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A New Noxious Weed – “Public Enemy Number One”


A weed that has been popping up in the Big Horn Basin has a lot of people worried.

It’s called Palmer amaranth, a weed that’s hard to control, resists many herbicides and has a tremendous root system.  The weed can also grow up to 10 feet tall, shading crops.  And farmers say it can clog up their harvesting equipment.

Josh Shorb, supervisor of the Park County Weed and Pest Control District, tells Cody Enterprise,  “It affects all core sectors of the ag industry, so it has us really worried,” he said. “It’s going to really impact Park County and the Big Horn Basin.”

One plant can produce 250,000 tiny seeds.

Shorb recently cited a report from a sugar company saying the weed wreaked such havoc in Colorado last year that 1,000 acres of sugar beets were abandoned.

“Yield losses have been reported up to 91% in corn and 75% in sugar beets,” according to the Wyoming Weed and Pest Council.

The discovery of Palmer amaranth in the Big Horn Basin occurred last summer, in a Washakie County sugar beet field. This year, Shorb said, it was found under several bird feeders at the Park County Animal Shelter east of Cody.

The invasive annual weed is native to the southwestern United States and northern Mexico, according to the state weed council. It was first identified as a problem noxious weed in the southeastern U.S., where it caused major problems in the region’s cotton industry, and has since spread to nearly every state and into Canada.

Palmer amaranth thrives in regions used for row crop production due to those areas’ high levels of soil disturbance plus fertilizer and irrigation they receive.

It has an extremely fast growth rate of approximately two to three inches per day, and its resistance to multiple classes of herbicides, including glyphosate, make it difficult and expensive for producers to control.

It’s also a highly competitive weed — the most competitive of the pigweed species.

Palmer amaranth has an extended germination and emergence window that could extend from April through September.

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 are the best ways to prevent the 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 three 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.

The public is encouraged to become familiar with identifying Palmer amaranth and to search for it in places such as crop fields, borders, ditches and road rights of way. Avoid entering areas where the plant is suspected or confirmed.

Those who must enter an infested area should always clean their vehicles, equipment and clothing prior to exiting the area, and be certain machinery is clean when moved from field to field.

Anyone who suspects the noxious weed has infested an area should notify Park County Weed and Pest about its location.

“We can go out and identify it,” Shorb said.

He’ll be hiring extra staff this summer to help search for the weed.

“We’ll be actively looking for it.”

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