Fraxinus pennsylvanica (green ash) is commonly used for reforestation of agricultural lowlands in the midwestern and eastern United States. We evaluated the effects of herbicide (untreated, glyphosate, and sulfometuron methyl) and tillage (tilled and no-till) on F. pennsylvanica success and composition of associated volunteer vegetation 3 years after the treatment applications and reforestation of a formerly cultivated field in southern Illinois. Tillage had no effect on F. pennsylvanica growth, whereas both herbicides increased all measures of tree performance. The response of associated native and non-native species cover was affected by an interaction between the herbicide and tillage treatments. In the presence of herbicide, cover of native species was greater in the no-till treatment, whereas non-native cover was higher in the tillage treatment. Both native and non-native cover were unaffected by tillage in the absence of herbicide, and there were no differences in cover among the herbicide treatments in the presence of tillage. Total diversity was higher in the tillage treatment than the no-till treatment, and diversity was lower in the sulfometuron methyl herbicide treatment than the control and glyphosate herbicide treatment. Lower diversity in the sulfometuron methyl treatment was attributed to greater cover of a native perennial grass, Andropogon virginicus (Broomsedge), which was inversely related to total diversity. We conclude that a single glyphosate herbicide application can enhance F. pennsylvanica growth and conserve associated species diversity in this system. Furthermore, species-specific responses of the associated vegetation should be included in management considerations, particularly if silvicultural treatments influence dominance and diversity in the establishing understory community during reforestation.