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Vardiman: Land managers should be aware of problematic weeds across the landscape

by Wyoming Livestock Roundup

Invasive weed species are a burgeoning problem for Wyomingites – both in croplands and on the range. In particularly cheatgrass, ventenata, medusahead wildrye, palmer amaranth and glyphosate-resistant kochia should be of concern for Wyomingites. 

While some species, like cheatgrass and kochia, are more prevalent across the state, other invasive species, including ventenata, medusahead and palmer amaranth, are new threats that producers should watch out for. 


At the end of May or beginning of June, if producers see a dark purple weed begin to dry down across their landscape, often cheatgrass comes to mind. 

“Cheatgrass is a nasty weed, but it could also be ventenata,” explains University of Wyoming (UW) Extension Educator Jeremiah Vardiman. “I have heard people who have ventenata wish they had cheatgrass instead.” 

While the species are very similar, Vardiman says ventenata is less palatable and provides very little spring grazing ability. 

“In the seedling stage, ventenata looks almost identical to cheatgrass. It is also a winter annual plant that germinates in the fall and produces thousands of seed heads in one year,” he explains. “Areas with ventenata grass drop forage production by up to 50 percent.”
To distinguish ventenata from cheatgrass, Vardiman says the seed head is key. 

The panicle of ventenata is also open and loose. Ventenata has a pyramidal panicle, meaning the petioles and seeds are distributed in a pyramid shape. 

Additionally, the awns of ventenata are bent and twisted.

“If producers are out on the landscape and they see what they think might be ventenata, contact Extension or Weed and Pest,” Vardiman emphasizes. “We have seen the plant in Sheridan and Johnson counties, and we want to do what we can to avoid its spread.” 

Weed specialists are concerned the plant may spread via livestock, in either wool, hair or the rumen. 

“So far we have seen ventenata mostly in rangelands, but all producers should be on the look-out for it,” he says.


Another species of concern is medusahead wildrye, which is also a major problem in several western states. 

“Medusahead wildrye has rigid, stiff awns, even when it’s green,” Vardiman describes. “The awns only get tougher as the plant matures.” 

The seed head has characteristic long awns, but Vardiman says medusahead is often confused with native plants.

“The palatability of medusahead is really terrible,” he says. “Even when it’s green, it isn’t desirable to livestock, so it could be a real problem if it gets onto our rangelands.”

Palmer amaranth

In cropland systems, Vardiman says a new concern is palmer amaranth. The species is similar to red-root pigweed, which is commonly found in the Big Horn Basin. 

While it hasn’t been found in Wyoming, Nebraska corn producers have seen infestations, and Vardiman adds, “This is a weed we definitely don’t want.” 

“Palmer amaranth is aggressive and fast-growing,” he says. “It can get taller than corn in certain cases, and it can get as big around as a person’s wrist.” 

The annual broadleaf weed looks very much like pigweed, and Vardiman comments, “We might think it is red-root pigweed on steroids.”

Palmer amaranth originated in the dry deserts of Mexico and the Southwest, and Vardiman notes it was introduced to the irrigated cropland of the Midwest through cotton meal, where it took off. 

“Palmer amaranth was used to only six inches of water or less, so when it saw 40 inches of water in the Midwest, it went gangbusters,” he says. “If palmer amaranth gets into a field, it has been known to break equipment, including cutter bars on combines and swathers.”

He adds, “Palmer amaranth has a robust, woody stalk as it matures, and it’s hard to get rid of.” 

To identify palmer amaranth, Vardiman says the plant has obligate leaves that are oval or egg-shaped and evenly dispersed. The leaves might have a v-shaped watermark as it matures. 

“The biggest indication for a sure-fire way to identify palmer amaranth is the petiole,” says Vardiman. “The petiole of the plant is longer than the leaf blade.” 

Additionally, the leaf tip comes to a hair-like point, and the stalk of the plant does not have any hairs on it. 

“If anyone sees palmer amaranth, they should turn it in to their local Weed and Pest or Extension office as soon as possible,” Vardiman emphasizes.


Glyphosate-resistant kochia is not new to Wyoming, and the plant has been documented in the Big Horn Basin. 

“We can identify glyphosate resistant kochia when we see strange patterns of kochia show up in our fields. We can see where the tumbleweed rolled through the field and distributed the seeds,” Vardiman explains. 

When glyphosate resistant plants are seen, Vardiman notes all the plants are approximately the same size, and they are not present in straight lines. 

“If we see straight lines in a field, that is likely not resistance,” he explains. “In those cases, it’s more likely we had sprayer skip.”
Vardiman emphasizes, “It’s important to keep all these plants in mind as we do weed management in our fields and on our land.”

Vardiman presented during 2019’s WESTI Ag Days, held Feb. 12-13 in Worland. 

Saige Albert is managing editor of the Wyoming Livestock Roundup. Send comments on this article to

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