AbstractPlasticity in growth, reproductive energy allocation (RA), and reproductive output were investigated inHelianthus annuusL. cultivar.Russia(Compositae) grown under varying densities and soil nitrogen levels. The role and behaviour of pollinators in seed production was also examined.Exceedingly marked plastic responses were detected in individual biomass, the patterns of resource allocation to total reproductive structures (RA) and also to propagules, fecundity, reproductive outputs, and propagule size and weight under changing densities and soil nitrogen levels. Plants cultivated at higher densities exhibited proportionately lower individual biomass, lower RA, lower fecundity, lower seed output, and smaller seed size in response to increasing density and decreasing soil nitrogen levels. However, differences due to different N‐levels were not as great as those to changing density.One of the most significant findings was that seed production under limited resource availability, i.e., lack of ample solar radiation and soil nutrients, due to strong interference at higher density plots, is exceedinglycostlty, This was most clearly demonstrated by a very sharp increase in relative energy partitioning to a single propagule in response to the increased density and decreasing nitrogen levels, the relative energy cost to a single achcne (RA) increasing from one to twenty‐fourfold. Reproduction was also affected by pollinator‐limitations, and seed size showed a marked position e