Garlic mustard [Alliaria petiolate (M. Bieb.) Cava & Grande] is a problematic European invader of temperate Eastern North American forests. We studied its germination response at reduced oxygen (O₂) concentrations. Seeds of garlic mustard were exposed to different O₂ concentrations (20.9, 15, 10, 5, and 2.5 %) in two laboratory experiments. Germination rate increased, and it took fewer days to attain 50 % germination (t₅₀) at 15 % O₂ than at 20.9 % O₂. Germination declined at 5 % O_2, and seedlings became stunted at 2.5 % O₂. The ability of garlic mustard to thrive under low O₂ concentrations may allow it to invade a wide variety of suboptimal habitats, such as shady understories where leaf litter cover the forest floors and on compressed and compact soils. Its ability to grow at low O₂ levels may contribute to its invasiveness.