Smooth brome (Bromus inermis) invasion into tallgrass prairie has led to development of methods of control. Prescribed burning is used by the US Fish and Wildlife Service (USFWS) to manage prairie according to a provisional model developed by Willson and Stubbendieck (2000). The model recommends conducting a prescribed burn at the onset of elongation of smooth brome. The USFWS uses the 5-leaf stage as a phenological cue, signaling the initiation of elongation. Variability in smooth brome development limits the reliability of this method. Our objective was to develop an alternative method to determine when smooth brome populations reach the targeted 50% elongation by correlating accumulated growing degree days and population level plant phenological stages (mean stage count) throughout sites in North Dakota, South Dakota, and Minnesota. A linear regression model was used to determine the onset of elongation in the smooth brome population, regardless of leaf stage variation. Field and greenhouse studies confirmed accumulated growing degree days predicted the initiation of elongation. We also compared smooth brome response to different seasonal burn treatments, determining it could be decreased by burning at other times. As part of the USFWS Native Prairie Adaptive Management program, results will be used to assist management decisions regarding the timing of control.