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Leafy spurge (Euphorbia esula L.) is an exotic, deep-rooted, perennial weed native to Europe and Asia. It was first reported in the United States in Newbury, Massachusetts, in 1827, where it likely established from contaminated soil left from ship ballasts. This invasive weed quickly spread westward across North America, accelerated by multiple reintroductions from contaminated crop seed including oat (Avena fatua L.), smooth brome (Bromus inermis Leyss.) and alfalfa (Medicago sativa L.) brought by European settlers. Leafy spurge is now abundant on the northern Great Plains of the United States and the prairie provinces of Canada, where it often forms stands dense enough to displace native plants and restrict cattle grazing.