A number of studies have suggested a link between hybridization and invasion. In this study, we experimentally test the potential for hybridization to influence invasion through a greenhouse common garden study. Diffuse knapweed (Centaurea diffusa Lam.) was introduced to North America with admixture from spotted knapweed (Centaurea stoebe subsp. stoebe L.). Comparisons between North American diffuse knapweed (including hybrid phenotypes) and native (European) diffuse knapweed in a common garden did not reveal enhanced performance or increased phenotypic variance, suggesting that pre-introduction hybridization or, more generally, post-introduction evolutionary change, has not significantly contributed to the invasion of diffuse knapweed. In contrast, early generation hybrids [artificially created Back Cross 1 (BC1) plants] exhibited increased variance for eight of the examined traits, and greater leaf and reproductive shoot production when compared to North American diffuse knapweed. Individual BC1 lines differed for several traits, suggesting the importance of the cross for drawing conclusions from such comparisons. When compared to the parental species (diffuse and spotted knapweed), the BC1 plants were not transgressive for any of the measured traits. Overall, these findings suggest that if diploid spotted knapweed is introduced to North America, interspecific hybridization has the potential to result in even more aggressive invaders.