]. Garlic mustard gets its name from the garlic smell released by the leaves when crushed; the leaves, as well as the flowers and seeds, are edible and add a mild garlic or mustard flavour to dishes. Make sure to you ask your neighbours if they have used herbicides on their lawn before you try adding garlic mustard leaves to your salad or pesto!
Mature garlic mustard plants are easy to identify. The heart-shaped leaves have saw-toothed edges, prominent veins, and long stalks connecting them to the main stem. Leaves are unpaired and alternate in their position on the stem. Its height varies, but in rich soil can reach up to 1 metre tall. The flowers are somewhat inconspicuous: small and white, each with four sepals and petals (both free), and clustered at the tips. As it is typical of the mustard family, the flowers have six stamens (male part) with the two outer stamens shorter than the four inner stamens. These flowers only develop in the plant’s second (and final) year of life (Images 1 and 2). In its first year of growth, the leaves are more rounded and can look like some other common Ontario herbs like violets or wild ginger [4]. Your other senses will come in handy here – you can tell young garlic mustard apart from the rest because its leaves will still have the distinctive garlic smell when crushed.
Despite its abundance in North America today, garlic mustard did not exist here until around the 1800s, when it was purportedly introduced by European settlers who cultivated it for food and medicine. It is a good source of vitamin A and C and was used as an antiseptic to treat ulcers and relieve itching caused by insect bites and stings [2], so it makes sense they would have wanted this useful plant with them in this strange new world. However, since its introduction to North America, A. petiolata has spread vigorously and crowded out some of the native flora, preferentially invading areas of moist soils and shade. Dr. Rob Colautt i of the Queen’s University Biology Department has been investigating the effects of this species on the soil microbial communities as a possible explanation for its rapid invasion [https://www.ecoevogeno.org/research.html]. His research has found that A. petiola does alter some nutrient-cycling bacteria in the soil [4]. We are excited to see the results of his ongoing work!
Herbarium specimens can tell us much about the history of garlic mustard. This specimen from the Fowler Herbarium (Image 3) was collected in Sweden in 1886; back then, the scientific name for this plant was Sisymbrium alliaria (now a taxonomic synonym). As more species and molecular tools are used in taxonomic studies, the names of species sometimes change. Taxonomy is constantly being revised and families, genera and species are continually being re-assigned as a result. Herbaria must be periodically updated with the most recent information – here, we can see that the genus and species was revised by Assistant Curator A.E, Garwood in 1980, and again in 1990 to what is currently the accepted name of the species: Alliaria petiolata (M. Bieberstein) Cavara & Grande. Our efforts to digitize the specimens housed by the Fowler Herbarium have also given us a chance to revise any outdated information in the collection.
References
- Colautti, R.I. Research. Colautti Lab. https://www.ecoevogeno.org/research.html
- Grieve, M. 1984. A Modern Herbal. Penguin. New York. ISBN 0-14-046-440-9
- Koch, M., Al-Shehbaz, I.A., and Mummenhoff, K. 2003. Molecular systematics, evolution, and population biology in the mustard family (Brassicaceae). Annals of the Missouri Botanical Garden 90, 151 – 171.
- Lavoie, K., Antunes, P.M., and Colautti, R.I. Effects of Alliaria petiolata invasion on soil microbial community structure inferred from bacterial 16S and fungal ITS metabarcodes. (in prep.)
- Newcomb, L. 1977. Newcomb’s Wildflower Guide. Little, Brown and Company. New York. pp. 138.
Opinicon Natural History 
The most difficult thing for me about Garlic Mustard is that the West Virginia White, a butterfly that is at risk in North America and globally, often lays its eggs on the plant, which subsequently die. The usual host plants, toothworts (especially two Cardamine species), are small, native mustards often more widely dispersed and the butterfly reportedly tends to stay close to home base. Garlic mustard can be prolific, with small colonies suddenly seeming to burst out and overtake areas in certain years. It is easy to remove, but appears to produce a tremendous number of seeds per plant. Toxic chemicals released by the roots inhibit nearby plant growth and it displaces native flora. As much as I love plants of all kinds, we do the butterfly and native plant species a real favour when we stay on top of garlic mustard removal in our areas.
Thanks for bringing that up! One can certainly see how quickly it overtakes an area when left unattended. I didn’t know about the West Virginia White butterfly, but it’s all the more reason to take control of our invasive species! Someone on Instagram brought this site to our attention http://www.invadingspecies.com/ which allows you to report occurrences of invasive species in Ontario!
I wanted to add a thank you for your informative description!