Erigeron lemmonii A. Gray is extremely rare, but not endangered. There are fewer than 1000 individuals, but they compose an apparently stable, heterozygotic, diploid (2n = 18) population. The plants are perennial and endemic to one small location (about 0.5 km²) in Scheelite Canyon, Huachuca Mountains (Cochise County), Fort Huachuca Army Base, part of the Sky Island region, Arizona. Erigeron lemmonii grows from crevices in cliffs, ledges, and rock faces where it can form mats by spreading vegetatively from the roots as they proliferate, especially in microsites where soil accumulates. Our goals were to (1) investigate floral anatomy for reproductive structures (inflorescences, florets, and pollen), (2) record flowering phenology, and (3) determine the species' breeding and mating system. Inflorescence anatomy and development are typical for Erigeron with ray (pistillate) and disc (bisexual) florets. Stigmas of the disc florets do not fully exsert and reflex, though they do present and expose the small (17-m-diameter), spiny, and mostly viable (85%) pollen by pushing it from the anthers below to the florets' entrances during anthesis. Such characteristics are appropriate for entomophily. Erigeron lemmonii has an extended (5–6 month) bimodal (May and August–September) sexual reproductive period. Each capitulum lives about 3 weeks (bud to cypsela-set) and each floret lasts about 5 days. Despite the small population, E. lemmonii is obligately xenogamous and genetically diverse, reliant on insect pollinators to produce wind-dispersed cypselae. It also reproduces clonally. We compare our results to the findings of others on congeners with similar growth habits. Our findings relate to management and conservation of this plant because, to persist, its habitat must support the diversity of pollinating insects on which it is reliant for sexual reproduction.