Windows of action for controlling palmer amaranth (Amaranthus palmeri) using emergence and phenology models

Amaranthus palmeri S. Watson is a competitive weed native to North America with many herbicide-resistant biotypes that have been spreading around the world. Due to its fast growth, farmers need to optimise control timing to reduce the risk of escapes. This study tracked A. palmeri emergence and phenology using days or growing degree days (GDD, T-base = 15 degrees C; thermal time or hydrothermal time). While A. palmeri has been observed emerging throughout the summer growing season, this study found that 90% of total season A. palmeri emerged before July in the absence of a crop canopy. Using thermal time, emergence could be predicted in different locations and years: reaching 10%, 50% and 90% at 77, 278 and 593 GDD from January 1, respectively. From the time of emergence, 10% of A. palmeri were 10 cm tall after 148 GDD, showed first signs of inflorescence by 212 GDD, and open florets by 419 GDD. Also, 50% of A. palmeri plants had reached the respective stages by 244, 394 and 796 GDD. Using the probability of A. palmeri to reach different phenological stages over time as a function of emergence prediction, critical control windows were determined based on thresholds for risk of escapes. Many tactics and times of action are important for managing this weed. Information about A. palmeri biology indicates these actions could be timed more effectively using weather data and predictive models.