Native woody species often demonstrate inherited latitudinal variation in cold hardiness. How long does it take for such variation to evolve in introduced species? We compared cold hardiness in the native Populus deltoides subsp. monilifera (plains cottonwood) and the introduced Tamarix ramosissima X chinensis (saltcedar). We collected cuttings of 25 individuals of each species from 15 sites in the central US ranging in latitude from 29°N to 48°N. Cuttings were planted in a common garden in Fort Collins, CO, latitude 41°N, on August 16, 2005. About once per month between September 2005 and June 2007, we exposed stem sections of these plants to a range of cold temperatures and determined the killing temperature by freeze-induced electrolyte leakage and direct observation. Although Tamarix was slightly more cold hardy in the early fall and late spring, Populus hardened off more rapidly and deeply. In midwinter, Populus was unharmed by cooling to -70°C, while Tamarix was killed at -30 to -40°C. There is strong inherited latitudinal variation in cold hardiness for both species.  Northern individuals survive colder temperatures earlier in the season than southern individuals.  Analysis of 9 microsatellite DNA loci shows a north-south genetic gradient in Tamarix in the central United States; southern Tamarix is more closely related to T. chinensis and northern Tamarix is more closely related to T. ramosissima.  Hybridization between these two Tamarix species has apparently introduced the genetic variability necessary for rapid (< 150 y) evolution of the latitudinal gradient in cold hardiness.