Dioecy is an uncommon reproductive strategy with a higher prevalence on oceanic islands. While alleles required for forming solely staminate or pistillate flowers can be transmitted vertically from ancestral species during island radiations, horizontal transfer among plant species through gene flow is also possible. Reproductive barriers may determine which species can interbreed and transmit sex-determining alleles. We investigated these isolating mechanisms in five reproductively diverse pairs of sympatric species of Schiedea that are endemic to the Hawaiian Islands and either show evidence of past hybridization or share a pollinator. We characterized post-pollination, prezygotic barriers such as pollen tube growth, as well as capsule formation, seed production, germination, and survival. We hypothesized that these barriers would increase with phylogenetic distance, and with style length. To directly measure pollen tube growth, we made reciprocal interspecific crosses and intraspecific crosses and counted stained pollen tubes at the middle and bottom of the style at 4 and 24 hr. To measure pollen competition, we pollinated each species with a mix of equal numbers of conspecific and heterospecific pollen grains and scored the proportion of hybrid offspring.  Compared with conspecific pollen tubes, the number of heterospecific pollen tubes at the bottom and middle of the style at 24 hr decreased with phylogenetic distance, but this effect was not evident after 4 hr. In distantly related species, heterospecific pollen tubes had a lower growth rate than conspecific pollen tubes at 24 hr. All measured stages of early acting reproductive isolation increased with phylogenetic distance, with hybrids among closely related species showing heterosis in survival. The proportion of surviving hybrids from mixed pollinations decreased with phylogenetic distance and with maternal style length. Low pre- and post-zygotic reproductive barriers in closely related sympatric Schiedea species could potentially facilitate the transfer of sex-determining alleles across species boundaries during the evolution of this island lineage, although hybridization is impeded in species with different style lengths. Ongoing genomic studies may unearth the evolution and transmission of dioecy in Schiedea.