by Carolyn Bernhardt
Key points:
- Palmer amaranth, a noxious weed in Minnesota, poses a significant risk to the state's agriculture and, by law, must be eradicated whenever it is found in Minnesota. The weed’s aggressive growth, prolific seed production, and herbicide resistance make Palmer amaranth a serious threat to Minnesota crops. If allowed to grow, it may surpass its native relative waterhemp as the region’s most troublesome weed.
- Recent MITPPC-funded research shows that Palmer amaranth is more adaptable to Minnesota’s relatively wet climate, showing resilience to both drought and partial flooding conditions, which challenges earlier assumptions that it would struggle in the Upper Midwest.
- The researchers grew Palmer amaranth and waterhemp in controlled greenhouse conditions that mimicked Minnesota’s climate, testing how the plants responded to different levels of water availability. The results showed Palmer amaranth was surprisingly adept at adjusting to the stress.
Scientists once believed Minnesota’s cooler, wetter climate helped stall the fast-growing, seed-churning, herbicide-defying menace to row crops, Palmer amaranth. But recent research funded by the Minnesota Invasive Terrestrial Pests and Plants Center suggests that assumption may be dangerously outdated.
As a postdoctoral associate at the University of Minnesota, Sachin Dhanda, PhD, who is now an assistant professor of weed ecology at South Dakota State University (SDSU), teamed up with Debalin Sarangi, PhD, an assistant professor and extension weed scientist in the Department of Agronomy and Plant Genetics at the College of Food, Agricultural and Natural Resource Sciences. The team tracked how Palmer has adapted to Midwest conditions.
"Palmer Amaranth is, according to the Weed Science Society of America, the number one troublesome weed in the United States," Dhanda says. "It can grow up to 2 to 4 inches in one to two days during peak growth."
Palmer amaranth is a native to the arid southwestern United States. Starting in 2023, the experts studied the weed’s ability to compete with native relatives like waterhemp under simulated climate stress. The project wrapped up earlier this summer with early findings suggesting that Palmer could be more flood-tolerant than scientists previously thought. These findings raise fresh concerns about how the aggressive weed will behave as Minnesota sees more extreme rain events due to climate change.
"We’re trying to understand if Palmer Amaranth has the potential to establish more strongly in northern states or if it’s just a matter of time,” says Sarangi.
The researchers investigated whether environmental barriers, such as wet springs and cold winters, have contributed to the limited spread of Palmer amaranth in Minnesota. They also explored whether competition from better-adapted native species, such as waterhemp, has been a factor, or if the spread is simply in an early lag phase before a larger outbreak.
"Our main concern is how Palmer Amaranth will behave under Minnesota’s different climatic conditions, which have more rainfall and cooler temperatures compared to the southern states," Dhanda says. The team conducted a controlled greenhouse study to simulate Minnesota’s environmental conditions and compare the growth of Palmer amaranth and waterhemp. They tested multiple populations of each species under five different water stress treatments—including constant flooding, flash flooding, normal watering, moderate drought, and severe drought—and measured growth rates and physiological responses to assess adaptability and competitiveness.
Despite being native to arid regions, the weed demonstrated surprising resilience to both drought and flooding, outperforming waterhemp in several stress scenarios. These findings challenge a long-held assumption among weed scientists—that Palmer would struggle in the Upper Midwest’s wetter conditions while waterhemp would maintain dominance. While the results are based on controlled environments and need further validation in the field, they raise serious concerns for land managers. If Palmer amaranth continues to prove this adaptable, it could soon surpass waterhemp as Minnesota’s most troublesome weed—posing a major threat to crops like sugar beets, potatoes, and dry beans, which already have limited herbicide options.
Early detection and rapid eradication are especially critical, particularly as long-term funding and support from various public agencies could take a dive. And while the greenhouse study provided valuable insights, it also came with several challenges. One of the biggest hurdles was inconsistent germination across seed populations.
Sarangi reiterates that the greenhouse study was also a simulation. “When you put this out in nature, there are some other factors interacting,” he says. For example, unlike field soil, pots lack complex soil layering and capillary water movement, which may influence the plant’s access to moisture. The study also tested only two populations of each species due to space and resource constraints. “If I have unlimited resources, unlimited people,” Sarangi says, “I'll do it for 10 Palmer Amaranth populations, 10 Waterhemp populations.”
Still, the study offers firsthand data on how Palmer amaranth might behave under Minnesota’s unique environmental conditions—information that many local farmers currently lack. “Most of them have no firsthand experience dealing with Palmer Amaranth,” says Sarangi. “They’re dying for some information.”
By showing that Palmer can survive not just cold but also waterlogged conditions, the research directly challenges the common assumption that the weed can’t thrive in the Upper Midwest. The study also contributes to the growing body of evidence that Palmer amaranth is remarkably adaptable, capable of surviving in both desert-like and flooded environments. This opens the door for further genetic studies to explore what gives this weed such resilience—information that could potentially inform crop breeding efforts in the future. In addition to its scientific contributions, the study supports outreach efforts led by the Minnesota Department of Agriculture’s Palmer Task Force, helping land managers identify and manage Palmer before it takes hold.
Future research, the team says, will also focus on the early detection of Palmer amaranth using machine learning tools, such as drones, robots, cell phones, and satellite imagery, to identify plants at just 1–2 inches tall. Researchers are also exploring integrated weed management strategies, including the use of cover crops and Harvest Weed Seed Control to sustainably reduce weed populations and delay herbicide resistance. Education and outreach remain priorities, as well, with workshops and blogs planned to help growers recognize and manage Palmer amaranth.
“MITPPC’s funding was used to train a new generation of weed scientists and generate information that will ultimately help the stakeholders make informed decisions," says Sarangi. The support funded Dhanda’s salary as a postdoc. Now, his faculty position at SDSU is in full swing. “It’s a similar situation in South Dakota — we’ve already found Palmer amaranth in around 10 to 12 counties,” Dhanda says. “So, same challenge, different state. We’ll definitely collaborate.”