Abstract Premise Declines in pollinator populations can reduce pollination services to plants, resulting in lower seed production. In response to these reductions, plants could increase the probability of pollinator visitation by plastically extending floral longevity. However, whether extended floral longevity increases seed production as pollinators decline depends on the limits to and costs of plasticity in longevity, both of which could be affected by drought stress. Methods To test whether drought stress affects the limits to and costs of plasticity in floral longevity in response to pollinator decline, we exposed Lobelia siphilitica to droughted and well‐watered treatments and measured floral longevity and the number of seeds produced by flowers pollinated on day 1 vs. day 5 of the female phase. If floral longevity is shorter in the droughted treatment, then drought stress could limit the expression of extended longevity. If delaying pollination until day 5 reduces seed production more in the droughted treatment, then drought stress could increase the cost of extended longevity. Results The droughted treatment reduced floral longevity by ~18% but did not affect the number of seeds produced by flowers pollinated on day 1 vs. day 5. Instead, delaying pollination until day 5 reduced the number of seeds by ~24% in both the droughted and well‐watered treatments. Conclusions Drought stress does not affect the cost of plasticity in floral longevity but could limit the expression of extended longevity. Consequently, whether extended floral longevity could increase seed production as pollinators decline may depend on human‐induced changes in precipitation.