Tamarix is a taxonomically difficult Old World genus that has become naturalized and invasive in the Americas and Australia. We examine the morphology and taxonomic history of 12 putative U.S. invasive Tamarix species, and investigate current invasions using chloroplast and nuclear sequence data. We test molecular phylogenetic hypotheses regarding the relationships of putative invasive taxa, and conclude that there are four invasive entities in the U.S., two of which are T. aphylla and T. parviflora. The sequence data also identify an invasive entity consisting of genetically indistinguishable T. ramosissima and T. chinensis, and another consisting of genetically indistinguishable T. gallica and T. canariensis. There is evidence of introgression between T. ramosissima, T. canariensis, and T. gallica, which is a likely source of confusion in the identification of some Tamarix invasions.

Communicating Editor: Jeff H. Rettig