Simple reintroduction of native plant species to degraded sites without prior removal of invasive species might prevent successful restoration.  This study tested the effect of the invasive Microstegium vimineum (Japanese stiltgrass) on the regeneration of the native herb Ageratina altissima (white snakeroot, formerly Eupatorium rugosum) in a post-agricultural woodland.  Field counts of naturally occurring A. altissima individuals of 12 size classes were performed in plots of high (>80%) M. vimineum cover and in plots of low (<20%) M. vimineum cover. The number of A. altissima individuals under 50cm in height in plots of high M. vimineum cover was significantly lower than in plots with low M. vimineum cover.  The number of A. altissima individuals over 50cm in height was not significantly different between high and low M. vimineum cover plots. We then tested the effect of M. vimineum cover on the survival and reproduction of planted A. altissima individuals.  Twenty A. altissima individuals were planted in each of five field plots with 100% M. vimineum cover and in five plots in which all M. vimineum cover had been manually removed.  Growth and survival of A. altissima were not significantly different between treatments.  However, the proportion of flowering individuals was significantly different; approximately twice as many plants flowered in plots with zero M. vimineum cover than in plots with 100% M. vimineum cover.  This supports the conclusion that M. vimineum decreases the reproductive potential of A. altissima, preventing recruitment and restoration of new individuals in areas of dense M. vimineum cover.