Knowledge of plant mating system attributes is essential to the conservation of endangered species. The purpose of this study was to examine the functional consequences and conservation implications of protandry, andromonoecy, and mating patterns in the critically endangered and clonal plant, Ptilimnium nodosum. We conducted intra- and interfloral phenological observations to quantify opportunities for geitonogamy and calculated floral sex ratios of sequentially blooming umbel orders. We also provide information on P. nodosum's mating system. Flowers are self-compatible, but strong intrafloral protandry precludes autonomous selfing (< 1% seed set). Weak interfloral protandry combined with vegetative reproduction, however, provided multiple opportunities for geitonogamy which characteristically decreased with umbel order. Although low, mean seed set was comparable among open pollinated (25%), manually outcrossed (26%), and geitonogamously selfed (21%) pollination treatments, suggesting a mixed-mating system dependent on insect-mediated pollination.The significant decrease in mean seed set among higher umbel orders in open-pollinated plants is likely a consequence of low floral density and lack of pollinators late in the flowering season. Moderate reproductive output and high seed germinability suggest that environmental conditions, not mating system, regulate the distribution and abundance of P. nodosum and high immediate fitness gained by self-compatibility and vegetative reproduction plays a critical role to the species' short-term persistence.