This designation is used for indigenous species of special concern because of characteristics that make the species particularly sensitive to human activities and natural events. Several factors contribute to the plant's rarity including human encroachment into wetland habitats, displacement by aggressive invasive species such as Lythrum salicaria , and insect seed predation. Sidalcea hendersonii is gynodioecious. This is a mating system whereby populations consist of separate coexisting hermaphroditic individuals and female individuals. Hermaphroditic Sidalcea hendersonii flowers contain both functional anthers and ovaries, are self-compatible but protandrous.
Female flowers have functional ovaries but nonfunctional stamens.
We investigated the genetic and ecological factors contributing to the maintenance of females in British Columbian populations of Sidalcea hendersonii - see Marshall 1998. Crossing experiments indicated that male sterility is controlled by a dominant nuclear allele. High frequencies of female plants in the majority of populations surveyed, in combination with the nuclear determination of sex, elevates the theoretical requirements for female fitness in this species. Females did have higher fitness, producing more surviving offspring than hermaphrodite plants in an experimental population. However, no inherent fitness advantages were evident in natural populations where females and hermaphrodites did not differ in viable seed production.