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Biological invasions change native plant communities, but theory predicting whether introductions create naturalized or invasive species is lacking. Focusing on either plant traits or interactions of introduced plants with native biota creates unreliable results, and improvements may require integration of trait- and interaction-based approaches. To assess the importance of plant traits and herbivory on invasiveness, we incorporated herbivore effects in comparisons of growth and phenology of invasive Phragmites australis and its native congener P. australis subsp. americanus. Our results were influenced by venue (field or common garden), with extended life span and optimized leaf-age structure of introduced P. australis indicating greater potential for resource capture. Attack by introduced gallflies affected expression of plant traits, but we found no consistent effect of aphid attack. Origin did not affect leaf emergence or stem height, but preferential gallfly attack stunted native P. australis and delayed senescence. Greater resource capture and lower attack by nonnative herbivores could give introduced P. australis an advantage over the native subspecies. Our results demonstrating the importance of plant traits as well as their modification by interactions with natural enemies questions whether the outcome of plant introductions can be predicted.