Invasive plants commonly establish and spread along roadsides, and understanding the mechanisms underlying this pattern is essential for risk assessment and effective management. Stinkwort, Dittrichia graveolens (L.) Greuter, a recent California invader, primarily inhabits roadsides but is beginning to spread into nearby plant communities. We tested whether D. graveolens grows best along roadsides due to a preference for soil conditions or is limited to these disturbed environments by plant competition. Lab and greenhouse experiments showed no germination advantage in engineered fill (simulating roadside soils), and D. graveolens grew slightly better in field topsoil. Competition trials with two annual grasses (soft brome, Bromus hordeaceus (L.) and Italian rye grass, Festuca perennis (L.) Columbus & J.P. Sm.) showed strong growth suppression of D. graveolens by both grasses, but competition was stronger in field topsoil than engineered fill. Engineered fill limited growth for all species, suggesting that roadside soils may provide a refuge from competition. In two years of field experiments, we examined mechanisms of competition by comparing responses to (1) removal of thatch, aboveground biomass, and above- and belowground biomass, (2) shading, and (3) water and nutrient addition and protection from belowground competition via trenching. Belowground competition in particular strongly affected D. graveolens performance. Our findings suggest that competition confines D. graveolens to disturbed roadsides; management should prioritize reducing bare, disturbed areas and enhancing plant competition in areas vulnerable to invasion.