Silicon, the second most abundant element in the Earth's crust, affects plant defenses across large geographic scales, which is an interesting yet mostly unexplored issue in non-native species. We investigated silicon-mediated variation in defensive/nutritional traits and palatability to the generalist herbivores Mythimna separata and Locusta migratoria in 16 geographic populations of the invasive grass Spartina alterniflora collected from native and introduced ranges. We found that silicon supplementation generally increased silicon accumulation in Spartina leaves by 76%, and enhanced their physical/chemical defenses (jasmonic acid +28%, flavonoids +12%, toughness +9%) and reduced nutritional quality (carbon content −2%, C:N ratio +5%), ultimately impeding the growth of both generalists (M. separata: −28%, and L. migratoria: −43%). Introduced populations, independent of silicon treatments, had significantly higher silicon content in leaves than native populations. However, silicon supplementation increased more leaf silicon in low-latitude introduced populations (+81%) than in high-latitude introduced populations (+63%). Moreover, the mechanisms of silicon accumulation affecting generalists differed between plant provenances: enhancing quantitative and qualitative chemical defenses in native populations, but improving quantitative chemical or structural defenses in introduced ones. These results suggest a rapid evolution of defense strategies mediated by silicon in introduced populations. Our findings provide insights into how non-native plants utilize silicon to enhance defenses, emphasizing the importance of metalloid defenses in invasion success.