Palmer amaranth breaching Ohio’s borders

By Matt Reese

Farmers have read about the horrors of hiring hand-weeding crews to clean up fields where herbicide resistant weeds have run rampant. They have heard the stories of failed crop fields due to weed pressure and there are plenty of tales of woe from the South about weed nightmares.

Palmer amaranth from a southern cotton field.

While there are many seedy weed characters behind these scary stories, there is one that rules them all. A dark general of weeds that can seize ahold of fields and rule with an indomitable iron fist that can wipe out farm profitability and productivity, glyphosate resistant Palmer amaranth has breached Ohio’s southern border on its destructive march to the north. Earlier this summer, this nightmare weed was spotted in a large field near Portsmouth in extreme southern Ohio. The weed, which has typically been prevalent in Southern states, is moving north, with several other suspected cases statewide. New infestations of Palmer amaranth have also been found farther north, in Michigan and Indiana.

Seed Consultants, Inc. director of agronomic services Bill Mullen, who works extensively with farmers in southern Ohio and around the state, said the weed should be a serious concern for farmers.

“There is a new weed coming — Palmer amaranth. It has been discovered down there around Portsmouth, Ohio around some of the river bottoms. We’re going to have to stay right on top of Palmer amaranth. When it becomes resistant to glyphosate, people will have to change their program to try and control this broadleaf weed,” Mullen said. “It is not going to be a small weed. It gets tall and crowds out any existing corn or soybean seedlings that are trying to come through. It will be very competitive against our growing crops. If we don’t take care of it early, the small areas that it is in will only get bigger. It could prove to be really limiting to yield potential as we go forward.”

Palmer amaranth can grow three inches a day and release nearly a half-million seeds per plant. And because the weed is glyphosate-resistant, many growers in Southern states, in addition to spraying, have had to hire crews to go into their fields to chop down the weeds with hoes and pull them by hand. Losses for growers dealing with this aggressive weed have been in the millions.

Palmer amaranth taking over a cotton field.

“These plants throw out a lot of seed. If we don’t control the weed quickly, we’ll see issues with it for many years down the road because of the seed it produces,” Mullen said. “If we get into hand weeding resistant weeds, I question if we can profitably grow corn or soybeans. If it comes down to that, we’re just going to have to set aside some money to control this weed. You may have to go to continuous corn where you have more weed control options.”

Ohio State University Extension experts are warning Ohio growers to take measures to prevent its further spread statewide, said Mark Loux, an OSU Extension weed specialist. The concern about glyphosate-resistant Palmer amaranth, that has required entire cotton and soybean fields to be mowed down in the South, is that if the weed gets established Ohio, it will be even harder to control than the glyphosate-resistant weeds already present statewide, he said.

“Not only is Palmer amaranth resistant to glyphosate, this weed’s rapid growth, large size, extended duration of emergence, prolific seed production and general tolerance to many herbicides makes it a much more formidable weed to deal with than the pigweed species we already have here in Ohio,” Loux said. “Among the weeds that we already deal with, Palmer amaranth is going to require pre- and post-emergence applications, possibly multiple post-emergence applications.

“It’s already resistant to two of the main types of herbicides we use in soybeans, glyphosate and ALS inhibitors, and the weed has to be less than four inches tall when spraying in order to get control of it.”

The good news is that with early diagnosis of infested fields and quick action, the potential spread of herbicide resistant Palmer amaranth in Ohio can be slowed or stopped, he said.

“This is one of those rare occasions where we have enough information to hopefully prevent additional infestations of an extremely aggressive weed, and there could be serious long-term consequences for farm profitability for failing to do so.”

As a part of the effort to monitor the resistant weed situation around the state, Loux is encouraging farmers participate in a couple of projects.

1.Take a survey about postemergence management of giant ragweed in Roundup Ready soybeans.  The survey found at the following link is brief (10 questions) and completely anonymous:

2. Extension is starting a project to determine whether pigweed populations in Ohio have developed resistance to glyphosate. The first step of this is to collect seed from any population of pigweed that survived POST glyphosate application in Roundup Ready crops. This includes redroot/smooth pigweed, waterhemp, or any related species. Contact Extension with the location of fields that can be sampled from, or to collect and mail seed. For the latter, please provide field location and your contact information. Contact Bruce Ackley for more information at 614-292-1393 or

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