Herbaceous plants in the family Acanthaceae are widely distributed in coastal belt forests throughout eastern and southern Africa. They typically grow in dense stands that dominate the forest understorey. In addition, some species are monocarpic and have highly synchronized reproduction events followed by mass diebacks. We hypothesise that recruitment opportunities for coastal dune forest trees in South Africa are limited by the presence of Isoglossa woodii, a monocarpic Acanthaceae that flowers every 5-10 years. This species is predicted to compete with tree seedlings and prevent the entry of new recruits into the population. Furthermore, I. woodii is hypothesised to act as an ecological filter that differentially affects the growth and survival of seedlings from different tree species. Over a seven-year period, we assessed I. woodii effects on tree seedling growth in experimental plots with intact I. woodii understorey and with I. woodii removed by cutting. Fencing exclosures were also used to evaluate the effects of vertebrate herbivory on I. woodii and tree seedlings. Our results showed that I. woodii stem density declined as a consequence of self-thinning over the study period, while the height of the herb increased. Overall height of the I. woodii understorey was lower in unfenced plots, indicating that vertebrate herbivory controls its growth. We found fewer tree seedlings in plots with an intact I. woodii understorey and fewer seedlings in unfenced areas. The presence of I. woodii also had effects on tree species diversity and composition, lending support to the argument that this species selectively filters tree seedlings and therefore influences coastal dune forest regeneration.