Remains of ancient agricultural activity, including many plots of abandoned farmland, can be found in most national parks in the southern portion of the province of Québec (Canada). Abandoned farmland sites in the Frontenac National Park (situated at the junction of the Cantons-de-l’Est and Chaudière-Appalaches regions) were studied to identify the factors driving their floristic composition. At the end of the summer of 2012, vegetation was sampled in 165 plots on 40 sites. Indicator species and redundancy analyses were performed. A total of 209 species were identified, of which 141 were exotic. This corresponds to more than half of all the plant species present in the entire park. Indicator species of recently abandoned sites (<25 years) were all herbaceous exotics, while those of long abandoned sites (≥25 years) were mostly native grasses. Floristic composition was mainly explained by the time since cessation of agricultural activities and soil pH. It is recommended that much of the abandoned farmland be encouraged to return to forest, but that some be kept in an herbaceous stage to maintain the rich flora of the area.