According to the original optimal reproductive allocation theory, plants should shift from vegetative growth to reproductive allocation abruptly and completely. Some plants do this, and it is also considered a good strategy for crop plants to maximize yield, but most plants shift gradually. Modified versions of the theory predict such a gradual transition from growth to reproduction. We hypothesize that kin selection can also alter the predictions of optimal allocation theory. We investigated the theoretical implications of both positive and negative kin selection on the timing of plant reproductive development using mathematical models. Under reasonable assumptions of costs and benefits, plants under kin selection are more likely to shift from growth to reproduction in an abrupt way when the initial value of the ratio between reproductive and vegetative biomass is high. Supported by empirical observations, our theoretical predictions have important implications in linking life history and energy allocation as well as for improving yields in agriculture.