ABSTRACT Most typical rock outcrop plants of the eastern United States occur either on calcareous or on non-calcareous outcrops, but not both. Often this is because their growth is inhibited in soil from the non-native substrate, as shown in this study for the granite outcrop endemic sedge Cyperus granitophilus when grown on limestone soil. For those rock outcrop species that can exploit both calcareous and non-calcareous substrates, it might be expected that they would do so by substrate specialization, with each population growing better on its native substrate than the other substrate. However, in most previously tested species, populations from both substrates grow well on the same substrate and both are inhibited on the second substrate. Populations found on the second substrate, though inhibited, are nevertheless able to maintain good health and grow sufficiently to maintain themselves there. In this study we show that the widespread sedge Cyperus aristatus (5C. inflexus), a regular component of limestone, sandstone, granite, chert, and serpentine rock outcrop communities in the eastern United States, exploits both calcareous and non-calcareous substrates by substrate specialization, something previously shown in only one other eastern United States rock outcrop species.