Friday, May 1, 2009

Alliaria petiolata: foliage

This is an extremely invasive species in Toronto. Alliaria petiolata, known in English as "garlic mustard" and in French as alliaire officinale ou herbe à ail is a biennial native to Eurasia and northern Africa. It is one of the worst invasive plants in Toronto.

Garlic mustard was originally brought to Canada from Europe as a food plant. Unfortunately, it has flourished too well. Garlic mustard is not edible to Canadian wildlife, it emerges early in the spring before many native plants, it produces large numbers of seeds, and it harms mycorrhizal fungi that native plants need to grow properly. These factors give it an advantage over native plants, with the result that it can invade forests and outcompete them. There are areas of the forest in High Park that are completely overrun with garlic mustard, at the expense of native plants. Garlic mustard is invading the forest around the Ontario Science Centre as well, endangering their beautiful white trilliums. Because Canadian wildlife won't eat garlic mustard, replacing native plants with garlic mustard means less food for wild animals and therefore a loss of animal as well as plant biodiversity.

For these and other reasons it is important for Toronto gardeners to be able to recognize garlic mustard and remove it. In the first year, garlic mustard forms a low rosette of heart- or kidney-shaped leaves with a jagged edge. The leaves have a distinctive garlicky smell when crushed. In the second year, the plant produces a stem 30-60 cm tall with clusters of small, white, four-petalled flowers. The flowers are followed by skinny upright seed pods, but please don't let it get to that stage if you can help it. After setting seed, the plant dies, which is good news. (There are some good photos of garlic mustard leaves, flowers, and seed pods on this page by the Plant Conservation Alliance Alien Plant Working Group.)

To control garlic mustard, remove the entire plant in the first year, or remove at the very least the flowers in the second year (in erosion-prone sites you may want to leave the roots of second year plants in place, but cut the plants off close to the ground, otherwise more flowers can form in the leaf axils). Don't leave the flowers or unripe seedpods lying around as even if cut off from the root they can still ripen and produce viable seed. You can eat the plants if you like; here's a recipe for garlic mustard pesto (I haven't tried it myself yet).

Garlic mustard seed in the soil remains viable for years, so eradicating it is a long-term project. So let's make it a May tradition to enjoy a walk outside eradicating garlic mustard!

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