Giant ragweed has been increasing as a major weed of row crops in the last30 yr, but quantitative data regarding its pattern and mechanisms of spreadin crop fields are lacking. To address this gap, we conducted a Web-basedsurvey of certified crop advisors in the U.S. Corn Belt and Ontario, Canada.Participants were asked questions regarding giant ragweed and cropproduction practices for the county of their choice. Responses were mappedand correlation analyses were conducted among the responses to determinefactors associated with giant ragweed populations. Respondents rated giantragweed as the most or one of the most difficult weeds to manage in 45% of421 U.S. counties responding, and 57% of responding counties reported giantragweed populations with herbicide resistance to acetolactate synthaseinhibitors, glyphosate, or both herbicides. Results suggest that giantragweed is increasing in crop fields outward from the east-central U.S. CornBelt in most directions. Crop production practices associated with giantragweed populations included minimum tillage, continuous soybean, andmultiple-application herbicide programs; ecological factors included giantragweed presence in noncrop edge habitats, early and prolonged emergence,and presence of the seed-burying common earthworm in crop fields. Managinggiant ragweed in noncrop areas could reduce giant ragweed migration fromnoncrop habitats into crop fields and slow its spread. Where giant ragweedis already established in crop fields, including a more diverse combinationof crop species, tillage practices, and herbicide sites of action will becritical to reduce populations, disrupt emergence patterns, and selectagainst herbicide-resistant giant ragweed genotypes. Incorporation of acereal grain into the crop rotation may help suppress early giant ragweedemergence and provide chemical or mechanical control options forlate-emerging giant ragweed.