The US Fish and Wildlife Service (USFWS) uses the five-leaf developmental stage as a signal to the initiation of elongation in smooth brome (Bromus inermis Leyss.). In areas where certain plant community criteria are met, conducting a prescribed burn at elongation onset has reduced smooth brome populations. However, leaf stage identification presents USFWS managers with challenges due to the variability of smooth brome development in tallgrass prairies of the Northern Great Plains. The objective of this research was to develop an alternative method to determine when smooth brome populations reach the targeted 50% elongation by linking accumulated growing degree days and population-level plant phenological stages (mean stage count). We determined smooth brome phenological stages at sites in North Dakota, South Dakota, and Minnesota and calculated the corresponding number of growing degree days (using the base temperature of 0 °C). Linear regression models, correlating phenological stage and growing degree days, determined onset of elongation in the smooth brome population, regardless of leaf stage variation. The average accumulated growing degree days (1256 AGDD) and corresponding standard deviation (±155 AGDD) can be used to predict when 95% of smooth brome populations in northern tallgrass prairies reach 50% elongation. As part of USFWS Native Prairie Adaptive Management program, results will be used to assist management decisions regarding the timing of defoliation in an effort to enhance native plant communities where smooth brome is the dominant invader.