Functional group composition can positively influence plant community invasibility through niche vacancy when the resident community does not sufficiently overlap with the invaders. Phenology plays a key role in determining functional group composition, as it defines the temporal scale on which niche overlap occurs. Defoliation may regulate the effect of functional group composition by releasing resources, resulting in interactive effects. In a manipulative experiment, we assembled native communities with different functional group compositions: perennial warm- and cool-season grasses growing either separately or together. These communities were subjected to two defoliation regimes to test the hypothesis that niche overlap determines the invasion success of Lolium multiflorum, an annual grass belonging to a functional group under-represented in the native flora of the region. We also hypothesized that higher defoliation intensity would increase community invasibility, promoting invasion and reducing the differences observed between communities under lower defoliation intensity. We evaluated this in two ontogenetic stages of the invader by counting seedlings and measuring above-ground biomass at the end of the growing season. The evidence shows that community invasibility is mediated by functional group composition through phenological overlap, although priority effects were also important. Defoliation intensity promoted seedling establishment similarly across all communities but did not affect the resulting above-ground biomass at the end of the growing season. Instead, it negatively impacted the performance of the invader growing in monoculture. Considering the variable effects on each ontogenetic stage is important for developing targeted management strategies at the appropriate temporal scale.