Rare plant species can be at risk of hybridization, reduced genetic variation, and genetic assimilation when a numerically abundant congener co-occurs in or invades their habitat. We investigated hybridization between a species in decline, Asclepias purpurascens L., and its common and widespread congener, Asclepias syriaca L. A total of 40 morphological traits were measured on 60 flowering plants from a mixed population in Connecticut. Cluster and principal component analyses identified two distinct clusters of parental species taxa and a third cluster representing putative hybrids. Although leaf traits of putative hybrids were more similar to A. syriaca, floral traits and the morphological space on the principal component analysis were more similar to A. purpurascens. This suggests that the population contains a mix of F1 and advanced generation hybrids with the possibility of introgression into A. purpurascens. Although putative hybrids are intermediate for most traits, pollen counts reveal reduced fertility of presumed hybrid that might influence mating behaviors and increase the likelihood of backcrossing with A. purpurascens. We suggest a combination of morphological traits that better identify the two parental species as well as hybrids in the field. Conservationists should consider options of managing A. syriaca where A. purpurascens needs protection from competition to prevent genetic assimilation of the latter.