Pigweed Identification

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.

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. There are several Extension resources available to help with pigweed identification including:

  • “Palmer amaranth seedling identification,” Purdue University, 7.5 minute video

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 has publications on the management of Palmer amaranth and waterhemp. 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: Palmer amaranth resistance and waterhemp resistance.

Certified Crop Advisor Training Series

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Here is another resource brought to you by University of Wisconsin Extension! Free, online certified crop advisor training videos are available now. In addition to CCA training, they are great for adding to your general knowledge about integrated pest management, soil science, and field and forage crops.

Vince Davis has created a series of weed science videos that can be accessed directly from the videos page.

In the toolbox: Invasive Plant Control Database

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The Midwest Invasive Plant Network has published an online search tool to find information on invasive species control- Invasive Plant Control Database. From this website, go to Resources and then to Tools. The MIPN database works best if you already know the invasive plant species that you have. To help with identification, the Herbarium at the University of Wisconsin-Green Bay has a very nice image catalog of invasive species in Wisconsin, including the counties where they have been found and species descriptions. Another great resource that categorizes the invasive species by habitat is hosted by Wisconsin’s Department of Natural Resources.

Still unsure of the plant species, try University of Wisconsin’s weed identification tool or go to the PlantDOC website to find your local Extension agent and submit a digital image for identification. The Extension Specialist for Invasive Plants at UW-Madison is Mark Renz and his contact information is here.

From the field: weed seedling identification

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Know thy enemy!

Spring has been slow to warm in Wisconsin but the weed seedlings are here.

The University of Minnesota has produced an excellent 2-page guide to broadleaf and grass seedling identification. You will need Adobe Acrobat Reader, a free program, to view the guide. Otherwise, check out their web page.

Michigan State University has an in-depth handbook for the North Central region available electronically as a pdf.