Many communities invaded by one non-native plant species also are invaded by others due to a shared response to environmental factors that promote invasive species generally, such as fragmentation, disturbance, and proximity to seed sources. Direct comparison of these co-invasive species in their shared communities therefore is necessary for understanding the ecology of invaded communities, particularly if management resources must be prioritized. We compared ecological characteristics of two of the most important co-invasive herb layer species in forests of the eastern United States, Alliaria petiolata (garlic mustard, a C₃ biennial herb native to Europe) and Microstegium vimineum (Japanese stilt-grass, a C₄ annual grass native to east Asia). Both are invasive across a wide geographic range, establish dense stands under varying canopy covers, and are associated with lower native herb layer diversity and abundance. However, A. petiolata has attracted much greater attention from the scientific and management communities. Our research had three related components aimed at understanding which species may be a greater threat. We conducted a greenhouse experiment with shaded and unshaded growing conditions, in which M. vimineum grew much greater shoot biomass (although total biomass was similar) and photosynthesis rates for M. vimineum generally exceeded those for A. petiolata. We also did a large field experiment using transplants grown from seed. Microstegium vimineum had greater survivorship, less insect herbivory, larger mass, and higher photosynthesis rates than first-year A. petiolata. In addition, we studied a co-invaded natural community. Both species' percent cover were variable across the site but had no relationship to photosynthetically active radiation (PAR) levels. Microstegium vimineum had higher cover in 2001 and by 2005 had increased dramatically over A. petiolata in co-invaded plots. In the summer months M. vimineum had greater photosynthesis rates (measured in situ with a Li-Cor 6400) and were matched only by A. petiolata's early spring rates. These results indicate that in forests where they co-invade M. vimineum may have greater potential for spread than A. petiolata and deserves increased attention from both scientists and forest managers.