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Silphium perfoliatum (cup plant) is native to North America and is increasingly used as a bioenergy crop in Germany. Spontaneous occurrences of this species have already been detected in several European countries. To assess the possible risk to biodiversity by spreading of S. perfoliatum, we investigated the competitiveness of this species against the native and highly competitive Urtica dioica over four years in a field experiment in Bayreuth (Germany). S. perfoliatum grew well among U. dioica, although its biomass was strongly reduced by surrounding U. dioica. Projection area, plant height, and reproductive potential were less or similarly reduced by surrounding U. dioica as by the intraspecific competition. Moreover, S. perfoliatum significantly suppressed the growth of the competitive U. dioica. A settlement and establishment of S. perfoliatum in the native flora of Central Europe and a suppression of uncompetitive plant species are therefore conceivable. [ABSTRACT FROM AUTHOR]