About Weed Emergence Scouting Tool (W.E.S.T.)


Description

The Weed Emergence Scouting Tool (W.E.S.T.) is a product developed by the Midwestern Regional Climate Center (MRCC), in collaboration with weed scientists and Extension educators at Purdue University, to provide county-level estimated first and peak emergence date information for Giant Ragweed and Waterhemp across the Midwestern United States. Weed emergence dates are estimated based on empirical relationships between weed phenology and Growing Degree Day (GDD) accumulations. This tool can be used to answer such questions as:

  • How many growing-degree days have accumulated from January 1st to the current date in Midwest counties?
  • Is Giant Ragweed or Waterhemp forecast to reach its first emergence date in Midwest counties within the next six days? If so, in how many days?
  • Is Giant Ragweed or Waterhemp forecast to reach its peak emergence date in Midwest counties within the next six days? If so, in how many days?
  • On what date did Giant Ragweed or Waterhemp reach its first and/or peak emergence in Midwest counties?
  • How have GDD accumulations progressed over the current season in a specific county?

Definitions

  • Giant Ragweed: Commonly found in agronomic crops and roadsides, this weed is a highly competitive summer annual weed, and is native to North America belonging to the species Ambrosia trifida.
  • Waterhemp: Commonly found in agronomic crops, this weed is a moderately competitive summer annual weed, that produces large amounts of weed and is native to the United States belonging to the species Amaranthus tuberculatis.
  • Daily Growing-Degree Day (GDD): A measure of how much heat has accumulated over a day relative to a base temperature. In this tool, daily GDD is calculated by county by subtracting the daily mean temperature (the arithmetic mean of the daily maximum temperature and the daily minimum temperature) from 48°F; if the resulting value is negative, daily GDD is zero.
  • GDD Accumulation: The number of daily GDD that has accumulated since a given date in a county. GDD accumulation in the W.E.S.T. starts on January 1st of the current year.
  • First Emergence Date: The date on which Giant Ragweed or Waterhemp is expected to first germinate in the current year based on an empirical relationship with GDD Accumulation. The GDD Accumulation corresponding to first emergence is assumed to be 120 for Giant Ragweed and 425 for Waterhemp.
  • Peak Emergence Date: The date on which Giant Ragweed or Waterhemp is expected to reach peak germination in the current year based on an empirical relationship with GDD Accumulation. The GDD Accumulation corresponding to first peak emergence is 250 for Giant Ragweed and 550 for Waterhemp. Weeds can have multiple peaks emergence periods throughout the year, but the W.E.S.T. only tracks the first peak date.
  • Midwestern United States: In this tool, the Midwestern United States (or Midwest) is defined as the nine states of Illinois, Indiana, Iowa, Kentucky, Michigan, Minnesota, Missouri, Ohio, and Wisconsin.

Data Source and Processing

For observed Daily GDD, the tool uses the Parameter-Elevation Regressions on Independent Slopes Model (PRISM) data-set, which has a nominal resolution of 4 km x 4 km. The PRISM data-set is archived in the Regional Climate Center Applied Climate Information System (RCC ACIS). The tool accesses RCC ACIS using an API, which provides county-level Daily GDD, with a base temperature of 48°F, based on PRISM temperature data. The observed Daily GDD is then added to the GDD Accumulation of the previous day to arrive at a new GDD Accumulation value for the current day.

To calculate forecast Daily GDD, the tool first obtains gridded hourly temperatures from the National Oceanic & Atmospheric Administration (NOAA) National Digital Forecast Database (NDFD). NDFD fields have a nominal resolution of 2.5 km x 2.5 km at the Equator, and the temperature field is available out to 156 hours (i.e., at most 6 full calendar days). Forecast daily maximum and minimum temperature values out to 6 days are obtained from the hourly temperature values. Then, the arithmetic means of forecast daily maximum and minimum temperatures are calculated to obtain forecast daily mean temperature values. The resulting forecast daily mean temperature values are then interpolated to obtain county-level forecast daily mean temperature values. Forecast Daily GDD is calculated in a similar manner as observed Daily GDD. Forecast GDD Accumulation for Day 1 is calculated by adding the Daily GDD value of the day after the current day to the GDD Accumulation value for the current day. In turn, the forecast GDD Accumulation for Day n is calculated by adding the Daily GDD value of n days after the current day to the GDD Accumulation value for n-1 days. At this time, the highest value of n is 6 days.

All data are obtained and calculations made at approximately 2100 UTC every day.

Empirical Relationship between GDD Accumulation and Weed Phenology

Past research from Iowa State University provides some general guidelines that connect GDD accumulations to the timing of first emergence for Giant Ragweed, Waterhemp, and numerous other weeds (ISU Extension, 2000). According to ISU Extension, Giant Ragweed will tend to emerge with fewer than 150 GDD while Waterhemp requires more than 350 GDD to begin emergence.

To further refine these relationships and to inform peak emergence relationships, Purdue Extension educators collected field observations at eight locations across Indiana from March to July 2023. Based on those observation, the following empirical thresholds were identified for use in the W.E.S.T. product:

Giant Ragweed

  • First emergence at 120 GDD (base 48)
  • Peak emergence at 200-300 GDD (base 48); mid-point 250 GDD

    • Waterhemp

  • First emergence at 425 GDD (base 48)
  • Peak emergence at 500-600 GDD (base 48); mid-point 550 GDD

    • Caveats and Limitations

    Local conditions may vary. All emergence dates denoted in this tool are estimates based on limited empirical observations. Further research is needed to test the accuracy of these relationships outside of Indiana. Anyone interested in collecting and sharing weed emergence observations for their location should contact Melissa Widhalm (mwidhalm@purdue.edu).

    It is likely that other environmental factors, such as soil temperature, play a role in weed emergence patterns. The W.E.S.T. currently only uses GDD accumulation relationships to estimate emergence dates. Future research and tool development aim to incorporate additional factors that will improve tool accuracy.

    Acknowledgments

    This work was supported by the USDA National Institute of Food and Agriculture, Crop Protection and Pest Management Program through the North Central Integrated Pest Management Center (2022-70006-38001). Ongoing technical support and development is provided by the Midwestern Regional Climate Center, which is a partnership between NOAA’s National Centers for Environmental Information (NCEI) and Purdue University in West Lafayette, Indiana.

    Feedback

    You can submit feedback about the Weed Emergence Scouting Tool online here or via email to mwidhalm@purdue.edu.