Vulpia (Vulpia spp.) and barley grass (Hordeum murinum ssp. leporinum Link) are invasive annual grass weeds of agricultural and natural ecosystems. Understanding how gap dynamics influence their growth and survival can improve management of these species in permanent pastures and minimise their negative effect on these ecosystems. We investigated the effects of gap size, timing of gap appearance, grazing method (continuous or rotational grazing), and species sown in the gap (vulpia and/or barley grass) on vulpia and barley grass establishment, growth, and survival. Total establishment of vulpia and barley grass increased with gap size, but grazing method and inter-specific seedling competition had little effect on their establishment. Survival and tiller production were generally greater in larger gaps, under continuous grazing, and for barley grass than for vulpia, but were not affected by sowing the weed species in mixtures. Timing of gap appearance had little effect on survival or tiller production. Individual plant biomass of the weed species was generally greater in larger gaps and for barley grass, but was not affected by grazing method or timing of gap appearance. There was little difference between grazing method or gap size in surface soil moisture and temperature. Photosynthetically active radiation recorded in gaps at the soil surface in rotationally grazed pastures was less than 20% of that in continuously grazed pastures and less in medium than in large gaps. Pasture height was also less under continuous grazing. From this study we concluded that these annual grass weeds can readily establish throughout autumn and winter in gaps, regardless of the grazing regime imposed. However, minimising bare ground during this time can significantly reduce their establishment. Post-establishment, rotational grazing was more effective than continuous grazing in suppressing their growth and survival throughout the growing season, most likely because of greater vegetation height, perennial grass content, and competition for light under rotational grazing. If annual grass weed populations are to be suppressed, it is necessary to use appropriate grazing management throughout the growing season, rather than focussing on the seedling establishment phase alone.