Flower-visiting insects are important crop pollinators, but their populations in agricultural landscapes are declining. One reason is the decreasing quantity and quality of floral resources. Deterioration of the situation caused by an increasing production of energy crops like maize, a pollen-only resource, may be mitigated by alternative crops such as Silphium perfoliatum L., which produces pollen and nectar over a long-lasting flowering period (July–September). The aim of this study was to assess the floral resources of S. perfoliatum. We calculated pollen grains and nectar sugar mass per inflorescence as well as inflorescences per plant using experimental plots and commercial fields in Germany. In addition, we analyzed free and protein-bound amino acids in pollen as well as sugars and free amino acids in nectar. The amount of pollen and nectar sugar per inflorescence decreased with increasing stem branching. The production of pollen and nectar sugar, however, was greatest in the second half of August due to the high number of inflorescences per plant at that stage. About half of the farmers harvested S. perfoliatum before the end of flowering to gain higher methane yields. With an early harvest around the beginning of September about one sixth of total pollen and one fifth of total nectar sugar production were lost to pollinators. Pollen and nectar showed high amounts of certain essential amino acids, particularly histidine, but total amino acids were low in concentration. Hence, S. perfoliatum should be complemented with different bee fodder plants to ensure a well-balanced diet.