Low species diversity accompanied high populations of Canada thistle [Cirsium arvense (L.) Scop.]. Two perennial grasses and two rushes grew with Canada thistle; annual plants did not. Canada thistle litter, ground roots, and ground foliage added to soil in greenhouse bio-assay tests reduced growth of redroot pigweed (Amaranthus retroflexus L.) and green foxtail [Setaria viridis (L.) Beauv.] more than cucumber (Cucumis sativis L.) or barley (Hordeum vulgare L.). The addition of nutrients did not mask the toxic effect. Ethanolic extracts of Canada thistle roots and foliage were similar in their ability to reduce radicle growth of barley, cucumber, green foxtail, and redroot pigweed in petri dish studies. There was no significant difference between water controls and controls adjusted to the average pH and osmotic potential of the extracts. Germination of barley and cucumber seed was not affected. Comparisons between ethanolic extractions and soil incorporation of plant residues with presumably non-allelopathic plants revealed that cucumber and barley extracts reduced redroot pigweed radicle growth whereas barley and green foxtail extracts increased green foxtail radicle growth. These effects were not observed when these plant residues were mixed in soil.