Species invasions have several stages: 1) immigration of propagules, 2) establishment, and 3) spread and replacement of native species. The unconstrained growth of a species contributes to competitive ability during establishment and spread in bare soil conditions such as exist in newly restored wetlands. Phalaris arundinacea, an invasive perennial grass, is problematic in wetlands across temperate North America. Studies have shown that other wetland species have slower growth than P. arundinacea during the first 2 y following germination, implying that P. arundinacea invasiveness may be related to high rates of biomass production during establishment. We conducted a uniform planting study to observe P. arundinacea growth over two growing seasons. Phalaris arundinacea grew rapidly during establishment, producing a total biomass of 124 g·plant⁻¹ in four months and 525 g·plant⁻¹ at the end of two growing seasons. Root:shoot ratios were < 1 during the first four months of establishment, but were > 2 for the rest of the study. The shift in root:shoot ratio suggests P. arundinacea may pre-empt other species during establishment by first securing above ground space and then spreading rapidly belowground. Phalaris arundinacea vegetative growth likely facilitates dominance of this species when growth is unconstrained. Species-specific information about unconstrained growth is important to developing a predictive understanding of early establishment of wetland plant communities in bare ground conditions, i.e., in newly created, restored, and disturbed wetlands.