In summer 2023, a new noxious weed was identified in two locations in the Big Horn Basin, according to a Weed and Pest department release. One was found in a Washakie County sugar beet field, and another under a bird feeder at the Park County Animal Shelter.

Palmer amaranth (Amaranthus palmeri) is an invasive annual weed native to the southwestern United States and Northern Mexico. Palmer amaranth was first identified as a problem noxious weed in the southeastern United States where it caused major problems in the region’s cotton industry and has since spread to nearly every U.S. state and Canada.

Palmer amaranth thrives in regions used for row crop production due to these areas high level of soil disturbance. Palmer amaranth has an extremely fast growth rate of approximately 2 to 3 inches per day and commonly reaches heights of 6 to 8 feet, greatly inhibiting productive crop growth. Many populations have developed resistance to multiple classes of herbicides, including glyphosate, making it very difficult and expensive for producers to control. It also is a highly competitive weed, and has been shown to be the most competitive of the pigweed species.

Palmer amaranth has an extended germination and emergence window that could extend from April through September. It is a very prolific seed producer, producing up to 250,000 seeds from one plant. Yield losses have been reported up to 91% in corn and 75% in sugar beets.

Palmer amaranth has the potential to cause significant economic loss to Big Horn Basin producers. Based on an analysis of several studies conducted by the University of Wyoming, Palmer amaranth could potentially cause up to $16 million worth of yield loss in sugar beets and up to $11 million of yield loss in dry beans statewide. Early detection and a rapid control response is the best way to prevent this weed from becoming established enough to cause this level of economic loss.

Palmer amaranth can be difficult to distinguish from similar amaranth species. In some cases, genetic testing is required to determine species. However, there are certain physical characteristics that can be looked for to aid in proper identification.

• Redroot and smooth pigweeds have fine hairs on their stems and leaves. Palmer amaranth does not have these hairs.

• Green leaves are smooth and arranged in an alternative pattern that grows symmetrically around the stem. There is a small, sharp spine at the leaf tip. Some Palmer amaranth leaves have a whitish V-shaped mark on them.

• Seedhead spikes on female Palmer amaranth plants are much taller, up to 3 feet long, and pricklier than waterhemp, redroot and smooth pigweed spikes.

• The petiole, or leaf stem, will be longer than the leaf blade on Palmer amaranth versus redroot pigweed which will be shorter than its leaf blade.

All members of the public are encouraged to become familiar with identifying Palmer amaranth and to actively search for it in crop fields, borders, ditches, road rights of way, etc. Avoid entering areas where the plant is suspected or confirmed. If you must enter an infested area, always clean vehicles, equipment and clothing prior to exiting the area. Be certain machinery is clean when moved from field to field. If you suspect Palmer amaranth on your property, please contact your Local Weed and Pest Control District for control recommendations.