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Restoration of grassland ecosystems is essential for mitigating global losses of biodiversity and is typically initiated to foster persistent, long‐term increases in biodiversity. Yet, evaluating long‐term impacts of restoration on biodiversity is rare, especially across sites restored using consistent methods. Evaluation of restoration outcomes is particularly important for increasing predictive capacity in restoration ecology to determine the level of restoration effort that is required to achieve both short‐ and long‐term restoration goals. We conducted a multisite study that explicitly compared the impacts of no intervention ("passive" or "natural" recovery), low intervention (seeding native plants), and moderate intervention (seeding native plants and using fire management) at 32 restored prairies differing in the age of restoration (3–23 years). Grasslands with natural recovery have equivalent native plant species richness compared to sites with low and moderate restoration intervention, however, they have significantly lower‐quality vegetation, as measured by Floristic Quality (mean C). We found that managing restored prairies with fire maintains native plant richness over time and is correlated with higher vegetation quality and presence of seeded species. Seed mixes with a high mean C score are positively correlated with plant community quality. However, seed mixes with more species are negatively correlated with the proportion of seeded species present. We found that while the degree of restoration intervention has no effect on the number of native plant species, greater levels of assisted recovery are required to produce restored prairies that resemble high‐quality remnant vegetation and, especially, to maintain these successes over the long term. [ABSTRACT FROM AUTHOR]