Stabilizing selection is expected to rapidly erode additive genetic variation, but considerable genetic variance persists in many traits linked to fitness. Theoretical work has demonstrated that fluctuating selection combined with overlapping generations may allow maintenance of additive genetic variation, but evidence for this mechanism in nature is scarce despite many species having overlapping generations, and most environments being variable. Using data from long term studies on Carlina vulgaris and Carduus nutans, we explore the role of temporal stochasticity in maintaining genetic variation in the flowering strategy using the adaptive dynamics framework, incorporating into our analysis the role of parameter uncertainty through bootstrapping. We show that coexistence of alternative flowering strategies can occur in a protected polymorphism through relative non-linearity of competition. However, coexistence is not evolutionarily stable and we discuss other aspects of these species that may prevent loss of genetic variance. We briefly discuss the role of relative non-linearity of competition in maintaining genetic variance in other life history traits.