Palmer amaranth and waterhemp are highly competitive pigweed species with widespread herbicide resistance that continue to spread through Pennsylvania. They are confirmed on over 40 farms across the state, with the highest concentrations in southeastern counties. While their emergence was delayed this year due to cool May temperatures, pigweeds have now begun emerging in Pennsylvania. Now is the time to scout fields for Palmer amaranth and waterhemp, in order to control them with effective herbicides before they reach 4 inches tall. Because of widespread resistance to glyphosate (Roundup) and the Group 2 herbicides (ALS-inhibitors), herbicide programs should include multiple other effective modes of action.
Very young Palmer amaranth and waterhemp may be identified by carefully examining their stems and leaves. We have compiled many resources onto an invasive pigweeds webpageincluding identification, management, and timely updates on these pigweeds. Please refer to this webpage for in depth information on the best management practices for these weeds. This includes a 5-part video series on identification and management, identification photos, and athat highlights some key points about these species. As new and more information is acquired, we will post them on this webpage. The Integrated Weed Management Resource Center also has valuable resource on invasive pigweeds.
Palmer amaranth - seedling; notched tip, no hairs, broad ovate shaped leaves, no waxy sheen (A. Hager, University of Illinois)
Waterhemp seedling - egg shaped cotyledons, notched tip, no hairs, narrow lanceolate leaves with waxy sheen (A. Hager, University of Illinois)
Palmer amaranth emerging in Berks County. Photo taken June 13, 2017