The recent expansion of the reed Phragmites australis in western Atlantic salt marshes has become a conservation concern. Historically, Phragmites was restricted to the terrestrial border of marshes, but now it aggressively invades lower elevations. To explore factors influencing this expansion, we examined (1) the effects of physical factors and competitive interactions on the performance of Phragmites and (2) the role of clonal integration on the ability of Phragmites to invade low marsh habitats. We transplanted Phragmites into vegetation along an elevation gradient, with and with- out neighboring plants. Phragmites died when transplanted to the lowest marsh zone but survived and expanded in higher marsh zones. This suggests that the low oxygen availability characteristic of waterlogged soils limits Phragmites growth in the low marsh. Neighboring vegetation reduced Phragmites growth in all zones, and severing Phragmites rhizomes invading the low marsh reduced the survivorship and photosynthetic rate of ramets. These results suggest that Phragmites may invade low marsh habitats by initially es- tablishing itself in the high marsh and then expanding into lower, less favorable habitats using clonal integration. This has important implications for understanding the changing dynamics of New England salt marsh plant communities and developing management strat- egies for the control of Phragmites.