A flexible mating system may enable self‐compatible plants to prevent self‐fertilization under certain circumstances. Campanulastrum americanum is a self‐compatible, protandrous herb. Although within‐plant pollen transfer is likely and self pollen can produce a full set of seeds, selfing in natural populations is rare. We investigated explanations for the high outcrossing rate. One possibility was that self pollen contacts the stigma rarely. We surveyed floral displays in nature to gauge the potential for within‐plant pollen transfer. Floral displays in nature are large enough to enable frequent geitonogamous pollinator visits and subsequent self‐pollination. Alternatively, the high outcrossing rate could result from cryptic self‐incompatibility, a mechanism that favors outcross pollen over self pollen. To determine whether outcross pollen has a seed‐siring advantage over self pollen, we pollinated maternal plants with equal mixtures of self and outcross pollen. Genotyping the offspring revealed that outc...