Pigweed Problems: U Of I Weed Scientist Concerned About Rapid Spread Of Two New Weeds In The State With Resistance To Common Herbicides

Published online: Feb 08, 2024 Articles John O'Connell, University of Idaho
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MOSCOW, Idaho — It’s been less than two years since University of Idaho Extension weed scientist Albert Adjesiwor’s fears were realized and a pair of problematic pigweeds – Palmer amaranth and waterhemp – were first found in Idaho. 

Yet it’s already clear to Adjesiwor that the weeds have established a firm foothold throughout the Magic and Treasure valleys. He acknowledges the window to eradicate the two invaders has closed, and his focus must shift toward educating farmers about how to manage them. And managing Palmer amaranth and waterhemp is no easy task.

The weeds tower over crop canopies – Palmer amaranth can grow more than 10 feet tall – and both species have developed broad resistance to common herbicides, making them extremely tough for growers to kill. Their seeds can survive in soil for up to five years, making it critical for farmers to make certain weeds that escape herbicide application are killed using any means possible before going to seed. 

In potatoes, a density of one Palmer amaranth plant per three feet of crop row can cause yield losses of about 50 percent. Adjesiwor saw one heavily infested potato field patch in Bruneau that he suspects was a total loss. 

If the weeds are too rampant in a field to be hand pulled, Adjesiwor advises mowing them, before applying mixtures of herbicides. Adjesiwor has seen evidence that the weed seeds entered Idaho in birdfeed, in cottonseed meal fed to cattle and on farm equipment brought in from surrounding states. 

“These weeds are tough to control, and they develop resistance to most herbicides we use,” Adjesiwor said. “My fear is they might have come in from states where they have already developed resistance to the herbicides we use in Idaho. Even if they are not resistant to most of the herbicides we use, it will not take long for them to develop resistance in the state because we apply the same herbicides every year.”

Sounding The Alarm

From Burley in the Magic Valley to Parma in the Treasure Valley, Palmer amaranth has become widespread, even growing in roadway medians. 

Adjesiwor couldn’t find Palmer amaranth or waterhemp when he drove throughout the region scanning farm fields shortly after he started working at the UI Kimberly Research and Extension Center in 2020. He had a suspicion, however, that it would only be a matter of time before one or both of the weeds would be discovered in Idaho, knowing they were present in surrounding states. 

Hoping to keep the problem at bay for as long as possible, he published an Extension bulletin in 2021 offering background and physical descriptions of the plants. Palmer amaranth has egg-shaped leaves and especially long leaf petioles. Waterhemp has slender foliage. Both weeds have separate male and female plants. 

In 2022, Adjesiwor brought potted samples of the weeds to field days, seeking to raise awareness among growers and agronomists. After one of the field days, he got a call from a Glenns Ferry farm about a pigweed that couldn’t be killed by glyphosate, which is the active ingredient in Roundup herbicide. The weeds proved to be Idaho’s first known waterhemp. He received another call a short while later about pigweeds in Homedale, which Adjesiwor confirmed as the state’s first known Palmer amaranth. 

“Probably they were here for a while, but people needed to know what they looked like,” Adjesiwor said. 

Reports of the two pigweeds began to flood in after Clarke Alder, an Amalgamated Sugar Co. weed scientist, investigated a company crop consultant’s report of a glyphosate-resistant pigweed in an Owyhee County farm field in late July 2023. 

“As soon as I got there, I knew what I was looking at and I called Albert Adjesiwor immediately,” Alder recalled.

Alder, Adjesiwor and Oregon State University (OSU) weed scientist Joel Felix collaborated to develop an identification guide for the weeds and launched a regional scouting effort. Alder recruited 20 Amalgamated Sugar crop consultants to keep an eye out for the weeds and report GPS locations of anything suspicious. They’ve also encouraged agronomists from other companies to assist. As a result, they’ve now located 71 populations of the two pigweeds in southern Idaho. 

Resistance Challenges

Adjesiwor has been collecting seeds from mature Palmer amaranth and waterhemp plants throughout the region, planting them in a greenhouse to increase seeds and evaluate the populations for resistance to common herbicides. 

None of the waterhemp seeds germinated, as they were likely harvested before they were ripe and viable. He’s been screening Palmer amaranth samples, however. He’s already confirmed they have broad resistance to glyphosate, and he anticipates he’ll also find resistance to Group 2 herbicides, which include imidazolinones, sulfonylureas, sulfonamides and triazolopyrimadines. 

Glyphosate resistance is especially concerning to Adjesiwor, given that all of the sugarbeets raised in Idaho have been engineered to withstand glyphosate applications. Adjesiwor emphasized that both weeds are most effectively managed with pre-emergence herbicides. 

“Growers are not used to managing these weeds,” Adjesiwor said. “My fear is they will not know they have them until they come up. In potatoes there are only a few options to manage Palmer amaranth and waterhemp once the potato plants come up. When sugarbeets come up, growers have no options.”

Responding To The Threat

Adjesiwor, Pamela Hutchinson, a cropping systems weed scientist at the UI Aberdeen Research and Extension Center, and collaborators from OSU and Washington State University have applied for a $27,000 grant through the Northwest Potato Research Consortium.

The funds would launch a tri-state effort to scout for the two pigweeds, gather seeds and raise plants from them to screen for resistance to common potato herbicides. They would also support a graduate student to help with spraying and evaluation. Hutchinson will aid in the search for Palmer amaranth and waterhemp in eastern Idaho, where it’s possible the weeds haven’t been found only because people haven’t looked for them. 

“We’ve seen it in southcentral Idaho and we expect it will move over here,” Hutchinson said. “That’s why we’re doing this survey. We’re trying to get ahead of it.”

Adjesiwor and other stakeholders will request additional funding from chemical companies to collect, store and test weed tissue samples, which would provide growers quicker, in-season herbicide-resistance results while also enabling them to record data without allowing weeds to go to seed. 

On Jan. 10, Alder and Amalgamated Sugar petitioned the Idaho State Department of Agriculture to have Palmer amaranth and waterhemp listed as noxious weeds. Palmer amaranth is native to the desert Southwest, and waterhemp is native to the Midwest, which is why they haven’t been listed federally as noxious weeds. They’re both designated as noxious weeds in several other states, however. 

A noxious weed designation would enable the state and counties to devote resources toward aiding growers in the weed battle, including spraying the weeds on roadsides. 

“Glyphosate has been a massive tool for us since 2008. The problem is these weeds are already resistant to glyphosate, and they need to be controlled at a young stage of their lifecycle,” Alder said. “We have to be very proactive with these weeds in particular because of the limitations of control measures and because of their lifecycle, otherwise the repercussions can be quite detrimental to sugar beet crops – 70 percent-plus yield losses in the worst-case scenarios.”

Albert Adjesiwor, Assistant Professor and Extension Specialist/Weed Science, (208) 423-6616, aadjesiwor@uidaho.edu.