Application of glyphosate to cut stumps is broadly viewed as an effective way to control invasive woody shrubs like the buckthorn species (Rhamnus cathartica L. and Frangula alnus Mill.). Since the primary cost associated with this control method is labor, identifying factors that improve efficacy is important. The objective of this research was to determine the effect of glyphosate concentration and soil moisture on the control of buckthorn using the cut-stump method. More than 600 mature buckthorn plants were cut and treated with glyphosate at concentrations between 0 and 41% active ingredient during October in soils varying from dry to moist. While increased glyphosate concentration improved control rates, soil moisture played a dominant role. In moist soils, the impact of glyphosate concentration was insignificant (z=-1.723, p=0.085), and between 40% and 60% of treated buckthorn plants escaped control at all glyphosate concentrations. In dry soils, however, control rates increased significantly with higher concentrations of glyphosate (z=-8.84, p < 10–15), achieving up to 98% control with 41 % glyphosate. Root exudates of two three-year old field-grown buckthorn seedlings that were treated with high glyphosate concentrations in dry soils using cut-stump treatments contained detectable glyphosate comparable to control rates of mature plants. These results suggest that using the highest labeled concentration of herbicide in relatively dry soil conditions will provide optimum effectiveness with control rates up to 98%.