<p>Diversity within species is evolutionarily important, but precisely how and under what conditions it affects ecological interactions is still coming into focus. Addressing this requires us to investigate the effects of intraspecific diversity across a wide variety of contexts, including in species types that are underrepresented in prior research. In this study, we sought to determine the separate and interactive effects of genotypic diversity and nutrient addition on <i>Solidago sempervirens</i> (seaside goldenrod), a common but not dominant salt marsh plant species. Using a trait survey, feeding preference experiment, and common garden experiment, we asked 1) whether traits related to growth strategy and susceptibility to herbivory differ among <i>S. sempervirens</i> genotypes and 2) how soil nutrients and genotypic diversity of intraspecific competitors affect plant growth strategy and susceptibility to herbivory. We found that traits related to growth strategy varied among genotypes in the lab and that genotypes adopted different growth strategies. In the common garden, genotypic diversity also affected plant growth strategy; plants grew more leaves in genotypic polyculture and more shoots in genotypic monoculture, even as total aboveground biomass did not differ. This was likely because the optimal competitive strategy differs when competitors’ traits are similar versus dissimilar. Diversity did not affect the amount of herbivory that plants experienced in the common garden despite differences among genotypes in the feeding preference experiment. Nutrient addition led to more shoots and greater herbivory, but there were no interactions between nutrient addition and genotypic diversity. The lack of interaction indicates that in this situation, the effect of genotypic diversity does not vary with nutritional context. Whether nutritional context alters the impact of genotypic diversity may depend on the magnitude of nutrient stress or characteristics of the focal species.</p>