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Watch out for Palmer amaranth in the Pacific Northwest

Posted by Joel Felix, Oregon State University | April 21, 2022

As I write this article, the 2022 cropping season is in full swing in the Treasure Valley of southwestern Idaho and eastern Oregon. Onions have already been seeded and emerged in some fields! Other crops including sugar beet and potato are being planted as well. Wheat fields appear completely green and soon sprayers will roll up and down the fields spraying to control weeds. Other than wheat, the other fields are devoid of weeds, which is emblematic of the spring season. But soon this will change. The normal suspects, including glyphosate-resistant kochia, will soon show their ugly heads and the ‘war against weeds’ will ensue. Weed management has become a challenge lately with the emergence of herbicide-resistant populations and low availability of hand labor compounding the problem. During summer 2021, glyphosate-resistant kochia was observed in almost every sugar beet field in the lower Treasure Valley area that encompasses Ontario, Oregon. Now, to make matters worse, there are indications that a new weed could be lurking in the fields of the Pacific Northwest (PNW) region – Palmer amaranth!

Palmer amaranth (Amaranthus palmeri) is a member of the pigweed family. Think of Palmer amaranth as redroot pigweed or Powell amaranth on steroids! Palmer amaranth has been nicknamed ‘the king of weeds’ largely because of the damage it is capable of causing. A single Palmer amaranth plant growing per foot of corn row reduces corn yield by 40%–80%. Our recently published bulletin –Pigweeds: Current and emerging threats in the Pacific Northwest – could help you get acquainted with this weed. Briefly, Palmer amaranth has a diamond or lance-shaped leaf; the length of petioles (the stem-like structure that connects the leaf blade to the main stem) is typically longer than the leaf blade; leaves tend to have a V-shaped watermark, often referred to as a chevron; stems are smooth and often lack hairs; male and female flowers develop on different plants (dioecious); the seed-head is longer than other members of the pigweed group (Figure 1); a single Palmer amaranth female plant is capable of producing 500,000 seeds when there is no competition!

Palmer amaranth.
Figure 1. A photograph of a suspected Palmer amaranth plant taken during summer 2021 in an area south of the City of Weiser in Washington County, ID. (Photo courtesy of Bonnie Davis, Washington County, ID Weed Control Superintendent).

Palmer amaranth has a long history of herbicide resistance that dates back to 1989 when the first plants resistant to the Group 3 herbicide Treflan (trifluralin) in cotton and soybean fields were identified in South Carolina. Populations resistant to 9 herbicide groups (Groups 2, 3, 4, 5, 9, 10, 14, 15, 27) have since been identified in alfalfa, corn, soybean, sorghum, peanuts, orchards, and other settings in various states. Palmer amaranth is currently the number one weed problem of crops in the southwestern, southeastern, midwestern, and the Great Plains regions of the United States.

The future of agriculture in the PNW could be upended by the appearance of Palmer amaranth in the region, with a potential to greatly disrupt weed management programs in various crops. In fact, it appears that Palmer amaranth is already in the PNW region, albeit in isolated areas. A photograph (Figure 1) taken by Bonnie Davis, Washington County, ID Weed Superintendent seems to be of Palmer amaranth plant. The photograph was taken in an area south of the City of Weiser, ID. We will monitor the surrounding fields during summer 2022.

What could you do to avoid expansion of Palmer amaranth in the region?

The first action is correct identification of the weed. The bulletin mentioned above contains images that could help identify Palmer amaranth. Most weed introductions can be kept to a minimum with vigilance and proactive scouting of fields to uproot plants before setting seeds. Consider bringing plants suspected of being Palmer amaranth to your Extension Office and sending photographs to weed scientists at Oregon State University, University of Idaho, or Washington State University. Always include georeferencing information to enable easy identification of the area.