Allelopathy is one explanation for non-native plant invasion, but native plants also can be allelopathic. We tested the allelopathic potentials of the non-native, invasive grass Microstegium vimineum (Japanese Stilt-grass) and the native herb Ageratina altissima (White Snakeroot), which both can dominate the herb layer in central New Jersey forests. Aqueous extracts from roots and shoots of both species negatively affected the speed of germination and the percent germination of Lettuce and Radish seeds in Petri dishes, and White Snakeroot shoot extract had the strongest effect. In a factorial experiment in pots of forest soil that combined extract treatments with activated carbon addition (to manipulate allelochemicals) and soil sterilization (to investigate indirect allelopathic effects via the soil microflora), Lettuce and Radish seedling establishment was reduced by extracts, especially from the native White Snakeroot. However, growth of surviving seedlings was unaffected by the extracts or their interactions with carbon or soil sterilization. These results show that a native species had stronger allelopathic potential than an aggressive, non-native invader from the same forest and that allelopathy was effective on the earliest developmental stages of the target plant species. In addition, activated carbon and sterilization interacted to directly influence plant growth. Growth was greater in sterilized than unsterilized soils, but only when carbon was added, suggesting caution in using these techniques in allelopathy studies.