Pale swallow-wort (Vincetoxicum rossicum) and black swallow-wort (V. nigrum) are herbaceous perennial vines introduced to the Northeastern U.S. from their native ranges in Eastern Europe and the Iberian Peninsula, respectively. Both species invade forest understories and old fields in unmanaged natural areas, and are becoming increasingly problematic in agricultural and horticultural habitats. Published reports have identified these species’ potential allelopathic ability, as well as the fungicidal and cytotoxic effect of secondary metabolites, as possibly playing a significant role in their invasibility and increased competitiveness. Preliminary findings confirm these reports, with a significant decrease in root elongation of large crabgrass (Digitaria sanguinalis) seedlings in particular when grown in the presence of seedlings of both swallow-wort species. Additionally, we are examining inter- and intra-population phenotypic plasticity as well as genetic diversity in populations collected across N.Y. State, in an attempt to identify the contribution of these factors to the swallow-wort species’ invasibility. By comparing standard measures of phenotype and genetic distance between and within swallow-wort populations, we can further characterize the N.Y. populations of both species of swallow-wort. Characterization will allow us to examine the recent spread of both species and their potential species similarities and differences, including reproductive success, which may involve successful hybridization of the two species in increasingly overlapping ranges.