AbstractSeveral flower traits can affect plant reproductive fitness via pollinator attraction, herbivory defense, and thermal regulation of the pistil. In this study, we focus on thermal regulation of the pistil after flowering. We experimentally investigated the functional significance of the withering corollas that remain attached to the calyx tubes ofOxalis strictaL. We studied thermal regulation of the pistil by removing corollas and comparing the plants with and without corollas, under regulated dark and light periods, with an ambient temperature during the dark period lower than that during the light period. In plants lacking corollas, the pistil temperature was significantly lower than in control plants (with intact corollas) by approximately 2°C. Although fruit set in the corolla‐removed plants was not significantly different from that in control plants, the temperature threshold for 50% fruition in the corolla‐removed plants was significantly higher than that in the controls. Furthermore, the seed number, total seed weight, and single‐grain weight were significantly lower in the corolla‐removed plants than in control plants. The estimated annual number of reproductive cycles (from June to October), total seed number, and total seed weights were also lower in corolla‐removed plants. These findings indicate that the withering corolla remains play a role in thermoregulation of the pistil, and thereby enhance reproductive success. Our study is the first to validate one of the assumed ecological roles of the withering remains of plant corollas.