Sexual dimorphism in dioecious plants often occurs as a consequence of the different resource requirements of females and males, especially during reproduction. The contrasting reproductive roles of the sexes can influence the phenology of growth, plant size and flowering time, with implications for the intensity of competitive interactions within and between the sexes. Here, we investigate the influence of contrasting nutrient regimes and intra-sexual and inter-sexual competition on the expression of sexual dimorphism in life-history traits and biomass allocation throughout the life cycle of the dioecious annual Rumex hastatulus Baldw. (Polygonaceae). Development of a sex-specific marker enabled us to quantify the influence of competition on sex-specific differences in mortality and vegetative traits. We were particularly interested in determining whether the overall performance of the sexes might differ between the two forms of intraspecific competition, potentially providing evidence for sexual specialization in resource acquisition and niche differentiation. Our results indicated that although patterns of sexual dimorphism were dynamic they were largely insensitive to nutrient conditions. We found that intra-sexual competition was more severe than inter-sexual competition differentially affecting mortality and most traits during the vegetative and particularly the reproductive stage of the life history. Female traits values generally increased more under inter-sexual than intra-sexual competition in comparison to males. Our findings are consistent with temporal niche differentiation resulting from sexual specialization for different resource requirements and provide evidence for the 'Jack Sprat effect'.