The Threatened plant Spalding's catchfly (Silene spaldingii S.Watson) is known to exhibit prolonged dormancy in which individuals survive belowground for one or more years; detection of all aboveground plants is essential for accurate estimates of prolonged dormancy. We conducted 2 long-term demographic studies of Spalding's catchfly in west central Idaho (2002–2013), following 947 plants in permanent plots for 10 consecutive years. To detect all plants emerging aboveground, we monitored twice each growing season—soon after emergence and at flowering—and searched closely for small, inconspicuous stages. Demographic estimates were based on stage-based transition matrix and mark—recapture analyses. Over 99% of aboveground plants were present and detectable at early monitoring; approximately one-third were rosette plants, most of which represented established plants present in previous years. Annual dormancy was approximately 10%. Detectability declined considerably by flowering (time dependent) and affected the small, ephemeral rosette stage disproportionately (stage dependent). At flowering, 48% of the plants present aboveground early in the season disappeared or became undetectable or unidentifiable. If we had monitored only at flowering time, we would have considerably underestimated population size, overestimated dormancy, and missed most recruitments. Early monitoring was critical for detecting all aboveground plants, obtaining unbiased demographic estimates, identifying a major vegetative stage class, demonstrating retrogression from larger stemmed to smaller rosette plants, and identifying rodents as a major threat. Our results have implications for other plants with inconspicuous, ephemeral, or dormant stage classes and those with long growing seasons in harsh environments where detectability of aboveground plant tissue may decrease over the growing season.