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Groundwater in many industrial regions is highly contaminated with heavy metals such as zinc (Zn) and cadmium (Cd), posing risks to ecosystems and human health. Therefore, we tested a surface flow constructed wetland planted with reed canary grass (Phalaris arundinacea L.) as a remediation strategy. The selection of plant species is critical for achieving continuous metal removal from constructed wetlands. Most Cd and Zn accumulating plants reported in the literature do not grow under permanent flooding, are invasive, or do not tolerate the climatic conditions in Germany. P. arundinacea is one of the highest-yielding cool-season grasses characterized by a high translocation ratio of Cd and Zn. However, information on metal tolerance and removal from highly mineralized waters under field conditions is lacking in the literature. P. arundinacea cv. Lipaula was cultivated in intermediate bulk containers (IBCs) filled with a sand-and-gravel substrate (~730 individuals per square meter). Groundwater with high concentrations of zinc (300 mg/L), cadmium (4.5 mg/L), and sulphate (1500 mg/L) was supplied from experimental wells near the study site in Duisburg, Germany. Over two consecutive phases of 7 and 6 weeks, the IBCs were irrigated either with uncontaminated tap water (reference) or with groundwater diluted with tap water—ranging from 1:30 and 1:15 in the first phase to 1:1 and undiluted groundwater in the second. Each treatment was threefold replicated. Treatment effects were evaluated by morphometric plant parameters as well as fresh and dry biomass (FM, DM) and by analysis of Zn and Cd in plant tissue (ICP-MS). Moreover, element retention in the substrate was evaluated by NH4NO3 extracts. Plants treated with undiluted groundwater developed slight chlorosis but produced more biomass, with tendencies toward increased shoot (1550 ± 200 g/m²) and root (1600 ± 150 g/m²) dry mass and longer root systems. The plant contained up to 2529 mg/kg Zn and 17.6 mg/kg Cd (DM) in the shoots and up to 3778 mg/kg Zn and 68.8 mg/kg Cd (DM) in the roots. Considering total metal uptake (metal concentration × dry biomass), this corresponds to a potential removal of 33 kg Zn/ha and 215 g Cd/ha via the aboveground biomass over the entire 13-week growth period. Our findings demonstrated that P. arundinacea tolerates high levels of heavy metals and represents a promising phytoremediation plant species for heavy metal removal in constructed wetlands.