Palmer amaranth is on the move in Iowa

Palmer amaranth is on the move in Iowa

Some new areas of infestation of this tough-to-control weed have been found in western Iowa.

Talking to farmers in meetings this winter, Iowa State University Soybean Research Center agronomist Clarke McGrath shared what he thought was accurate and encouraging news—Palmer amaranth was still being pretty well contained in the known locations across the state. Unfortunately, in late February he found that in Harrison County in western Iowa this new invasive weed has increased its range and seed bank.

Related:Palmer Amaranth: Understanding the Profit Siphon in your Field 

PALMER AMARANTH: Also known as Palmer pigweed, this extremely aggressive, fast-growing weed is a serious problem in the southern U.S. and is showing up in more fields in Midwest states. It’s been identified in several locations the past few years in Iowa.

After discussions over the last few weeks with growers and retailers in the area of the original Palmer infested fields near Modale, “we suspected there may have been a significant Palmer infestation in another field in the vicinity of the original field,” says McGrath. “When the snow was gone the last week in February, a local agronomist and I hiked out in the mud to see what we were dealing with.” McGrath, who also serves as an ISU Extension field agronomist, is based at Harlan in western Iowa.

They found some new infestations of Palmer in western Iowa
After hiking in a mile or so, they found what they were looking for, a large area of Palmer amaranth (and waterhemp) that had a very prolific year in 2015. In some unharvested soybeans, there were hundreds of Palmer plants, some approaching 6 feet tall and standing well in spite of the winter weather. A lot of weed seed was produced in this unharvested area of the field.

There was also evidence of Palmer in parts of the harvested portion of the field, meaning some Palmer seed likely went through the harvesting equipment. “As of now, we don’t know where the grain or the harvesting equipment went after running through the Palmer or if the equipment was cleaned of the weed seed before the machinery was used in another field,” says McGrath.

More hiking led to finding more Palmer in areas to the west and south of this field. The two agronomists found some in an adjoining cornstalk field, so there is the likelihood that Palmer weed seed contaminated the harvest equipment and the grain as well. Scouting of the original Palmer field and its borders through last summer showed control was pretty solid in 2015; it was nearly “Palmer free.” They saw some Palmer, but it was easier to see it this last trip in February since Palmer stands so well compared to other weeds and grasses that tend to lodge and lean over after a few months of winter. There was mature Palmer amaranth that had set seed in border areas and along lanes.

Results of ISU weed specialist’s trek around Iowa last fall
ISU Extension weed management specialist Bob Hartzler last fall also took a look at some fields and areas in Iowa where Palmer had previously been identified the past few years. That included some western Iowa Palmer infestations.

* In Fremont County: Hartzler was disappointed in the control of Palmer amaranth here for two reasonsFirst, it is a very small field, less than 2 acres. The second is that the field is used for show plots for a seed company. It wouldn’t take much effort to adopt the zero threshold approach for Palmer amaranth management at this location. While there weren’t large numbers of Palmer amaranth present in the field, there were enough to sustain the population and act as a seed source to spread the weed to new fields. There also was a healthy population of Palmer in an adjacent ditch.


* In Page County: On the final stop of the day when Hartzler was searching last fall, he found a few escapes in the crop field and adjacent areas. But the Palmer amaranth population was down significantly compared to 2014. Pigweed population in the area was dominated by waterhemp rather than Palmer amaranth in 2015, whereas in 2014 the weed population was split equally between waterhemp and Palmer, if not skewed towards Palmer amaranth.

In the coffee shop, it is known as palmer pigweed. In university circles, it is referred to as Palmer amaranth. Whatever you want to call it, this weed is the No. 1 weed to watch. Download our free report Palmer Amaranth: Understanding the Profit Siphon in your Field

Looks like Iowa farmers still have upper hand on this new weed
“Our experience over the last few years with these few Palmer fields, and some insight on this topic from Bob Hartzler, leads me to believe we have the upper hand right now in controlling this invasive new weed, and for a few reasons,” says McGrath.

First, Palmer amaranth may not be completely adapted to Iowa’s soils and environment at this time. But it will adapt with time, sort of like waterhemp was an unknown weed in Iowa and then the population increased in many fields statewide, in a hurry.

Second, the ability of growers to manage the Palmer infestation fairly well in some of these fields indicates Palmer amaranth is unlikely to run rampant across the state in the next few years. “We can keep it suppressed if we watch for it and eradicate it early rather than letting the weed seed population build up in a field,” says McGrath.

Keep a close watch on your fields for Palmer amaranth
Hartzler says one thing probably working in Iowa’s favor is it often takes weeds several years to adapt to new environments, a period referred to as the “lag phase.” However, he doesn’t downplay the need for vigilance regarding this new weed threat.

Palmer amaranth is more competitive than waterhemp, thus when and if Palmer becomes more widely distributed in Iowa, it will have a much bigger economic impact than our current No. 1 weed problem, says McGrath. “Palmer amaranth needs to be aggressively attacked at this time while it is at a disadvantage competing with waterhemp, other weeds and the crop.”

Proper identification of this hard to control weed is critical
In summary, McGrath offers the following points:

* Palmer is spreading slowly in Iowa, but this new find in Harrison County and some of the variables with the weed seed being transported off the farm on equipment and in grain means “we need to continue to watch and scout for Palmer amaranth, especially in western Iowa.”

* Growers are doing a good job watching for this weed, as was the case with this new infestation. “At winter meetings farmers asked to see what a Palmer seed head looks like,” says McGrath. “Occasionally they said they need to go look at something similar in their field borders, terraces and grass waterways. Palmer isn’t at the top of their list of weed control concerns right now, but it is on their radar and they’re watching for it.”

* This spring and summer, scout for Palmer amaranth in your fields. Use effective herbicide groups and application rates of residual herbicides. Hit your weeds early with post herbicides. Then scout some more. If you have a few survivors that look like waterhemp or Palmer you are wondering about, contact a local agronomist for help with identification.”

For more information on identifying and managing Palmer amaranth and other weeds, visit

TAGS: Soybean
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