Background: Weedy non-native species have long been predicted to be more phenotypically plastic than native species. Question: Are weedy non-native species more plastic than natives? Organisms: Fourteen perennial plant species: Acer platanoides, Acer saccharum, Bromus inermis, Bromus latiglumis, Celastrus orbiculatus, Celastrus scandens, Elymus repens, Elymus trachycaulus, Plantago major, Plantago rugelii, Rosa multiflora, Rosa palustris, Solanum dulcamara, and Solanum carolinense. Field site: Mesic old-field in Dryden, NY (42°27?49?N, 76°26?40?W). Methods: We grew seven pairs of native and non-native plant congeners in the field and tested their responses to reduced competition and the addition of fertilizer. We measured the plasticity of six traits related to growth and leaf palatability (total length, leaf dry mass, maximum relative growth rate, leaf toughness, trichome density, and specific leaf area). Conclusions: Weedy non-native species did not differ consistently from natives in their phenotypic plasticity. Instead, relatedness was a better predictor of plasticity.