<p>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. We conducted a manipulative experiment, assembling native communities with contrasting functional group compositions and two defoliation regimes, to test the hypothesis that niche overlap determines the invasion success of a cool-season annual grass, <i>Lolium multiflorum</i>, a functional group under-represented in the native flora of the region, and that defoliation modulates such effect. We evaluated invasibility in two ontogenetic stages of the invader by counting seedlings and measuring above-ground biomass at the end of the growing season. We found that phenological overlap between cool-season grasses and the invader throughout its entire life cycle reduced community invasibility, while warm-season grasses reduced invasion through phenological overlap during seedling establishment and through a priority effect by acquiring soil resources before the invader growth. Defoliation intensity promoted seedling establishment but did not affect the invader’s final above-ground biomass, except in the control treatment, suggesting that invasion resistance experienced by the invader simultaneously shifts across ontogenetic stages and depends on the balance of intra- and interspecific competition. Accounting for variable effects of functional group composition and defoliation intensity across the invader’s ontogenetic stages is important for developing targeted management strategies at the appropriate temporal scale.</p>