Like many open habitats, sustainability of oak savannas in midwestern North America depends on periodic disturbances such as fires to curtail encroachment by tall woody plants. An uncertainty in restoring and sustaining oak savannas is how frequently fires must occur to maintain the groundlayer plant diversity savannas are known for and what levels of tree canopy and sapling layer encroachment trigger shifts in groundlayers. In an oak savanna undergoing restoration in northwestern Ohio, we examined how groundlayers changed with temporal variation in tree (≥10 cm in diameter) and sapling (<10 cm) layers and prescribed fires by remeasuring permanent plots up to 17 times from 1988 to 2021. Groundlayer cover was maximized when tree basal area was <13 m²/ha (35% tree canopy cover), there were fewer than 100 trees/ha, and fire had occurred since the previous growing season. Illustrating attrition in groundlayers above these thresholds, two-thirds of savanna groundlayer cover disappeared when tree density exceeded 100/ha and over 2 y passed without fires. Through savanna species persisting at low cover, species richness endured longer between fires (4+ years), doubled during periods with at least one fire in 3 y, and increased by a third when saplings were sparse (<80 stems/ha). Savanna groundlayers during the 34 y study fluctuated with intermittent increases and decreases associated with dynamics in trees, saplings, and time since fire. Although they require a major commitment because their benefits are so transient, frequent, low-severity prescribed fires appear capable of sustaining savanna groundlayer diversity indefinitely under prevailing conditions.