A replacement series experiment was conducted in the field to quantify the interaction between Canada thistle and buckwheat, and to determine if allelopathy was the mechanism of interference. Plant biomass data indicated that buckwheat only responded to intraspecific competition and not interspecific competition from Canada thistle. Canada thistle responded to interspecific competition from buckwheat plants but not to intraspecific competition. The absence of one of the interspecific competition components (no measurable effect of Canada thistle density on buckwheat) indicates that the value for niche differentiation is probably greater than unity, signifying that there is no mutual antagonism between buckwheat and Canada thistle. Because allelopathy is one mechanism that causes mutual inhibition, data do not support the hypothesis that buckwheat interferes with Canada thistle by allelopathy.
