The worst-weed rankings for corn become more consistent across regions when rated by their difficulty to control. When defined in this manner, all three experts rank common waterhemp as worst.

“In Illinois, waterhemp tops our list of worst weeds, no matter what the crop,” Hager says. “I list it first because of its predominance across the state and because it’s ideally suited to adapt to today’s modern farming practices: It does well in limited tillage situations; it has an extended period of emergence; one female plant can produce one million seeds; and machinery can transport the small seeds from field to field.”

Owen agrees that waterhemp has an uncanny ability to stymie efforts to stop it, compared to other weeds. “It’s come to greater prominence in Iowa recently because of its ability to adapt to whatever product farmers use to control it,” Owen says. “Nationwide, waterhemp has evolved resistance to ALS-inhibitor herbicides, PPO-inhibitor herbicides, glyphosate, HPPD inhibitors, photosystem II inhibitors and now to 2,4-D in Nebraska.”

Any solution to waterhemp has to start with an integrated approach, Hager says. “Farmers will need to use more than just one herbicide to control it,” he says. “They should begin by planting into a weed-free environment, either by using some form of preplant tillage and/or a soil residual herbicide that is applied at the appropriate rate for soil type.”

When using postemergence herbicide products, weed height is an important factor for effective control, Hager emphasizes. “For waterhemp, a timely application should occur at or before a 4-in. weed height,” he says.

Farmers also should scout after treatments to see how well they worked, he adds. “Very few soil residual herbicides have activity that will last the whole season,” he says. “And with waterhemp, very few postemergence herbicides will provide adequate control with just one application.”