Silicon (Si) concentrations in biofuel feedstock crops have a critical role in combustion processes. The purpose of this study was to quantify Si concentrations in plant biomass samples and to evaluate the contributing factors for Si concentrations. We determined total Si concentrations in Miscanthus x giganteus (M. x giganteus) collected from various research trial plots in the eastern U.S. and in Miscanthus spp. and Panicum virgatum, 'Cave-in-Rock' (switchgrass) from an additional eight trial plots established across Illinois. Whole aboveground plant biomass at each site were air-dried and ground. Total Si concentrations in plant samples were determined by dry-ashing plant tissue in a muffle furnace, followed by alkaline fusion and then colorimetric analysis. Average Si concentrations in statewide M. x giganteus plant samples ranged from 0.72% to 1.6% and samples from within Illinois ranged from 0.55% to 2.4%. The overall median value of concentrations in M. x giganteus samples among all sites was 1.08%. The median value in switchgrass samples (1.5%) was 1.4 times higher than that for M. x giganteus. Among six other Miscanthus spp. samples from the Urbana trial plot in Illinois, Si concentrations were about 1/3 that of M. x giganteus. Variation in Si concentrations tended to be associated with temperature and precipitation of the location where the biofuel crops are being grown. We did not find any relationship between soil type and plant Si concentrations. Long-term evaluations of soil mineral concentrations and additional environmental factors are required to better understand the contributing factors for Si concentrations. (C) 2011 Elsevier Ltd. All rights reserved.