The evolution of sexual dimorphism depends in part on the additive genetic variance–covariance matrices within females, within males, and across the sexes. We investigated quantitative genetics of floral biomass allocation in females and hermaphrodites of gynodioecious Schiedea adamantis (Caryophyllaceae). The G-matrices within females (Gf), within hermaphrodites (Gm), and between sexes (B) were compared to those for the closely related S. salicaria, which exhibits a lower frequency of females and less-pronounced sexual dimorphism. Additive genetic variation was detected in all measured traits in S. adamantis, with narrow-sense heritability from 0.34–1.0. Female allocation and floral size traits covaried more tightly than did those traits with allocation to stamens. Between-sex genetic correlations were all <1, indicating sex-specific expression of genes. Common principal-components analysis detected differences between Gf and Gm, suggesting potential for further independent evolution of the sexes. The two species of Schiedea differed in Gm and especially so in Gf, with S. adamantis showing greater genetic variation in capsule mass and tighter genetic covariation between female allocation traits and flower size in females. Despite greater sexual dimorphism in S. adamantis, genetic correlations between the two sexes (standardized elements of B) were similar to correlations between sexes in S. salicaria.