Alliaria petiolata (garlic mustard) is a European biennial herb that was probably introduced to North America in the 1800's. Since its introduction, it has spread throughout the northern U.S. and southern Canada and is now considered by most land managers and researchers to be a noxious invasive plant. The first herbarium record of A. petiolata appears to be from around the port of New York with a subsequent westward migration. Although intraspecific morphological variation appears to be low within some North American populations, researchers have documented considerable phenotypic plasticiy in silique size, mature plant size, and length of seed dormancy among populations growing under different environmental conditions. The purpose of this study was to investigate the relationship between morphological and genetic variation among disjunct populations of A. petiolata across its North American range. Seed was collected from populations in seven U.S. states and one Canadian province and sown in a common garden in Athens, Ohio. Seed germination percentages, germination rates, and rosette biomass were determined. Seed germination percentages and rates were also examined under laboratory conditions. Leaf tissue was collected from study population plants and intra- and inter-population level genetic variation was examined using inter-simple sequence repeat primers (ISSRs). These experiments showed low levels of genetic diversity both within and among A. petiolata populations, but a relatively high degree of morophological variability. Seed size and germination rates differed significantly among populations. Canada and Indiana populations had the lowest germination percentages (39% and 27%, respectively), but produced the largest rosettes. These data indicate that phenotypic plasticity, rather than substantial genetic variation owing to multiple invasions, may be important in allowing A. petiolata to readily adapt to its environment and invade such diverse habitats in North America.

Key words: Alliaria petiolata, Brassicaceae, genetic variation, ISSR, morphological variation, plasticity