Demographic models are increasingly used in the study of invasive weeds. Incorporating nonlinearity in plant demographic rates can change the interpretation of population dynamics and the development of effective control strategies. We estimated functional nonlinearity in demographic rates the invasive weed Alliaria petiolata [(Brassicaceae (M. Bieb) Cavara and Grande)] from natural populations in Michigan and Illinois. Analysis of neighbor density in mapped A. petiolata populations indicates the spatial scale of density dependence and shows that seedling recruitment, survival to the flowering stage, and fecundity are negatively correlated with initial neighbor density. These functions will be used to parameterize future models of A. petiolata and the projected outcomes of developing control programs.