Palmer amaranth (Amaranthus palmeri)
The original pdf of this National Pest Alert, suitable for printing, is available online here.
Palmer amaranth is native to the desert regions of northern Mexico and the southwestern United States, and has spread into the Midwest, the East Coast and portions of Canada. Palmer amaranth is a relatively new introduction in Iowa, Minnesota, South Dakota and Wisconsin. Palmer amaranth can be distributed by birds, through livestock feed, manure, grazing, wind, farm equipment etc. Amaranth expansion was accelerated in 2016 as an unintentional contaminant in some native seed mixes purchased by growers participating in conservation programs.
Palmer amaranth grows 1–3 inches in height per day, up to 6–8 feet tall and occasionally will reach 10 feet or more. Emergence occurs from early May through late summer and a single plant can produce over 200,000 seeds. Compounding the concern over the expansion of this highly aggressive, fast-growing weed is its resistance to herbicides. It is similar to waterhemp in its ability to rapidly evolve resistance to many herbicides used in weed management programs.
Identification of Palmer Amaranth Plants
A petiole longer than the leaf blade is the most reliable vegetative trait to distinguish the two pigweeds. Not all leaves on a Palmer amaranth will have this trait. Robert Hartzler, Iowa State University identification of Palmer amaranth seed is nearly impossible because the seed of different pigweed species look similar. Seed growers test each seed lot for germination and purity, and endusers can request the laboratory tests for each species in your seed mix. If the seed test indicates the presence of “pigweed” or “Amaranthus “Amaranthus spp.” then don’t purchase the seed unless the pigweed seed has been genetically tested and determined not to be Palmer amaranth. Many seed companies are now using a genetic test to positively identify Palmer amaranth seed from other pigweed species. Although the conservation seed industry is now aware of the Palmer amaranth issue, producers should still discuss seed purchases with local NRCS personnel and University Extension specialists to help ensure that the seed mix is amaranth-free.
Preventing Spread to Production Acreage
Lindsay Haines, USDA Natural Resources Conservation Service
Bob Hartzler, Iowa State University
Robert Nowierski, USDA National Institute of Food and Agriculture
Susan Ratcliffe, University of Illinois
Jill Schroeder, USDA Office of Pest Management Policy Palmer amaranth in conservation planting.
Robert Hartzler, Iowa State University