Technical Abstract: The “novel weapons” hypothesis states that some invasive weed species owe part of their success as invaders to allelopathy mediated by allelochemicals that are new to the native species. Presumably, no resistance has evolved among the native species to this new allelochemical (i.e. the novel weapon). In their native habitat, however, the plants that co-evolved with these invasive species have theoretically evolved defenses which obviate the allelochemical advantage. Previous studies have claimed that catechin is such a novel weapon of the invasive species spotted knapweed (Centaurea stoebe = C. maculosa). These studies indicated that (–)-catechin is more phytotoxic than (+)-catechin. Other studies have not found sufficient (–)-catechin in field soils to support this theory. We report that (–)-catechin and (+)-catechin are essentially equally, but poorly phytotoxic to a variety of plant species in bioassays without soil. In a dose/response experiment with Montana soils, we found the lowest dose for a growth reduction of two native Montana grasses (Koeleria macrantha and Festuca idahoensis) by a racemic mixture of (±)-catechin ranged from 25 to 50 mM, concentrations orders of magnitude higher than expected in nature. Autoclaving the soil before adding the catechin did not affect the activity of catechin. Our findings suggest that catechin is not a novel weapon of spotted knapweed, and that other factors or allelochemical(s) must be found to explain the success of this species as an invader in North America.