Sanders, S. and J. B. McGraw (Department of Biology, West Virginia University, Morgantown, WV 26505). Population differentiation of a threatened plant: variation in response to local environment and implications for restoration. J. Torrey Bot. Soc. 132: 561–572. 2005.—Intraspecific genetic variation in plants is frequently associated with adaptation to local environments. Detection of ecotypic differentiation can promote an understanding of a species' distribution and be an important consideration in restoration efforts. We performed a classical reciprocal transplant study using four natural populations of Hydrastis canadensis to test for localized adaptation. Hydrastis canadensis exhibited plasticity in response to site quality variation, and at the site level there was no evidence of local genetic adaptation or differential performance of plants derived from distinct source populations. However, the four H. canadensis sources responded differentially to microsites within the transplant site. A second study examined the importance of including multiple sources when introducing new populations for restoration purposes by comparing H. canadensis performance in populations that were mixtures of three natural sources versus populations that were monocultures of each natural source. We found that populations established with plant material from single sources performed better than those established with multiple sources. Collectively, our findings indicate that restoration efforts should involve multiple sources dispersed over multiple sites as a bet-hedging strategy to increase the likelihood of suitable source-site compatibility. However, within a given restoration site, these sources should be spatially separated, such that numerous populations are introduced, each comprised of only a single source.