Coastal sage scrub (CSS) is a target for restoration because it provides habitat for numerous special-status species and it has been impacted by urbanization, agriculture and invasion by non-native species. Many restoration designs have neglected the herbaceous understory component of CSS, although it may comprise the majority of vascular plant species in a natural CSS stand. The omission of an understory may promote invasion by non-native plants and reduce overall success. This study investigated the role of native seed addition, non-native species removal, gaps in the shrub canopy, and soil moisture, upon establishment of a native understory. Native biomass increased significantly with seed addition, and the abundance of experimentally seeded native species was positively correlated with soil moisture. Natives were not affected by competition with non-natives or the presence of gaps. Although all seeded native species germinated, only two of seven established successfully, perhaps due to very low rainfall. Non-native species were negatively affected by the addition of native seeds and had greater growth in gaps. We conclude that planting shrubs in a dense configuration to reduce gap size may reduce non-native species abundance in the understory while having little effect on the native understory. Seeding may be all that is required to establish a native understory, and may also be an effective method of suppressing non-native species.