Giant Ragweed Emergence Across the Midwestern United States.

Predicting weed emergence timing from the seed bank plays a critical role in scheduling early season post-emergence weed management operations to achieve the greatest efficacy.  This study used common source seed (Illinois) in 13 site years in Illinois, Michigan, Kansas, and South Dakota to examine the importance of site and climate variability by year on giant ragweed emergence.  Thermal time, hydrothermal time, and modified hydrothermal time models were used with the Weibull equation to fit emergence data. Thermal time used growing degree days (GDD) with a temperature base = 1°C accumulating from January 1 of each year.  Hydrothermal time incorporated soil dryness, when soil water potential was too low (ψ_L <-400 kPa), thermal time did not accumulate.  Modified hydrothermal time incorporated soil wetness as a parameter and thermal time did not accumulate if soil water potential was above -33 kPa. Regional emergence was not well estimated by calendar day, however, using thermal time for the model had an R² = 0.79, indicating that soil temperature had a large influence on emergence.  Incorporating soil dryness with hydrothermal time improved the regional estimate although minimally (R² = 0.805).  Emergence within a site differed enough among years that combining between year datasets often resulted in low R² values.  However, both the hydrothermal and modified hydrothermal time models often had R² values >0.95 for a single site-year.  Soil temperature and wetness prior to and during emergence had a large impact on start and end of emergence.  The data from the current research indicate that while not robust across the region, emergence forecasts for giant ragweed under differing soil management scenarios can be predicted.  If very wet conditions are prevalent in the spring, the modified model, using an upper boundary for water potential may be appropriate to better model seedling emergence.