Information about invasive species is often based primarily on anecdotal evidence, indicating the need for further information. Ranunculus ficaria is an ephemeral riparian plant species that is presumed invasive in the United States, despite the lack of any published information on its impacts. Mechanisms by which R. ficaria may affect native plant species include competition and allelopathy. We examined if R. ficaria negatively affected the growth and reproduction of the native Impatiens capensis and, if so, whether it is by allelopathy, nutrient competition or some combination thereof. We performed a fully-factorial field experiment, manipulating the presence of R. ficaria, nutrients and allelopathy (with the use of activated carbon). The presence of R. ficaria tended to negatively affect life span of I. capensis. In the absence of carbon, R. ficaria significantly decreased seed production, illustrating the negative impact of R. ficaria. In the presence of carbon, there was no effect of R. ficaria, suggesting that carbon may have ameliorated the negative allelopathic effect of R. ficaria. Nutrient competition did not show strong effects. Despite its widespread identification as an invasive species, this is the first study to demonstrate the negative impact of R. ficaria on a native species and the possible role of allelopathy in its success. Further, the negative impacts of this ephemeral species persist well beyond its early growing season, which calls into question previous widespread assumptions about R. ficaria exerting effects primarily on other ephemeral species.