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Vigilent Watch for Palmer Amaranth

Palmer Amaranth

Submitted by Craig Askim, Extension Agent, Agriculture and Natural Resources   

To date, Palmer amaranth has not been documented in North Dakota. We want to keep it that way. There has been a concerted education program in North Dakota from academia and industry for correct identification of Palmer amaranth. For review below are some reasons why we don’t want to let it get started and why growers should quickly destroy any plants if found.

Palmer Amaranth has one of the fastest weed growth rates known - >2 inches/day and has long emergence patterns from mid-May through August. This weed can exploit even slight canopy openings with the ability to produces from 1 to 1.8 million seeds/plant. The seed is short-lived and only 2% of seed is viable after 6 years but the sheer number of seeds produced by a female plant makes eradication difficult once established.

Female plants can grow to more than 10 feet tall with a 5-6 inch stem girth and with the seed heads more than 1 foot in length. Male plants senesce earlier than female plants and typically don't grow as large late in the season but can be fairly large and competitive through much of the season. Once male plants are done shedding pollen they will quit growing, whereas the females will continue to growth in height and stem girth. Male stems usually don't get more than about 1 inch in diameter vs female stems can get 3 to 4 inches in diameter.

A thick stand of Palmer amaranth will basically result in 75 to 100% yield loss of many crops because of competition and harvest difficulties. Identification of Palmer amaranth in the seedling stages is very difficult as redroot pigweed, Powell amaranth, and Water-hemp may have many similar features.

Palmer amaranth has dioecious reproduction, so individual plants are either male or female, which forces outcrossing and genetic diversity. This gives Palmer amaranth the ability to adapt and quickly spread herbicide resistance genes when selection pressure is applied.  Palmer amaranth is a prolific seed producer. Each plant can produce at least 100,000 seeds when they compete with a crop. In noncompetitive scenarios they can produce nearly a half million seeds.

Palmer amaranth seeds are rather small and thrive in no-till or minimum tillage fields. In those situations, seeds are allowed to stay in their ideal emergence zone: the top inch of soil. Humans easily transport the small seeds through grain, seed, or feed contamination; or on equipment such as combines. Palmer exhibits aggressive growth and competitiveness with crops.

Palmer populations have evolved resistance to multiple herbicide modes of action, including ALS inhibitors, triazines, HPPD inhibitors, dinitroanilines, and glyphosate. The majority of populations of Palmer amaranth are ALS-inhibitor- and glyphosate-resistant making it very hard to manage if populations get out of control.

Contributing source: Rich Zollinger – NDSU Extension Weed Specialist


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