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Garlic mustard: an invasive plant that threatens our flora

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First year rosettes of a garlic mustard
Alliaria petiolata
  • Alliaria petiolata
  • Alliaria petiolata
  • Alliaria petiolata
  • Alliaria petiolata
  • Alliaria petiolata

Garlic mustard (Alliaria petiolata) is an invasive species of European origin. It was introduced to North America in the 19th century by colonists who used it as a medicinal plant as well as a condiment.

As of today, it has invaded several natural environments in Québec, Ontario, the Maritimes and British Columbia. It is an aggressive exotic species, which constitutes a major threat to the biodiversity of our forests. It can quickly dominate the forest floor, at the expense of the native flora. Its expansion in Québec is fast, but unfortunately it often goes unnoticed because it is discreet and little-known.

Learn to recognize it (it is easily identifiable) and participate in the efforts to counter its invasion.

Identify garlic mustard

Garlic mustard is a biennial herbaceous plant of the cabbage family (Brassicaceae). It reproduces by seeds.

In the first year, garlic mustard germinates and produces a rosette of toothed, kidney-shaped basal leaves. These leaves are evergreen (they remain green during winter).

During the second year, garlic mustard develops one or more stems that go from 30 cm to more than 1.5 m in height. Triangular sharply toothed leaves are arranged alternately on either side of the stem.

From May to early June, small white flowers with four petals appear. This is followed by the formation of long and narrow fruits (seed pods) called siliques. Each one contains about twenty small black seeds. The siliques open and spread their seeds in July and August.

When crushed, the young garlic mustard leaves have a strong garlic smell.

A very competitive species

Garlic mustard has several competitive advantages which contribute to the success of its invasion:

  • Adaptability: It is very adaptable. It can live both in humid environments (wet woods, ditches) and dry ones (roadsides or paths), and can grow as much in the sun (fields) as in shaded areas. Its shade tolerance makes it very competitive in a forest setting.
  • Early growth: Its rosette of evergreen leaves allows it to take advantage of the first spring sun rays and thus start its growth hastily, which gives it an important edge.
  • Abundant seed production: It produces seeds in abundance: 200 to 1000 seeds per plant. A colony can produce up to 100,000 seeds per m2! The majority of garlic mustard seeds germinate after one year, but can remain viable for up to 15 years, forming a seed bank in the soil. Seeds are easily scattered by humans and animals.
  • Release of toxins into the soil: Garlic mustard roots produce glucosinolates. Those toxins harm mycorrhizal fungi in the soil, which affects plants growing nearby. The native vegetation then becomes scarce, which allows garlic mustard to expand further.
  • Low predation from herbivores: This invasive exotic species is very little or not at all grazed by herbivores such as white-tailed deers. The latter prefer the native vegetation, which becomes even more sparse.

Impacts of garlic mustard on biodiversity

Garlic mustard quickly forms dense groupings and can dominate the undergrowth in just 5 to 7 years. The size of the colonies can double every 4 years!

Flora

When invading a forest, garlic mustard can overtake native plants and threaten several species already considered at risk, such as American ginseng (Panax quinquefolius), an endangered plant in Québec.

Fauna

Because of its repercussions on flora, garlic mustard can also affect native fauna. This is the case with the small West Virginia White butterfly (Pieris virginiensis), considered rare and whose Canadian distribution is limited to sugar maple-hickory stands in southern Québec and Ontario.

The caterpillars of this butterfly feed exclusively on toothwort foliage, including two-leaved toothwort (Cardamine diphylla), a vulnerable species in Québec. However, the invasion of forests by garlic mustard reduces the toothwort populations, thus limiting food sources for the larvae. In addition, the West Virginia White sometimes confuses garlic mustard with toothwort and lays its eggs there. As the caterpillars do not develop on garlic mustard leaves, they eventually die.

How to fight against garlic mustard

Fighting garlic mustard takes sustained efforts, but everyone can help tackle this invasive exotic species. The earlier the interventions, the more likely they are to be successful!

Here's how you can take action:

1. Learn to recognize garlic mustard. Keep an eye out during your hikes. The plant is often found in ditches, along roads and paths.

2. Stay on trails while walking through the forest and keep your pets on a leash.

3. If you spot it, report its presence to park officials or your municipality. You can also record your observation using Sentinelle — a detection tool provided by the ministère de l’Environnement et de la Lutte contre les changements climatiques (MELCC). This tool (available only in french) allows you to map observations of invasive exotic species.

4. Be part of a collective garlic mustard pulling activity near you. Ask your municipality or consult this article about this invasive plant in our blog.

5. If the plant grows on your land, pull it by taking the following precautions:

  • Pull garlic mustard before it produces seeds, otherwise there is a high risk of scattering them.
  • It is easier to pull up a garlic mustard plant in damp soil after it rains.
  • The plants must be uprooted completely. It is important to grab the plant at the base and then pull to remove the root, which looks like a small 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. Following this period, it is possible to dispose of the bags during garbage collection.
  • Never compost garlic mustard!

6. Share this information with your friends.

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