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Seasonal variation in the accumulation and partitioning of nitrogen, phosphorus and potassium was determined in both the above-ground and below-ground dry matter of the potential energy crops Miscanthus × giganteus and Spartina cynosuroides. It is desirable from both economic and environmental perspectives that such crops should exhibit a high nutrient use efficiency and minimal nutrient losses to the environment. The N, P and K concentrations in the above-ground dry matter, at final harvest, were 5.0, 0.6 and 12.0 mg g−1 respectively in M. × giganteus and 3.0, 0.4 and 1.0 mg g−1 in S. cynosuroides. Both species exhibited the high N-use efficiency expected of C4 plants. Nitrate leaching was negligible. At the end of the growing season, nutrients were translocated to the rhizomes and, in the case of M. × giganteus, recycled to the soil in shed leaves. Consequently the nutrient content of the crop offtake was low. It was calculated that the N, P and K requirements of a M. × giganteus crop producing an above-ground harvest of 1.5 kg m−2 dry matter would be 9.2, 1.3 and 20.4 g m−2 respectively. The corresponding nutrient requirements for S. cynosuroides would be 7.5, 1.7 and 8.8 g m−2. Except for the K requirement of M. × giganteus, the N, P and K demands of both species were less than those of typical graminaceous crops, including maize.