Competition for soil water resources between newly planted Douglas-fir seedlings and aggressive early-seral plants, such as Senecio sylvaticus [L.] (Woodland groundsel, 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 site conditions has not been quantified. This project quantified these interactions at three varied sites across the Pacific Northwest: the Coastal Range, the Cascade foothills, and the fringe of south-central valley. We tested whether water competition between Senecio and Douglas-fir seedlings in areas of high abundance of Senecio caused increased water stress in the tree seedlings. Overall, this study showed greater percent cover of Senecio was associated with greater soil moisture depletion. This soil moisture depletion was correlated with increased Douglas-fir water stress. There were also significant differences between and within species for biomass partitioning and phenotypic responses across the sites. The results here showed that there is a strong influence of site conditions on the degree of sensitivity of Douglas-fir to Senecio presence; the dry site saw the greatest shifts in biomass partitioning, the most observable water depletion, and the greatest amount of measured water stress. These results can be useful for determining effective Forest Vegetation Management regimes that take into account the presence of Senecio and the likely impact of this presence on crop tree drought stress based on site conditions.