Crops grown for bioenergy production are a mandated component of the United States energy portfolio. Giant miscanthus (Miscanthus × giganteus) is a leading bioenergy crop similar in habit to the invasive plant giant reed (Arundo donax). To characterize the environmental tolerance of giant miscanthus, we compared the soil moisture stress tolerance of giant miscanthus and giant reed under glasshouse conditions. We subjected both species to soil moisture conditions of severe drought (−4.2 MPa), mild drought (−0.5 MPa), field-capacity (control), and flooded soils. These conditions were applied to two cohorts: one in which soil moisture conditions were imposed on newly planted rhizome fragments, and one in which conditions were imposed on established plants after 8 weeks of growth in field-capacity soil. After 16 weeks, we harvested all plants, measured above- and belowground biomass, and evaluated the reproductive viability of rhizome fragments. The total biomass of each species under flooded conditions was not different from the field-capacity control groups regardless of cohort. However, drought did affect the two cohorts differently. In the cohort treated after 8 weeks of growth, mild and severe drought conditions resulted in 56% and 66% reductions in biomass, averaged over both species, compared with the controls. In the cohort treated for the entire 16 weeks, mild and severe drought conditions resulted in 92% and 94% reductions in biomass. Rhizome fragments from both species and both cohorts showed 100% viability following flooded and control treatments; drought treatments reduced rhizome viability in both species, with a greater impact on giant miscanthus. Although giant miscanthus does not appear to have the potential to escape and establish in relatively dry upland ecosystems, it does show tolerance to flooded conditions similar to giant reed.