XVI International Botanical Congess

Bidens polylepis Blake (Asteraceae) frequently dominates sites of disturbance in Fairfax County, Virginia, forming nearly monospecific patches. We wished to examine the possibility that Bidens excludes Oenothera biennis L.(Onagraceae), from colonizing these disturbed areas. Oenothera seeds were sown in five soil treatments and grown in a controlled environment chamber. The soil treatments were (1) soil from a Bidens population, (2) soil from a Bidens population with fertilizer added, (3) soil from an comparable disturbed site without Bidens, (4) soil from an Oenothera population, and (5) greenhouse soil. After four weeks, Oenothera seedlings in soil from a Bidens population (treatment 1) had significantly smaller shoot biomass and seedling height than seedlings in all other soil treatments.