Respect the Rotation tour visitors listen intently to University of Illinois weed scientist Aaron Hager’s presentation on the challenges of controlling Palmer amaranth. Palmer amaranth weeds in the foreground have overtaken soybeans in a test plot focusing on herbicide timing and rates.
Respect the Rotation tour visitors listen intently to University of Illinois weed scientist Aaron Hager’s presentation on the challenges of controlling Palmer amaranth. Palmer amaranth weeds in the foreground have overtaken soybeans in a test plot focusing on herbicide timing and rates.
ESSEX, Ill. — Farmers going toe to toe against Palmer amaranth will lose.

“This thing will outgrow us. We don’t have enough spray rigs in this state. We don’t have enough folks that can operate them,” University of Illinois weed specialist Aaron Hager said at a Respect the Rotation field tour.

“The way we win the battle against Palmer is not going against the plant, but is by attacking this thing at the weakest point in its life cycle — when it’s a seed.”

The Kankakee County research plots facilitated by Bayer CropScience, tests various treatments in a field where Palmer amaranth was established about three to four years ago.

One of the biggest advantages that can be used to combat Palmer is that the seeds are not indefinitely viable in the soil

Unlike velvet weed or giant ragweed where seeds are viable in the soil for 30 or 40 years, Palmer seed is viable for no more than 10 years.

“If you can prevent these seeds from being back into the seed bank for three, four or five years, you can really begin to see these population numbers go down. That’s why we focus so much of our time and effort on talking about the seed introduction and seed prevention,” Hager said.

“The problem with preventing seed introduction is I don’t think we can do it. There are so many ways that these small seeds can move around quickly from area to area.”

Hoe Down

Researchers are therefore focusing on what can be done to prevent the plants from producing seeds.

“In some instances, a hoe is going to be the only thing that we have to do that with,” Hager said. “Nobody likes to do that on a hot sunny day, but if we can remove these plants before the seed population begins to build, five years later we’re going to look back and say that’s the best thing we ever did.”

Palmer amaranth has a higher growth rate and is more competitive than other pigweed species.

Growth rates approaching three inches per day, and yield losses of 78 percent in soybeans and 91 percent in corn attributed to Palmer amaranth have been reported in the scientific literature.

“Once these populations do become established, your herbicide cost will at least double. In many areas of Tennessee Palmer is now about a $100-per-acre weed,” Hager said.

Palmer infestations vary in the research plots depending on herbicide applications.

4-Inch Limit

Hager noted one plot that featured various glyphosate rates and application times.

The first application was when Palmer was about four inches tall, and a second application was done seven days later.

“What we found this year is that we started with 22 ounces, we then go to 44, 66, 88 ounces on up and found at this site that once we hit in the range of 44 and 66 ounces our control ratings were about 95 percent,” Hager said.

“The challenge that we ran into is we were only able to evaluate that experiment for about two weeks, because two weeks after either the early timing or the last timing we couldn’t see the ground between the rows. We had another flush of Palmer and very rapid growth rate.”

He said a minimum of three applications of herbicides would be required before a combine could be used to harvest the field due to the heavy Palmer population.

“If you did one application, you’d never put a combine in here. If you did two, you could put a combine in here, but you’d cuss yourself for not making three. With three you have a fighting chance and four applications would be even better,” he said.

“I don’t honestly remember how many germination events we’ve had in this particular population. I think it’s somewhere over about 12.”


Hager recommends the following guidelines for Palmer amaranth management:

n If you discover a plant that you think may be Palmer amaranth, you can verify its identity by sending a leaf tissue sample to the U of I for identification using molecular biology techniques.

n Plants confirmed or suspected of being Palmer amaranth should be physically removed from the field prior to flowering. Do not rely on herbicides for control. Physical removal can include hoeing or hand-pulling plants from the soil. If hoeing is used, be sure to sever the plant stem at or below the soil surface to reduce the potential for re-growth, and remove plants from the field as they will re-root from stem fragments.

n If Palmer amaranth plants are not identified until after brown-to-black colored seeds are present on female plants, we suggest leaving the plants undisturbed in order to avoid inadvertently spreading seed.

n Mark or flag areas where Palmer amaranth plants produced seed. These areas should be intensively scouted the following season and an aggressive Palmer amaranth management plan implemented to prevent future seed production.

* Do not mechanically harvest mature Palmer amaranth plants. Physically remove the plants prior to harvest and either leave the plants in the field or place in a sturdy garden bag and remove the plants from the field. Bury or burn the bags in a burn barrel as soon as possible.

* Fields in which Palmer amaranth seeds were produced should not be tilled during the fall or following spring. Leaving the seeds near the soil surface increases the opportunities for seed predation by various granivores.

* Herbicides that control waterhemp also control Palmer amaranth. An integrated herbicide program should include soil-residual herbicides applied at full recommended use rates of within two weeks of planting and followed by post-emergence herbicides applied before Palmer amaranth plants exceed 3 inches tall.

“From a management standpoint, there is no one single tactic that will effectively control this thing from reducing crop yield,” Hager said.

“This thing plays by its own rules. We have to adapt to its rules, it will not adapt to what we want to do. If there’s ever been a good example of a species that an integrated system is necessary for, this is it without a doubt.”