When waterhemp goes wild

Keeping this weed in check requires a comprehensive herbicide and management game plan.

Waterhemp control in corn and soybeans took a hit in 2025, as postemergence herbicide treatments with glufosinate and mesotrione became less effective.

The cause: Increasing herbicide resistance, larger farm size and weather wreaking havoc with application timing and weed uptake. Poor results make weed resistance diagnosis more difficult when low-level herbicide resistance combines with less-than-ideal spraying conditions.

Over the decades of repeated herbicide exposure, waterhemp has evolved to resist single and multiple sites of action among herbicide groups (2, 4, 5, 9, 14, 15 and 27), according to the Weed Science Society of America. And, just recently, University of Illinois weed scientists confirmed that waterhemp populations in the state have developed resistance to glufosinate (Group 10). Waterhemp species that resist multiple herbicide groups, known as non-target-site resistance, are increasingly being stacked on top of target-site resistance to create complex resistant populations.

“Last year, the weed control issue that stuck with me is farmers are spread too thin for timely spray coverage over all their acres,” said Wes Everman, Iowa State University Extension weed specialist. “One weather wrinkle, like extra rain or excessive wind, creates a cascading effect that keeps them out of fields in a timely manner.”

Everman also noted that temperature, humidity and soil moisture conditions all impacted weed growth and herbicide efficacy, in tandem with herbicide resistance. “I believe we need to pay closer attention to these weather conditions before we spray, especially with some of our herbicides like glufosinate,” Everman said.

For example, the glufosinate label indicates that high humidity, warm temperatures and bright sunlight are needed to improve performance on actively growing 3-inch-tall weeds (or up to 6-inch weeds with higher rates).

Avoid glyphosate mentality

Trying to shoehorn current postemergence herbicides into the glyphosate mold is increasing waterhemp control failure.

“Roundup [herbicide] had so much flexibility, with little worry about weed species, size, rates, nozzles — none of that. Just pour it in the tank and go,” Everman said. “We cannot do that with glufosinate, 2,4-D, dicamba, PPOs, HPPDs or really any post herbicide. That has been a challenging mentality for growers to overcome during the last 15 years.”

Everman often talks about a field-by-field strategy during his winter grower meetings. Managing smaller pieces of land and customizing layered control tactics can help improve waterhemp control.

“As you plan 2026, identify your weediest fields and tailor your herbicide strength to them. Begin with a strong residual preemergence program using two effective modes of action on waterhemp. This increases your flexibility to revisit your toughest fields at different crop stages. Weeds won’t get as big as quickly, so you can spread out your post-spraying workload.”
— Wes Everman

On less weedy fields, growers can reduce the preemergence program and then adjust the post-spray timing, he continues. The goal is to spread the application workload, so there’s less need to spray everything during a one-week window. Prioritize the worst fields first.

One size doesn’t fit all

An effective weed control plan means avoiding one-size-fits-all approaches. “Sure, it’s easy for growers and custom applicators to prefer the same tank mix for all preemergence acres and the same mix for all postemergence acres, not having to swap herbicide mixes field to field,” Everman said. “But this strategy fails to target our weeds, it doesn’t reduce our weed seedbank and it’s not cost-effective.”

A successful layered approach to reduce waterhemp plants’ ability to produce seed often goes beyond the jug. More farmers are trying and succeeding with cover crops. Some farmers are using primary tillage to reduce early weed flushes.

“I’ve had a number of guys tell me this winter that they are using a cultivator again,” Everman said. “And I’ve been surprised at the number of meeting attendees who tell me they walk soybeans to pull waterhemp, saying it can make a big difference.”

Corn challenges

Farmers are also paying closer attention to waterhemp in corn, as the once-infallible herbicides are now struggling to control it. Waterhemp resistance to HPPDs began occurring a decade ago. Now, there are some signs of resistance to Group 15 products. Everman says farmers should now worry about controlling waterhemp in corn.

Weeds ignore field planting order, growing degree days and fertility plans. “Sometimes we must adapt our mindset and alter our approach. By improving residual control of the initial flush of waterhemp, we gain a wider application window to spray under more optimal conditions,” Everman adds.