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Garlic mustard [Alliaria petiolata [(M. Bieb) Cavara & Grande; Brassicaceae] is a European native biennial herb, first recorded on Long Island, NY in the 1860s, and is expanding rapidly in northeastern and midwestern forests in the U.S. and in southern Canada. Garlic mustard flourishes in moist woodlands with moderate exposure to light, but it can grow in a diversity of other habitats. It is found in natural areas, woodlots, and along edges of agricultural fields and lawns throughout North America. Several life history traits likely contribute to the invasiveness of this species. It has a high inbreeding rate and can produce numerous seeds. It exhibits remarkable morphological plasticity to local environmental conditions. It can exude allelopathic chemicals (glucosinolates and their hydrolysis products) that can reduce seed germination and growth of some species, and that can affect mycorrhizal potential of soils. Garlic mustard has been shown to outcompete some ecologically and commercially important hardwoods in short-term experiments, and its presence in natural areas is associated with reduced native herb abundance and diversity. Garlic mustard can also negatively impact salamander populations that rely on litter dwelling animals for food, and it can endanger populations of the rare butterfly Pieris virginiensis by serving as an oviposition site by adults on which larvae can not survive. Because of its known or potential negative impacts in natural and agricultural ecosystems, garlic mustard is an important target for chemical and biological control efforts.