Timing of weed emergence varies both from year to year and also across the state. The table provides information on the initial date of emergence of five early weed species at locations across Iowa. Giant ragweed, one of the earliest emerging summer annuals found in crop fields, initiated emergence in the last week of March or first week of April in both 1998 and 1999. At most locations giant foxtail began emergence about 4 weeks after giant ragweed, but at the SE (Crawfordsville) location there has been only a 2-week difference in initial emergence of these species.
Iowa State University
Hartzler, Robert G. and Buhler, Douglas D., "Initial dates of weed emergence" (1999). Integrated Crop Management News. 2158.
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