Identification and Management of Palmer Amaranth in Grain Sorghum

In many parts of the United States and especially in the Midwest, Palmer amaranth has become one of the toughest weeds to control. Palmer amaranth is native to the southern part of the United States but can now be found in 39 of the 48 continental United States. The weed is an aggressive competitor in field crops as it can grow two to three inches per day and reach heights of six to eight feet at full maturity. It is one of several herbaceous pigweed Amaranthus species found in the Midwest (Figure 1).

When finding an unknown pigweed species, the first thing to look for is hair or no hair on the stem. Red root pigweed and Powell amaranth have hairy stems; the hairs of the young branches protrude. Palmer amaranth, waterhemp, and spiny amaranth have hairless (glabrous) stems. Spiny amaranth can be distinguished from Palmer amaranth and waterhemp by the presence of a spine at the point where the leaves attach to the stem. These spines reach up to ½ inch in length.

If the plant has hairless stems and there is no point present where the leaves attach to the stem, the weed is either Palmer amaranth or waterhemp and it can be difficult to tell the two species apart till they have flowered. From experience, it is possible to differentiate between vegetative Palmer amaranth and waterhemp, but the diversity of both species makes this difficult. The most common way to identify Palmer amaranth from waterhemp is the petioles of Palmer amaranth are usually longer than the leaf of the plant (Figure 2). The flowering structure is usually branchless, with thick, long stems (1 to 2 feet) at the top of the plant and shorter branches. Female plants bear seeds and have a rough flowering structure.

Management of Palmer amaranth in Grain Sorghum

If you haven’t planted grain sorghum before, Palmer amaranth and grass seem to be the toughest weeds to control. The best strategy is to start clean. Options rapidly become limited when grain sorghum seedlings emerge through the soil surface. As with most row crop weed control programs, the most effective control of Palmer amaranth is to use multiple effective modes of actions.

For preemergent options, herbicides from group 5 and 15 should be considered. The more herbicide groups you can stack on top of those basic two, the better. As always, follow directions on herbicide labels. For best results, start with a clean tilled field or use a burndown herbicide option to control actively growing weeds.

For post-emergent herbicide options, the options are very limited. All the available options should be applied when Palmer amaranth is under 4-inches. Post-emergent chemistry options include atrazine, Aim® EC Herbicide, 2,4-D, dicamba, bromoxynil and Huskie® Herbicide. These options can work in the right conditions but have limited effectiveness after Palmer amaranth is above 4-inches.

Lancaster, S. and Falk-Jones, J. 2022. Palmer amaranth control in grain sorghum. Agronomy eUpdates. Issue 914.  https://eupdate.agronomy.ksu.edu/article_new/palmer-amaranth-control-in-grain-sorghum-502-1/.

Lawrence, N. 2017. Management of ALS-Resistant Palmer amaranth and waterhemp in the Panhandle. CROPWATCH. University of Nebraska-Lincoln.  https://cropwatch.unl.edu/2017/management-als-resistant-palmer-amaranth-and-waterhemp-panhandle#:~:text=Management%20of%20ALS-/.Resistant%20Palmer%20Amaranth%20and%20Waterhemp%20in,to%20get%20worse%20in%20the%20near%20future.%20/.

Peterson, D.E. 2019 revision. (Horak, M.J., Peterson, D.E., Chessman, D.J. and Wax, L.M. 1994) Pigweed Identification. A pictorial guide to the common pigweeds of the Great Plains. Kansas State University. https://bookstore.ksre.ksu.edu/pubs/S80.pdf/. 

Hartzler, B. Palmer amaranth: ID, biology and management. Integrated Crop Management. Iowa State University Extension and Outreach. https://crops.extension.iastate.edu/encyclopedia/palmer-amaranth-id-biology-and-management/.

Websites verified 3/6/23.

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