Technical Abstract: Canada thistle is difficult to manage in farming systems with reduced reliance on herbicides, including organic and low-external input systems. Previous field studies found that defoliation or sudangrass interference suppressed Canada thistle. Our objective was to understand the factors causing suppression of Canada thistle. Greenhouse studies found increased defoliation frequency (up to 4 defoliations) decreased Canada thistle shoot height, shoot and root mass, and root-to-shoot ratio. Plants with larger root mass had greater shoot mass and shoot number (r = 0.87 and 0.73, respectively), indicating a probable interdependence of root size (carbohydrate reserves), bud density and subsequent shoot growth. Canada thistle shoot dry mass was 38.7, 2.76, and 0.39 g/pot in the defoliation only, sudangrass interference only, and defoliation + interference + surface mulch treatments, respectively. Sudangrass interference was effective by itself in suppressing thistle growth; combining interference with defoliation did not further reduce growth (i.e. 2.76 and 2.83 g/pot, respectively) In an experiment minimizing interspecific competition, we found no evidence of sudangrass having a phytotoxic effect on Canada thistle. Sudangrass competition or frequent shoot removal minimizes the photosynthetic capacity of Canada thistle, suppressing growth of troublesome perennial weed.