||C-4 herbaceous rhizomatous perennials have many positive attributes as potential and actual biomass energy crops on agricultural land. Two species from this group have become widely planted. Miscanthus x giganteus has been extensively studied and planted in Europe, and Panicum virgatum in N. America. To date, side-by-side comparisons of mature stands of these crops have not been reported in the peer-reviewed literature. We examined all peer-reviewed articles describing productivity of these species and extracted dry matter yields, nitrogen fertilization (N), temperature (growing degree days) and precipitation/irrigation. Only yields reported 3 or more years after planting were included. Despite being on different continents, trials spanned similar temperature, N and water ranges. Miscanthus x giganteus (97 observations) yielded an average 22 Mg ha(-1) compared to 10 Mg ha(-1) for P. virgatum (77 observations). Both crops showed a significant positive response to water and N, but not to temperature. Miscanthus x giganteus yielded significantly more biomass than P. virgatum across the range of all three variables. There were differences between the species in their apparent responses to these variables. Miscanthus x giganteus showed a stronger response to water, while P. virgatum showed a significantly stronger response to nitrogen. Since energetic viability, and profitability, of biomass crops hinges critically on high outputs of biomass energy for low inputs of money and fossil fuels, these results suggest that M. x giganteus holds greater promise for biomass energy cropping than does P. virgatum across the range of trial conditions under-taken to date.