As interest in invasive species management increases, new information with respect to invasive species abundance and distribution in invaded habitats is imperative. One essential type of information is demographic data. When invasive plants colonize a new habitat, their numbers may be low at first, but the population may undergo rapid expansion. We were interested in the effect of intraspecific density on the population dynamics and life history attributes of Alliaria petiolata a Eurasian biennial herb that has become an invasive pest in portions of North America. Thirty plots were established in a mesic second-growth deciduous forest in high, medium and low density patches of A. petiolata rosettes. Demographic data were collected for all A. petiolata cohorts present in the plots from 1996–1998. In June 1998 all first year rosette and second year mature individuals were harvested, dried and weighed. Stage-based population projection matrices were constructed in order to compare demography among plots and years, and models were used to predict trends in future population growth. There were significant differences among demographic parameters as a function of density and year. Survival to flowering in 1998 was greatest for plants in low density plots. These plants were also larger and produced more fruits than plants in either medium or high density plots. Initial differences among plots in plant density diminished and by 1998 there was no significant difference among density treatment plots in number of flowering plants or number of seeds produced. Seed bank formation ensures that, even under less favorable circumstances, A. petiolata can remain at a site for a number of years. Lambda values indicated that the number of plants in plots of each density is increasing, with the greatest increase in low density plots (λ = 1.45). As this study shows, due to abundant seed production, patches of low A. petiolata density in a newly colonized mesic forest can grow rapidly and in a few years form a dense stand.