Liz Bosak, Outreach Specialist, Department of Agronomy, University of Wisconsin-Madison
Weed identification at the seedling and immature stages can be difficult but is often necessary because scouting should occur before weeds reach 4 inches in height. At emergence before a full set of true leaves appear, pigweeds can be confused with other weed species such as wild buckwheat, eastern black nightshade, and ladysthumb. In addition, the pigweeds: Palmer amaranth, waterhemp, redroot pigweed, Powell amaranth, and smooth pigweed, are not easily separated by species at the immature stages. The first step is to look at the leaves and stems because Palmer amaranth and waterhemp do not have any hairs compared to Powell, redroot, and smooth pigweed, which do have hairs but they may not be obvious at the immature stage. If the plant looks like it may be Palmer amaranth or waterhemp, then the next step is to look at the leaf shape and petiole. Palmer amaranth has a more rounded leaf shape and a petiole that is longer than the leaf itself, https://youtu.be/NLGEwizXj-M?t=9m15s . For a list of resources available by species, guides for the most common weed seedlings, and links to the WeedID smartphone apps, take a look at the Weed Info page on the WCWS website (http://wcws.cals.wisc.edu/weed-info/ ). There are several Extension resources available to help with pigweed identification including:
- “Identification of the weedy pigweeds and waterhemps of Iowa,” Iowa State University, http://wcws.cals.wisc.edu/wp-content/uploads/sites/4/2013/12/weed_info_pigweed_identification_iowa_state_university.pdf
- “Pigweed identification: a pictorial guide to the common pigweeds of the Great Plains,” Kansas State University, http://www.ksre.ksu.edu/bookstore/pubs/s80.pdf
- “Identifying Palmer amaranth in the field,” Purdue University, 4 minute video, https://youtu.be/aVbgPGg0GO0
- “Palmer amaranth seedling identification,” Purdue University, 7.5 minute video, https://youtu.be/wNgRvvnPQJ8
The videos provide a nice introduction to identification, particularly, if using the printed guides. Pigweeds present a tough set of management challenges, for instance, very high growth rates, extended emergence over most of the growing season, and high seed production. The United Soybean Board’s TakeAction website, www.takeactiononweeds.com, has publications on management of Palmer amaranth (http://takeactiononweeds.com/wp-content/uploads/2014/01/palmer-amaranth-management-in-soybeans.pdf ) and waterhemp (http://takeactiononweeds.com/wp-content/uploads/2014/01/waterhemp-management-in-soybeans.pdf ). Populations of Palmer amaranth and waterhemp have been confirmed resistant to glyphosate in Wisconsin. To read more about glyphosate-resistant pigweeds in Wisconsin, please consult these fact sheets: http://wcws.cals.wisc.edu/wp-content/uploads/sites/4/2013/03/WCWS_206_palmer_amaranth_resistance_WEB.pdf and http://wcws.cals.wisc.edu/wp-content/uploads/sites/4/2013/03/WCWS_207_waterhemp_resistance_WEB.pdf .