Palmer Amaranth Confirmed in Turner County: Is this weed in your fields?

A white map of South Dakota with black lines depicting Counties. Numbers are throughout the map denoting the numerical order of Palmer amaranth infestations.
Figure 1. Estimation of the Palmer amaranth infestations in South Dakota. The numerical order represents the order in which the infestations were reported.

Written collaboratively by Eric Jones, Graig Reicks, Philip Rozeboom, Jill Alms and David Vos.

Palmer amaranth has recently been confirmed in Turner County. Currently, the estimation of Palmer amaranth infestations has been confirmed in 18 counties across South Dakota (Figure 1). 

Palmer amaranth is a weed native to the Southwest United States that has moved across the country, primarily in the Southern United States. Movement of Palmer amaranth can be traced to seed movement by wildlife, contaminated-machinery, -feed, -manure, and – crop seed. The weed is difficult to manage due to prolonged emergence, rapid growth, and copious seed production. In addition, Palmer amaranth is resistant to nine herbicide groups and multiple herbicide resistance is common (Heap 2025).

Identification

Left: A green Palmer amaranth weed surrounded by various other green plants. Right: Yellow waterhemp weeds with green to yellow plants surrounding.
Figure 2. At glance, Palmer amaranth and waterhemp look very similar. However, the left inset is Palmer amaranth, and the right inset is waterhemp. Careful steps for identifying these two weed species are needed.

Waterhemp, a close relative of Palmer amaranth, is well-known across South Dakota. At first glance, the two species look very similar (Figure 2). However, there are tricks that facilitate proper identification. The flowers are the most accurate way to differentiate the two species. Palmer amaranth possesses long bracts (modified leaves associated with flowers) that are sharp to the touch, while waterhemp possesses short bracts that are softer to the touch (Figure 3). While this is an accurate way for identification, this time is too late to manage the weed and seeds are likely present at this growth stage. Therefore, identifying plants in the vegetative stage ensures the suspect plant cannot produce seed. The leaf to petiole ratio is the most accurate way to identify smaller plants in the vegetative stage and management can still be effective. If the petiole (the structure that attaches the stem to leaf) is longer than the leaf, the plant is likely Palmer amaranth (Figure 4). If the petiole is shorter than the leaf, the plant is likely waterhemp. The leaf to petiole ratio is not 100% accurate (i.e., plants from either species can possess longer/shorter petioles if one searches hard enough), so examine several leaves and their attached petioles on each plant to confirm the majority is highly recommended.

Bracts

Left: A green Palmer amaranth weed surrounded by various other green plants. Right: Yellow waterhemp weeds with green to yellow plants surrounding.
Figure 3. Bracts (modified leaves on flowers) of Palmer amaranth (left) and waterhemp (right). Bracts of Palmer amaranth are larger and sharp to the touch while the bracts of waterhemp are smaller and softer to the touch. While an accurate way to identify the species, usually seeds are produced at this time.

Leaves and Petioles 

Left: green Palmer amaranth leaf surrounded by brown dirt. Right: a green waterhemp leaf being held by a hand.
Figure 4. Left: Leaf-to-petiole ratio to identify Palmer amaranth. The petiole is usually the length of the leaf or longer. Right: Leaf-to-petiole ratio to identify waterhemp. The petiole is usually half the length of the leaf.

Management

A bundle of brown Palmer amaranth plants.
Figure 5. Palmer amaranth plants were removed from the field and contained. This method of removal is laborious but ensures no seeds are being returned to the soil.

If Palmer amaranth is found in fields, plants should be removed, contained, and destroyed (Figure 5). If plants are simply uprooted, seeds will be returned to the soil and will have to be managed in future growing seasons. Informing South Dakota State University Extension of Palmer amaranth infestations helps us track where the weed has been found in the state and make recommendations based on the locale. While likely an uncomfortable situation, informing neighbors of Palmer amaranth infestations is a good idea so the management (and hopefully eradication) can be a community approach and much more effective than tackling the endeavor alone. There are already too many difficult-to-manage weeds (i.e., Canada thistle, kochia, leafy spurge, waterhemp), we do not need Palmer amaranth to be added to the list! 

Reference

  • Heap, I.  The International Herbicide-Resistant Weed Database.  Online.  Friday, September 5, 2025. Available  www.weedscience.org

Related Topics

Corn Weeds, Soybean Weeds, Wheat Weeds

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