| Summary: | <i>Alliaria petiolata</i> and <i>Hesperis matronalis</i> are wide-ranging non-native species in North America. <i>Ageratina altissima</i> is native to North America but has become a concern as an invasive species in Asia. A replacement series experiment was established to quantify the competitive interactions between these three species and to rank their relative competitiveness with each other. We assessed leaf count, chlorophyll content, and aboveground biomass with comparisons between replacement series mixtures and competition species. Overall leaf count and aboveground biomass were greatest in <i>A. altissima</i> and chlorophyll content was lowest in <i>A. petiolata</i>. Chlorophyll content and aboveground biomass were lower for <i>A. altissima</i> in competition with <i>A. petiolata</i> compared to <i>H. matronalis</i>. Leaf count for <i>A. petiolata</i> was lower in competition with <i>A. altissima</i> compared to <i>H. matronalis</i>. Aboveground biomass for <i>H. matronalis</i> was lower in competition regardless of the species compared to monoculture. There were also negative trends in biomass for <i>A. petiolata</i> in competition with increasing neighbors. However, for <i>A. altissima</i>, the negative trend in biomass was with <i>A. petiolata</i>, <i>H. matronalis</i> did not negatively affect <i>A. altissima</i> biomass. Our rank order of competitiveness was <i>A. altissima</i> > <i>A. petiolata</i> >> <i>H. matronalis</i>.
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