1. Species of the dioecious South African plant genus Leucadendron vary strikingly in their degree of sexual dimorphism, with males of dimorphic species typically possessing more and thinner branches and smaller leaves than the corresponding females.2. Many Leucadendron species are serotinous, with females of serotinous species maintaining seeds in transpiring cones for a number of years. Since cones must be kept alive, serotinous females presumably incur higher costs in terms of water loss than non-serotinous females and, particularly, males. Given that branching architecture is known to affect water conductance, with less resistance in less highly branched plants, we predicted a positive correlation between the degree of serotiny in a species and sexual dimorphism in branching.3. We tested the effect of serotiny and several other factors on the degree of sexual dimorphism in Leucadendron by conducting a comparative analysis of variation across 49 species using phylogenetic regression.4. There was a strong positive association between the degree of serotiny and sexual dimorphism. This association is consistent with the idea that the costs of maintaining cones may select females to be less highly branched than males.5. Synthesis. Serotiny is a strategy of maternal care that contributes to offspring survival after seed maturation. We found that, in Leucadendron, the evolution of increased serotiny corresponds with the evolution of more marked sexual dimorphism. This study exposes a hitherto unidentified association between sexual dimorphism and maternal care in plants and points to a possible additional reproductive burden carried by serotinous plants compared with those that release their seeds at maturity.