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:
“Identifying Palmer amaranth in the field,” Purdue University, 4 minute video
“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.
Wisconsin’s Yahara Watershed encompasses lakes Mendota, Monona, Waubesa, Wingra, and Kegonsa, and also includes the Lakeshore Nature Preserve.The UW-Madison Water Sustainability and Climate project created the Water Walk video series to provide a virtual tour of this beautiful area as well as to show how human activities impact the quality of water in the watershed.
Here is another WCWS video discussing strategies to manage herbicide resistance. Get great weed control and save a bag of soybean seed by using a residual herbicide. Check out more WCWS videos at http://wcws.cals.wisc.edu/videos.
The Weed Science Society of America recently held an Herbicide Resistance Summit that brought together leading weed scientists from around the world to discuss the future of herbicide resistance at the National Academy of Sciences in Washington D.C. All of the presentations are on YouTube.
If you have 1 minute or 30 minutes, take the time to hear what the Director of the Australian Herbicide Resistance Initiative, Stephen Powles, says about herbicide resistance in the United States.
Diversify Weed Management
The Transgenic Treadmill
Avoiding the Herbicide Resistance Train Wreck in the United States