Genotypic diversity within host-plant populations has been linked to the diversity of associated arthropod communities, but the temporal dynamics of this relationship, along with the underlying mechanisms, are not well understood. In this study, we employed a common garden experiment that manipulated the number of genotypes within patches of Solidago altissima, tall goldenrod, to contain 1, 3, 6 or 12 genotypes m−2 and measured both host-plant and arthropod responses to genotypic diversity throughout an entire growing season. Despite substantial phenological changes in host plants and in the composition of the arthropod community, we detected consistent positive responses of arthropod diversity to host-plant genotypic diversity throughout all but the end of the growing season. Arthropod richness and abundance increased with genotypic diversity by up to∼65%. Furthermore, arthropod responses were non-additive for most of the growing season, with up to 52% more species occurring in mixtures than the number predicted by summing the number of arthropods associated with component genotypes in monoculture. Non-additive arthropod responses were likely driven by concurrent non-additive increases in host-plant aboveground biomass. Qualitative differences among host-plant genotypes were also important early in the season, when specialist herbivores dominated the arthropod community. Neither arthropod diversity nor flower number was associated with genotypic diversity at the end of the growing season, when generalist floral-associated herbivores dominated. Taken together, these results show that focusing on the temporal dynamics in the quantity and quality of co-occurring host-plant genotypes and associated community composition can help uncover the mechanisms that link intraspecific host-plant diversity to the structure of arthropod communities. Furthermore, consistent non-additive effects in genotypically diverse plots may limit the predictability of the arthropod community based solely on the genetic make-up of a host-plant patch.