Boltonia decurrens is a plant that occurs only on floodplains of the Illinois River and in the area where the Illinois, Missouri and Mississippi Rivers merge. The number of naturally occurring populations has declined over the past 100 years. Boltonia decurrens is on the federal list of threatened species, and is listed as threatened in Illinois and as a species of concern in Missouri. Decline of the species is generally attributed to the loss of suitable habitat due to the alteration of the hydrology of the Illinois River. Without regular flooding, which provides appropriate habitat for the species, a B. decurrens population is replaced within three to five years by a variety of herbaceous species; its survival is dependant upon seedling establishment to renew extant populations and to colonize new sites. In this study, we assessed the effects of achene pattern (aggregated or random) and density (high, medium and low) on the germination of B. decurrens' achenes in the presence of three competitors: Polygonum pensylvanicum, Aster pilosus and A. lateriflorus. Percent germination of B. decurrens' achenes was significantly higher in random compared to aggregate treatments, indicating that there was no positive effect of conspecific aggregation of achenes on germination nor a negative effect of nearby competitor achenes. Boltonia decurrens' germination was highest at low density in the random treatment, compared to the same density in aggregated achenes, thus suggesting the possibility of an autoallelopathic effect. Although B. decurrens' germination was not affected by the presence of competitors, there were significant differences in germination among species: Aster lateriflorus had higher percent germination than P. pensylvanicum, A. pilosus or B. decurrens, and germination of B. decurrens was significantly higher than P. pensylvanicum or A. pilosus. Data from this study will provide insights into the role of competition during germination in floodplain communities and be useful to conservation personnel when they evaluate potential sites for reintroduction or plan control measures to target the species that pose the greatest threat to B. decurrens at a population site.