Technical Abstract: A number of studies have suggested a causative link between hybridization and invasion. In this study, we experimentally test for such a link through a greenhouse common garden study with artificially created Backcross 1 (BC1) diffuse knapweed hybrids. In comparison with diffuse knapweed plants of native (European) origin, BC1 plants exhibited increased variance for five of the examined traits, and greater leaf and reproductive shoot production. However, BC1 and European diffuse knapweed did not respond differently to competition. 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. While the increased variance and robustness suggest that hybridization could have contributed to this successful invasion, such differences were not apparent in comparisons between North American diffuse knapweed (including hybrid phenotypes) and European diffuse knapweed. Because North American hybrids resulted from a pre-invasion hybridization event, they may have lost this additional phenotypic variation prior to invasion. We conclude that hybridization may be more likely to contribute to invasion success if the hybridization event happens post- rather than pre-introduction.