Euthamia graminifolia and Solidago canadensis may be found growing in dense, essentially monospecific, stands. One hypothesis to explain this phenomenon is that both of these goldenrods, as do many related taxa in Asteraceae, release allelochemicals into the surrounding environment. To test this hypothesis, leaf and root/rhizome tissues of both species were screened for allelopathic activity. Aqueous extracts (50 g dried tissue in 2000 ml distilled water) were prepared for each tissue type for each species and a series of dilutions ranging from 0% to 100% of each extract was used for testing radish and lettuce seed germination and root growth. Leaf extracts from both species significantly inhibited seed germination and root growth. Root/rhizome extracts significantly inhibited root growth, but had no effect on seed germination. These results suggest that allelopathy may inhibit the growth and survival of competing species. In addition, both taxa may have a negative effect on yields of nearby crop species.