Three populations of a state-listed species of special concern, Honckenya peploides subsp. robusta, were investigated on the north shore of Niantic Bay, Connecticut, over two growing seasons. Information was developed for a plant conservation plan as part of a railroad bridge reconstruction project. In situ description of this subdioecious subarctic species included the density and distribution of plants growing in sand and shingle habitats. The seasonal timings of emergence, early growth, maturity, senescence, and dormancy were established for the plants at this location. Growth rate was most rapid immediately following emergence in April, slowed during flowering in May, increased again in June, and declined thereafter. A rust fungus not previously known to use H. peploides as an aecial host was documented. All three populations of H. peploides were comprised of male plants—an extreme case of spatial segregation of the sexes. One male plant produced only a few seed pods, indicating that local seed was limited, and thus not an option for propagation. The final conservation plan recommended transplantation of existing plants to a new location.