Both competitors and natural enemies can limit plant population growth. However, demographic comparisons of the effects of these interactions on introduced versus co-occurring, related native species are uncommon. We asked: (1) does plant competition, insect herbivory, or their combination reduce population growth rate, log lambda, of the Eurasian thistle Cirsium vulgare sufficiently to explain its limited invasiveness in western tallgrass prairie; and (2) how do the effects of these interactions compare to those for C. altissimum, its co-occurring, synchronously- flowering native congener? We developed integral projection models (IPMs) to estimate log lambda for both species, using parameter estimates from field experiments. Our models predicted that the growth potential (growth rate at minimal competition and herbivory) for the introduced thistle (log lambda = 3.5 (2.5, 4.6)) was twice as large as for the native thistle (log lambda = 1.6 (0.4, 3.1)); however, a high level of competition and ambient insect herbivory together reduced log k to similar values for both thistle species (C. vulgare: log lambda = -1.3 (-2.4, -0.3) vs C. altissimum: log lambda = -0.9 (-1.4, -0.3)). This suggests that the introduced thistle was more affected by competition and insect herbivory. For the introduced thistle, neither competition nor insect herbivory alone led to negative log k. In contrast, for the native thistle, high competition alone also led to negative population growth (log lambda = -0.8, percentile limits do not overlap with zero). Ambient herbivory alone prevented the spread for both thistle species (percentile limits include zero). Overall, the results show that interspecific competition followed by ambient levels of insect herbivory strongly constrained log k for both thistles, limiting C. vulgare invasiveness and C. altissimum abundance. The outcome highlights the importance of synergy between the two biological interactions in limiting plant population growth. Improved understanding of mechanisms limiting log k for weedy plants enhances our ability to predict when biotic resistance will contribute to invasive plant species management.