It’s a matter of when, not if, the weed palmer amaranth spreads into the Pacific Northwest, researchers say.

Several possible samples have already been sent to Colorado State University for identification, University of Idaho Extension specialist Albert Adjesiwor told the Capital Press.

“It’s a lot harder to control than most of the weeds we have,” Adjesiwor said. “If you’re struggling to control kochia, you’ll probably struggle more to control palmer amaranth.”

The Weed Science Society of America calls palmer amaranth the most troublesome weed in the U.S.

It has developed resistance to at least nine herbicide chemistries, according to the UI. Some populations can be treated with dicamba and 2,4-D. Other populations have already developed dicamba resistance.

The samples will be screened for resistance, Adjesiwor said.

The weed could be introduced into Idaho through cover crop blends or contaminated hay, or cottonseed meal imported from other states as livestock feed, the UI press release said.

Palmer amaranth has a hard seed coat that can pass intact through the digestive tracts of cattle, making manure a potential source of contamination.

Tainted birdseed and farm equipment are other potential means of introducing the weed.

Native to the desert southwest, palmer amaranth is widespread in the U.S., except for the Pacific Northwest, Montana and a few states in the northeast.

Palmer amaranth has a diamond-shaped leaf and its petioles — the stem-like structures connecting the main stem to the leaf blades — are longer than the leaf blades.

Palmer Amaranth commonly grows 6 to 8 feet tall, “wreaking havoc” on surrounding crop yields, the press release said.

Adjesiwor would like to see the Idaho State Department of Agriculture add palmer amaranth to the state’s noxious weeds list. That would allow the state to track and scout for the weed — and to respond rapidly when it surfaces.

Palmer amaranth appears similar to other pigweeds already occurring in the Pacific Northwest, Adjesiwor said.

The state Department of Agriculture contacted Washington State University weed science professor Drew Lyon several years ago about identifying different amaranth species by seed, which is “downright near impossible” without growing them out, he said.

Lyon said palmer amaranth has “tremendous potential” to irrigated crops grown under summer irrigation in the Columbia Basin.

Growers most need to learn to identify the weed, Lyon said.

“Right now we’re in the ‘Let’s keep our eye out for it and let people know who need to know if we find it’ (stage),” he said.

Adjesiwor is most worried about growers potentially having the weed and not knowing it.

“If we’re able to catch it very early in the infestation process, we can at least prescribe crop rotations or herbicide programs that would at least keep it under control and not get out of hand,” he said.

A UI master’s student recently began developing management strategies for herbicide-resistant weeds, using a two-year, $100,000 state Department of Agriculture grant.

Adjesiwor and UI released the publication “Pigweeds: Current Emerging Weed Threats in the Pacific Northwest” to help farmers identify the weed.