Because Typha × glauca often dominates wetlands where humans have stabilized the natural hydrologic regime, we 1) compared its expansion rates where water levels were stabilized vs. fluctuating and 2) explored the potential for stabilized water levels to allow plants to accumulate more phosphorus (P) and increase growth. In three Wisconsin marshes, the area dominated by Typha expanded linearly over time, but rates were higher where water levels were stabilized than where they fluctuated naturally (based on nine aerial photos from 1963 to 2000). In a large wetland (412 ha) behind a dam, Typha × glauca expanded 81,152 m²/year, and clone diameters extended 3.9 ± 0.61 m/year. In contrast, a mixed stand (mostly T. angustifolia) in an upstream wetland with fluctuating water levels expanded only 2,327 m²/year, and clones extended only 2.5 ± 0.75 m/year. While various factors could have caused these differences, a separate two-factor experiment in outdoor microcosms supported the hypothesis that stabilized water levels alone can enhance T. × glauca spread. The experiment indicated that both stabilized water levels and P additions increased P accumulation and growth of T. × glauca. Constant inundation (5–10 cm deep) allowed T. × glauca to produce 56% more biomass (61.6 ± 4.0 g) than a regime with two drawdowns (39.4 ± 1.9 g; p < 0.001). Plants under constant inundation accumulated 0.15 ± 0.007 g P, which was 36% more than with one drawdown (0.12 ± 0.004 g; p < 0.001) and 67% more than with two drawdowns (0.09 ± 0.005 g; p < 0.001). Also as expected, the addition of 2 g P/m² increased biomass 23% more than the control (57.8 ± 3.0 vs. 46.9 ± 3.0 g/plant; p  =  0.02). Our microcosm results suggest that unavailable P can shift to a form that T. × glauca can use. Thus, internal eutrophication can augment rates of T. × glauca invasion.