We present a model of plant sexual allocation that takes account of the timing of male and female function when resources take the form of a rate-limited photosynthetic "income" rather than a resource "pool." The model suggests that investment in male function imposes an opportunity cost with respect to female allocation by diverting resources from further vegetative growth that could have fueled later fruit and seed maturation. The optimal sex allocation predicted by the model is more female biased than the allocation expected if a plant instantaneously divides a common pool of reproductive resources. If fruiting tissue is photosynthetic, the optimal allocation is even more female biased. Temporal factors may help account for the frequently observed female bias in carbon allocation. Although the phenological factors that affect reproduction are certain to vary among species depending on their morphology, physiology, and habitat, such factors may frequently be important, and future studies of plant sex allocation would profit from explicit and detailed attention to the timing of reproductive events.