Garlic Mustard

Liza Drozdov – Halton Master Gardener

When the snow melts revealing the first green leaves in the garden, look closely before you get too excited–it might be an unwelcome pest. If that plant is a small basal rosette of green leaves with scalloped edges, growing close to the ground, it’s probably Garlic Mustard (Alliaria petiolata), an invasive weed.

Native to Europe and Asia, it was brought to North America by settlers, for use in cooking and medicine, but it escaped settlements and has steadily proliferated and colonized everywhere in the province.

Native to Europe and Asia, it was brought to North America by settlers, for use in cooking and medicine, but it escaped settlements and has steadily proliferated and colonized everywhere in the province. It’s a member of the Brassica family, like kale and broccoli and is very high in nutritional value. It contains fiber, beta-carotene, vitamins C and E, zinc, calcium iron and manganese, but it is bitter. Garlic Mustard can now be found all over Ontario and as it has been able to successfully invade the shady forest understory layer all over the province, it’s a real danger to our native woodlands.

Garlic Mustard is tolerant of most soils and light conditions from full sun to part shade, which enables it to quickly infest an area and overwhelm anything else growing. It’s prolific and aggressive and the Ontario Invasive Plant Council has identified Garlic Mustard as a threat to biodiversity since it displaces native woodland plants. The plants form a dense monoculture that suppress native flora, and they also release an allelopathic chemical that permanently alters soil chemistry and prevents the future growth of other plants.

A biennial, Garlic Mustard has a distinctive odour of onion or garlic when crushed. In its first year, it remains a low cluster of green leaves. In its second year, it will send up a single flower stalk with small white flower that will produce hundreds of seeds that germinate quickly. The seeds can also lie dormant in your soil for up to five years, so infested areas need to be monitored for years to make sure the established seed bank is depleted.  

New seedlings remain green through the winter, which gives them an advantage over all other plants since they get a head start photosynthesizing once the snow cover melts. But that also gives the gardener an advantage, since they’re easy to find now in early spring, when all other perennials in the garden are still dormant.

According to the Ontario Invasive Plant Council, individual plants can produce up to 150 seedpods with each pod containing 22 seeds; established colonies can produce over 105,000 seeds per square metre, so you can imagine how many dormant seeds are lying in wait! The best method for controlling Garlic Mustard is to prevent its establishment, so make sure you don’t allow it to bloom. Another alternative is to eat it–after all, that’s why the early settlers brought the plant to our shores.

Garlic Mustard only grows from seed–not through runners or offsets, so the best method of Garlic Mustard control is to cut the flower stalk before it blooms. The parent plant will die the year it blooms, since it’s a biennial. Make sure to dispose of cuttings in the garbage–not your compost, or you’ll spread the seeds throughout your garden.

New seedlings remain green through the winter, which gives them an advantage over all other plants since they get a head start photosynthesizing once the snow cover melts. But that also gives the gardener an advantage, since they’re easy to find now in early spring, when all other perennials in the garden are still dormant. Dig them out now and you’ll find Garlic Mustard will be easy to control.

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