Fire behavior associated with wild and prescribed fires is variable, but plays a vital role in how a plant responds to fire. Understanding the relationship between fire behavior and rangeland plant community response will help to improve the use of prescribed fire to achieve management objectives. Fire is an important ecological process in many rangeland ecosystems and can be used as a tool to maintain grassland plant communities and shift community composition. Purple threeawn (Aristida purpurea Nutt.) is a grass native to North America that has poor forage quality and the ability to form near monocultures. Therefore, the identification of tools to reduce purple threeawn abundance is desirable. We assessed the effects of summer and fall prescribed fire on purple threeawn plant basal area, tiller production, and plant mortality one growing season post fire in the northern Great Plains. Thermocouples and portable data loggers were used to measure the maximum temperature, heat duration, and heat dosage that individual purple threeawn plants experienced. Fire reduced basal area and tiller production 59% and 57%, respectively. Heat dosage (C-statistic = 0.69) and heat duration (C-statistic = 0.65) were good predictors of purple threeawn mortality. A restrospective analysis showed maximum temperatures were similar for fall and summer fires but heat duration and dosage were 44% and 21% greater for summer fires, respectively. Our results indicate that purple threeawn is a fire sensitive species. The ability to predict purple threeawn mortality could enhance the efficacy of prescribed fire as tool to restore purple threeawn and other Aristida-dominated plant communities.