Help remove invasive Garlic Mustard
Invasive Garlic Mustard crowds out native plant species like Milkweed, hurting Monarch butterfly populations
Invasive Plant of the Month Feature – Garlic Mustard
By Renee Sandelowsky and Helen Varekamp
Renee Sandelowsky and Helen Varekamp are Bayfield residents and local volunteers. They are writing a monthly series of articles, for the Canadian Coalition for Invasive Plant Regulation, about the benefits of planting native species of plants and removing invasive species. This month (April 2025), their focus is the invasive plant Garlic Mustard.
We used to plant invasive plants like Goutweed, Periwinkle, Russian Olive, Burning Bush, and Creeping Jenny because they were pretty and easy to grow and required no maintenance. We’ve learned, since then, that what makes invasive plants so easy to grow is also what makes them so dangerous.
Invasive plants are aggressive species. They crowd out native plants and they may change the chemistry of the soil. They disrupt the functioning of an ecosystem — the collection of living and non-living organisms that exist and function in a particular region.
So, why does that matter? Many insects evolved to use specific plants for food. The most obvious example is the Monarch butterfly. Monarch caterpillars can only eat milkweed – a plant which is toxic to most other insects. Without Milkweed, there might be no Monarch butterflies. When an invasive plant like Garlic Mustard crowds out Milkweed in the area, it depletes crucial food sources for the caterpillars and therefore contributes to the decline of Monarch butterflies.
We know that insect populations are in decline, but you may not know that many birds need insects to feed their young. A clutch of baby chickadees eats between 6,240 and 9,120 caterpillars before they fledge. If there aren’t enough caterpillars, the baby chickadees cannot survive.
We also know that more than one in four birds have disappeared in the last 50 years. In fact, wild bird populations in the continental United States and Canada have declined by almost 30 per cent since 1970. The reason for these declines? Land use intensification, pesticide use, climate change and invasive plants. Invasive and non-native plant species do not provide insects with the food and nutrients they need to survive, which in turn depletes the food supply for many birds.
Focus on Garlic Mustard – Alliaria petiolate
The focus of Canadian Coalition for Invasive Plant Regulation (CCIPR) is on plants sold through the nursery trades, but this month we are focusing on Garlic Mustard, since it is easy to recognize and can be controlled with ongoing vigilance. It grows abundantly along roadsides and wastelands, but also in people’s gardens and yards.
Garlic Mustard is an herb native to Europe. Garlic Mustard was brought to North America in the early 1800s for medicinal and culinary uses. It has since escaped into the wild and is now one of Ontario’s most aggressive forest invaders.
Garlic Mustard has two distinct life stages over its first two years. In the first year, it grows only a cluster of leaves shaped like a rosette, while a strong root system develops.
Plants that survive the winter produce flowers and hundreds of seeds in their second year. Dense stands produce more than 60,000 seeds per square metre. Seeds are easily spread by people, leaf blowers, and animals and the plant can grow in a wide range of sunny and fully shaded habitats, including undisturbed forest, riverbanks, and roadsides.
Garlic Mustard does not provide a valuable food source for native wildlife. It can displace wildflowers like Trilliums and Trout Lilies, while it hinders other plants by interfering with the growth of fungi that bring nutrients to the roots of the plants.
HOW TO IDENTIFY GARLIC MUSTARD
- Young leaves release a strong garlic odour when crushed.
- First-year plants produce a rosette of dark green, kidney-shaped leaves with scalloped edges.
- Second-year plants grow a stem up to one metre high with triangular, alternate, sharply toothed leaves.
- Second-year plants produce white flowers with four small petals in May.
- Narrow seedpods, 2-6 centimetres long, split open in mid-summer to reveal tiny black seeds.
Garlic Mustard resembles several native Ontario plants in the carrot, daisy and violet family. If in doubt, crush the leaf to determine whether it emits a strong garlic smell.
HOW TO MANAGE GARLIC MUSTARD
Managing Garlic Mustard is quite achievable with persistence and a multi-step approach:
- Manual Removal: Cut plants at the base in early spring before flowers emerge; recut as needed. If the plant has flowered, cut it back at the base and dispose of it to prevent seed formation. Dispose of plants in the garbage, not in the compost or municipal green waste system. Pulling the entire plant early in the spring is another option, but be aware that hand pulling Garlic Mustard may disturb the soil and stimulate germination of seeds in the soil. Seeds can stay dormant for up to five years.
- Smothering: Smothering may be a better choice for larger infestations. Cover infested areas with tarps or cardboard to block sunlight. This can be started any time of year and the process can take up to two years to complete. Check progress each season.
- Herbicide Treatment: Difficult-to-reach, or large areas may be treated by herbicide. Herbicide applications must be completed in accordance with the Pesticides Act, and all other applicable legislation. If you are considering a herbicide application, it is recommended that you consult a licenced and insured organization who offers these services.
When hiking, prevent the spread of invasive plants by staying on the trails and keeping pets on a leash. At home, do not use leaf-blowers near Garlic Mustard populations as they will propel seeds to other areas.
After removal, replant the area with regionally appropriate native plants. This step is essential to prevent reinfestation and to restore ecosystem health. Native plants will also attract pollinators and wildlife, improving biodiversity in your garden.
A good choice, as an alternative, would be Foamflower – Tiarella cordifolia.
LEARN MORE AND TAKE ACTION
Education is key to combating invasive plants. Learn to identify invasive species and choose native or non-invasive alternatives for your garden.
Focus on managing one or two invasive plants at a time and be patient — successful eradication takes time and persistence.
To learn more, visit:
- Ontario Invasive Plant Council
- Canadian Coalition for Invasive Plant Regulation
- Grow Me Instead Guide
- Garlic Mustard – Profile and Resources