Invasive Spotlight: Garlic Mustard
An invasive species can be any kind of living organism—a plant, insect, fish, fungus or bacteria—that is not native to an ecosystem and causes harm.
-National Wildlife Federation
Garlic mustard (Alliaria petiolata) is also known as Poor Man’s Mustard, Hedge Garlic, Garlic Root and Jack-by-the-Hedge. It is called garlic mustard because the leaves have a garlic smell when they are crushed.
Why is garlic mustard bad?
Classed as a non-native invasive plant, this species is a serious concern. It has spread throughout much of the United States over the past 150 years, becoming one of the worst invaders of forests in the American Northeast and Midwest. While it is usually found in the undergrowth of disturbed woodlots and forest edges, recent findings have shown that garlic mustard has the ability to establish and spread even in pristine areas. This spread has allowed it to become the dominant plant in the undergrowth of some forests, greatly reducing the diversity of all species. Garlic mustard is one of very few non-native plants to be able to successfully invade forest understories.
It can form dense stands in the understory choking out native plants by control light, water, and nutrient resources. Many of our native Spring wildflowers that occur in similar moist soil forest habitats are most impacted.
Garlic mustard is allelopathic (it exudes a compound from its roots that inhibits the growth of beneficial soil fungi (mycorrhizae) and prevents seeds from germinating.) and has inhibited growth of both grasses and herbs in laboratory settings (Michigan State University, 2008). Unsurprisingly, experimental trials have shown that removal of garlic mustard leads to increased diversity of other species, including annuals and tree seedlings.
Where did Garlic Mustard come from?
Garlic mustard originated from Europe and parts of Asia. It is first thought to have been introduced into North America for medicinal purposes and food, with the earliest known report dating back to 1868 on Long Island, NY.
Garlic mustard has a biennial life cycle, that is, it takes two years to fully mature and produce seeds. Seeds germinate in February to early March of the first year and grow into a short rosette by the middle of the summer. During the second year of growth, plants typically bolt and form upright, flowering stems in March and April, and the plant dies by June. At this point even though it has died, it seeds live on for the next generation.
It can cross-pollinate or self-pollinate and has a high seed production rate. The four-sided seedpods, develop in May, containing small black seeds lined up in a row. On average, a garlic mustard plant will produce 22 pods, each of which can contain as many as 28 seeds. Plant stands can produce more than 62,000 seeds per square meter to quickly out compete local flora, changing the structure of plant communities on the forest floor. The seeds generally germinate within one to two years, but may remain viable for up to five years in the seed bank. The fact that it is self fertile means that one plant can occupy a site and produce a seed bank. It’s not picky about conditions, and can grow in dense shade or sunny sites. Seed dispersal is mainly by humans or wildlife carrying the seeds.
First Year Plants
Crushed rosettes and new foliage have an odor of garlic
Low-growing rosettes with rounded, kidney-shaped leaves, scalloped on the edges
Basal rosettes stay green in fall and winter; spring growth starts very early
Leaves are not noticeably fuzzy or hairy (unlike most look-alike species)
Roots typically have a characteristic s-shaped bend
Second Year Plants
Typically grow to about 3 feet tall, but can be anywhere from a few inches to over 6 feet tall depending on conditions
Each plant usually produces one flowering stem. If a plant is cut or stepped on, many stems will form
Small, white 4-petaled flowers appear in early spring and are in clusters at the top of the stem
Upper leaves are more triangular, becoming smaller toward the top of the plant, coarsely toothed
Roots typically have a characteristic s-shaped bend
Prevention, Control and Management
Although edible for people, it is hardly touched by local wildlife or insects. There are few effective natural enemies of garlic mustard in North America. Herbivores, or animals that eat plant material, such as deer and woodchucks only remove up to 2% of the leaf area in a stand of garlic mustard (Evans et al. 2005). Thus, we can’t depend on or local herbivores to control garlic mustard, we must do so ourselves.
The best way to get rid of garlic mustard is manually; i.e. pulling it up and discarding it. Try to pull up the plants before they set seed, because the action of yanking the plant from the ground will spread the seed.
Removing garlic mustard by hand is not difficult if done when the soil is moist. The wet soil makes it easier to pull up the plants, and you're more likely to get all or most of the long tap root. You need to remove at least the upper half of the root to prevent a new stalk from forming.
Carefully and thoroughly clean off boots, clothes and tools before leaving the area to avoid carrying the tiny seeds to new sites.
Much as our District loves to promote composting, this is not the way to go with this plant. If you don’t get high enough temperatures for a long enough time, you won’t kill the seeds. Instead, bag them up and throw them out with your garbage. They will be deeply buried in the landfill.
The ultimate goal in removing garlic mustard is to prevent seed development and spreading until the existing seed bank is depleted. Unluckily for us, this may take 2- 5 years in any confined area.
Chemical applications can also be effective for controlling garlic mustard, particularly in areas too large for removal by hand. In dense stands where other plant species are not present, a glyphosate-based herbicide such as Roundup® can be an effective method for removal. Glyphosate herbicides are non-selective, so caution must be used when non-target species are in the area. Chemical applications are most affective during the spring (March-April) when garlic mustard is one of the few plants actively growing. Be wary, as it will also kill any of the few native Spring wildflowers that haven’t been choked out. Fall applications may be used; however other plant species still in their growing season may be harmed.
The best method for controlling garlic mustard, or any other invasive plant, is to prevent its establishment. Disturbances in the forest understory that would allow for rapid invasion should be minimized. This would include limiting foot traffic, grazing, and erosion-causing activities. Monitoring the forest understory and removing any garlic mustard plants as soon as they are introduced will help to prevent the establishment and spread of this invader. We have seen, that when we an area has honeysuckle removed, garlic mustard quickly moves in. Luckily it is much easier to control and remove the garlic mustard than it is honeysuckle.
If you can’t beat them, eat them!
Page of this newsletter from the Environmental Education Council of Ohio contains several recipes for garlic mustard https://eeco.wildapricot.org/resources/Documents/Fall19_HighRes.pdf