We present yield stress measurements of a concentrated suspension of Miscanthus x giganteus, a biomass substrate for the production of biofuels and chemicals. The particle porosity and the apparent solid packing fraction are quantified, and a simple relationship between volume and mass fractions proposed. The relationship between yield stress and solid concentration was measured with the maximum torque method of Dzuy and Boger (J Rheol 27: 321-349, 1983) for concentrations between 11 and 15 %, just below maximum packing. In the range of concentrations considered, the measured dependence of the yield stress on concentration appears to be significantly stronger than that reported for corn stover, a plant for which reliable data is available: the apparent power law exponent is found to be larger than 10, while the corresponding exponent reported for corn is in the range 4-6. We argue that differences in particle rigidity between corn and Miscanthus may play a role in explaining this result.