Competition for soil water resources between newly planted Douglas-fir seedlings and aggressive early-seral plants, such as Senecio sylvaticus [L.] (Senecio), can create drought conditions that impact tree seedling physiology, growth, and likelihood of mortality. However, the specific impact of Senecio on soil moisture dynamics and inducement of water stress in newly planted tree seedlings across varying site conditions has not been quantified. This study quantified these interactions at three contrasting sites across the U.S. Pacific Northwest: the Coastal Range, the Cascade foothills, and the fringe of south-central valley of Western Oregon. We tested whether competition between Senecio and Douglas-fir seedlings for soil water resources in areas of high Senecio abundance caused increased water stress in the tree seedlings. Senecio demonstrated a high degree of plasticity across sites increasing its lifespan and shoot:root in response to increased soil water resources. Senecio also had more than twice the root area of influence as Douglas-fir. Overall, greater Senecio abundance was associated with greater soil moisture depletion and this soil moisture depletion was correlated with increased Douglas-fir water stress. The magnitude of this response varied across sites; the dry site had the greatest shifts in Senecio biomass partitioning, the highest observable water depletion, and the greatest amount of Douglas-fir water stress. The presented results can be useful for determining effective forest vegetation management regimes by considering the impact of Senecio presence on Douglas-fir seedling drought stress across different site conditions.