Garlic Mustard – Ecology and Control Methods

by Christine Hanrahan

Garlic mustard (Alliaria petiolata) is recognized as a serious invasive plant species in North America. Native to Europe, the plant was brought here by settlers for food and medicinal purposes. It has been present in North America for over 140 years, apparently first recorded in 1868 on Long Island in New York State and in Canada (including Ottawa) in the late 19th century. Since then, it has had a long lag period, sitting quietly, doing nothing, until about 40 years ago, when it began to spread rapidly, alarming naturalists and biologists alike.

The plant is most abundant in southwest Ontario, but can be found all along the St. Lawrence Valley and, of course, throughout Ottawa.

Garlic mustard is found in forests, edge habitats, along trails, roadsides, and in just about any disturbed area. It grows in shade, sun, damp conditions and dry, although it prefers dry, wooded locations.

Biology of Garlic Mustard
This early spring plant grows rapidly. In the first year of development, only basal “rosettes” are visible, but by the second year, when they reach maturity, the plants can reach a height of 1 to 1.5 metres. A biennial plant (a 2-year life-cycle), it self-pollinates and is cross-pollinated (by insects) and spreads only by seed.

Seeds usually germinate in the second year, although they can remain viable in the soil for 5-10 years; seed viability apparently decreases over time. Long exposure to cold is needed for germination to occur; thus our winters are perfect. Individual plants can produce anywhere from about 350 (usual) to 8,000 seeds, and the germination rate is 40-100%. Although seedling mortality is high (only about 2-7% survive) and drought can kill off 95% of seedlings, none of this seems to prevent the spread of garlic mustard.

The problem with Garlic Mustard….
Garlic mustard is a habitat generalist; it seeds prolifically and has no natural enemies in North America. It displaces native plants (especially spring ephemerals) and decreases diversity, which in turn decreases available food for wildlife (pollen, nectar, fruit, foliage, roots). It probably also decreases suitable habitat for ground-nesting birds, such as ovenbirds and hermit thrushes, when it is the dominant understorey plant.

I read somewhere that in a highly infested area, seedling density may reach 17,000 in half a square metre. Even it only 2-7% survive, this is still a lot of garlic mustard. In one local site with a high diversity of native plants, particularly spring ephemerals, garlic mustard was first noticed about 12 years ago; within 5 years, it had almost completely dominated the area to the detriment of native flora.

It has been found to disrupt the symbiotic relationship between tree seedlings and arbuscular mycorrhizal fungi (mycorrhizae colonize root systems and are critical for nutrient and water uptake). In recent years, high concentrations of cyanide compounds have been discovered in garlic mustard, and these compounds may contribute to the reduction in mycorrhizal fungi.

Studies have shown that the growth of sugar maple (Acer saccharum), red maple (A. rubrum), and white ash (Fraxinus americana) is poor in soils invaded by this species. Worse, the negative impact of garlic mustard on the soil and on arbuscular mycorrhizal fungi continues for several years after the plant is removed from a site. Not all tree species are affected, and further studies are ongoing.

Garlic Mustard Control Methods
Control of this plant is both easy and difficult. It is easy to pull up, but difficult to completely eradicate. At the Fletcher Wildlife Garden (FWG), we’ve been pulling it out for years. Initially, we thought we’d eradicated it, as it didn’t reappear for several years. But disturbed soil facilitates seed germination, and it came back in areas where we’d been removing it and other invasives.

Management options vary depending on the size of the infestation. For a small stand, pulling is feasible. For large areas, methods proposed by those who deal with this problem regularly include herbicide use (usually 1-2% glyphosate) and controlled burns. Neither of these is particularly desirable, although both can have some impact.

Pulling is easy to do (especially when the soil is damp), quick, and requires no special tools. However, this disturbs the soil, which encourages further growth from the seed bank. You also need to ensure that the entire root is pulled up (see photo of the long roots below). Nonetheless, this is the easiest method for smaller infestations, such as we have at FWG.

Cutting is often suggested for large areas of garlic mustard, particularly if nothing else is growing with it. Scythes, weed whippers, even lawnmowers are three tools that work best for cutting many plants at once, particularly as they must be cut at ground level. This needs to be done after flowering (earlier, and they may re-sprout), but before seed production.

Hoeing up the basal rosettes could help prevent plants from growing and setting seed, but we’ve not tried this at FWG, yet.

No matter the method you choose and whether you have few or many plants, follow-up control is critical. Your goal is to prevent seed production AND deplete that seed bank. You will likely have to do follow-up work for at least five years and, in most cases, much longer. Studies suggest that single-year control efforts are worse than no management at all, because the resulting soil disturbance creates perfect growing conditions for garlic mustard, aids in growth and distribution, and exacerbates the problem. Without follow-up control, it is probably better to do nothing!

Disposal is important too. If you have cut or pulled plants when seedpods are just forming, don’t leave them on the ground, as seeds will continue ripening. Place them in plastic bags and dispose of in garbage. Not an ideal method, but composting in a small home composter, or leaving on the ground, will nullify all the work you’ve put into removal! Creating a big pile and covering it with a tarpaulin may work to control seed production if the tarp is well secured. Basal rosettes dug or hoed up, can be left on the ground.

Other Plants Similar to Garlic Mustard
When removing garlic mustard, remember that many other common plants can closely resemble their leaves and flowers. Early saxifrage (Saxifraga virginiensis) and toothwort (Dentaria diphylla), both have small white flowers similar to garlic mustard. Leaves of yellow avens (Geum allepicum), barren-ground strawberry (Waldsteinia fragarioides), violets (Viola), and non-native motherwort (Leonurus cardiaca) can be mistaken for garlic mustard.

Bio-control
In Europe, 69 species of insects are known to feed on garlic mustard. Efforts to find bio-control agents for North America began in Europe in 1998, and several weevil species look quite promising. One, Ceutorhynchus scrobicollis (a root miner), has been in quarantine at the University of Minnesota (with two other weevil species)for several years, and if all goes well, it will be reared for release sometime in the next few years. The two other weevil species in quarantine, C. alliariae and C. roberti, are shoot-mining weevils. A fourth, being studied in Switzerland as quite promising, is a seed feeder.

References
Friends of the Patapsco Valley and Heritage Greenway, Inc., 5th Annual Garlic Mustard Challenge.

Gerber, E., Cortat, C., Hinz, H.L., Blossey, B., Katovich, E., Skinner, L. 2009. Biology and host specificity of Ceutorhynchus scrobicollis (Curculionidae; Coleoptera), a root-crown mining weevil proposed as biological control agent against Alliaria petiolata in North America. Biocontrol Science and Technology, 19(1-2).

Mabry, C. 2007. Background and methods of control for garlic mustard. A Report Prepared for the Sierra Club.

Nature Conservancy of Canada. 2007. Control methods for the invasive plant garlic mustard (Alliaria petiolata) within Ontario Natural Areas. V1.0. NCC – Southwestern Ontario Region, London, Ontario. 16 pp.

Nuzzo, V. 1997. Alliaria petiolata. Exotic Pests of Eastern Forests, Conference Proceedings, April 8-10, 1997, Nashville, Tenn.

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