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Don’t Sleep on Waterhemp

Ignoring this weed will come back to bite you.

Palmer amaranth is the pigweed species that rightfully earns the most consternation from Midwest farmers. But don’t forget that waterhemp can be just as invasive and contains many of the same characteristics as Palmer.

That’s the take-home message from Brian Jenks, weed specialist at North Dakota State University, during the annual Ag Horizons Conference in Pierre, South Dakota, on November 26.

“From the Dakotas, through Iowa, Illinois, and Indiana and across the Midwest, waterhemp is becoming a huge problem,” warns Jenks.

He adds that many farmers treat waterhemp like redroot pigweed – an invasive species itself, but much easier to control than waterhemp. “It looks a bit like redroot pigweed, so people tend to treat it like redroot pigweed. My fear is that will come back and bite us.”

Why it’s a problem

Brian Jenks
Brian Jenks
Waterhemp emerges throughout the growing season, and is prone to herbicide resistance. Even if growers use a residual herbicide, or postapplication, the weed emerges and thrives in the summer months. “On top of that, the herbicides we’re applying post-emergence in soybeans aren’t working [on waterhemp] anymore. Our choices are fewer,” Jenks says.

If that sounds familiar to Palmer amaranth, you’re right.

The difference between the two weeds are subtle. Waterhemp has longer, narrower leaves than Palmer’s leaves, which are wider at the base. Palmer has petioles longer than the leaf blade.

“The real difference between the two is that Palmer grows quicker, and maybe a little taller. But the weed characteristics are the same,” Jenks explains. That means: prolific seed production, quick growth, and the scary fast ability to overtake a field.

What to do

In the western edge of the western Corn Belt, waterhemp infestations tend not to be severe – yet. But farmers should become familiar with the weed’s growth habits and appearance, scout fields, and remove waterhemp plants if they find them.

Those who have a more severe problem are advised to intensify crop rotations. Adding long season cash crops to a short-season system, or fall crops to a spring crop rotation, allows for different chemistries to be used at different times of the year. Small grains, such as wheat, canola, or oats, also tend to keep the pigweed species from thriving in the summer.

“Anything we can do to reduce the seed in the seedbank is beneficial. Diversity is the key,” he says. “Growers who have four to five or six crops in a rotation just tend to have fewer problems with the resistance issue than folks doing fewer crops.”

Dicamba-tolerant crops are a tool, but Jenks says these are a short-term solution.

“We know resistance will develop to dicamba. We had kochia that is tolerant to dicamba – even before dicamba beans came to the market,” he says.

“I think the easy ways are going to fail. These weeds will adapt to what we’re doing, if we keep going with the same rotation and same herbicides over and over.” 

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