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The relative importance of allelopathy and resource competition in plant-plant interactions has been vigorously debated but seldom tested. We used activated carbon to manipulate the effects of root exudates of Centaurea maculosa, a noxious weed in much of western North America, on root elongation rates and growth of the native bunchgrass Festuca idahoensis in order to investigate the relative importance of allelopathy in the total interference of Centaurea. In root observation chambers, Festuca root elongation rates decreased to approximate to 50% of the control, beginning 4 days before contacting Centaurea roots in silica sand. However, when activated carbon, which has a high affinity for adsorbing to organic compounds, was added to the sand the effects of Centaurea roots on Festuca root elongation were reduced. In other experiments, Festuca plants were 50% smaller when grown with Centaurea than with conspecifics in pure silica sand. However, Festuca grown with Centaurea in mixtures of sand and activated carbon were 85% larger than Festuca grown with Centaurea in silica sand without carbon. These results suggest that allelopathy accounts for a substantial proportion of the total interference of Centaurea on Festuca, shifting the balance of competition in favor of Centaurea. However, Centaurea outperformed Festuca even in the presence of activated carbon, demonstrating the importance of the combined roles of resource competition and allelopathy.