Invasion of the Garlic Mustard

Garlic mustard (Alliaria petiolata), a cool-season, shade-tolerant, obligate biennial herb, is currently one of the most serious invaders in forested areas of the northeastern and midwestern United States. Alliaria petiolata is one of the few non-native herbaceous species able to invade and dominate the understory of North American forests. Garlic mustard is not known as a weed on other continents.

Little long-term research has been conducted to document the impact of garlic mustard on native vegetation. However, sites invaded by A. petiolata frequently have low native herbaceous richness, and garlic mustard has been implicated in several studies as the cause of this low diversity. Garlic mustard invades sites independent of the presence or cover of native species, and species-rich sites seem to be more likely to be invaded than species-poor sites per one researcher.

Long-term presence of garlic mustard was also associated with a significant decline in cover of native perennial herbaceous species. Phytotoxic chemicals produced by A. petiolata may interfere with growth of native species, potentially through inhibition of mycorrhizal (beneficial fungal) activity.

The presence of garlic mustard interferes with oviposition (egg laying) of some rare native butterflies. The native hosts of the West Virginia white (Pieris virginiensis) are toothworts (Cardamine spp.). Eggs laid by females on garlic mustard hatch, but larvae are unable to complete development.

Text and image modified from the USDA Forest Service.

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