Palmer amaranth is a new weed for Iowa, having shown up for the first time several years ago. It's a tough-to-control weed that's been marching up into the Corn Belt from the southern U.S. over the past decade. Palmer amaranth has been confirmed in five counties in Iowa over the last two summers. So it's not widespread here -- not yet, anyway.
The only (known) infestation in Muscatine County is on a very sandy soil near the Cedar River and most of the emerged seedlings had at least four true leaves on May 4, 2015. That's the observation of Meaghan Anderson, Iowa State University Extension field agronomist in east central Iowa.
"This particular infestation in Muscatine County has been intensively managed for just over a year, resulting in a significant decrease in its presence at the farm. This is very encouraging. It tells us that with early discovery and good management, eradication of this difficult-to-control weed may be possible," says Anderson.
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While the seedlings in this picture appear quite distinctive with their long petioles and round leaves, ISU weed management specialists would not suggest that identification of a population at this seedling stage is easy or even possible. "The presence of Palmer amaranth in this area previously gives us confidence that the seedlings seen on May 4 are Palmer amaranth and not another of the many Amaranthus species present in the state," says Anderson. "The Palmer amaranth in this field is much larger than waterhemp observed in other fields."
YOUNG PALMER: This photo shows two Palmer amaranth seedlings with at least four true leaves on May 4, 2015. Photo was taken at the only known infestation of this weed in Muscatine County, which is one of only five counties in the state where Palmer has been identified so far.
ISU website can help you correctly identify this difficult weed
When scouting for Palmer amaranth, Anderson says the key vegetative characteristics you should look for would be a lack of hairs on stems, ovate-shaped leaves, and the presence of petioles longer than the leaf blades. Bob Hartzler, ISU Extension weed scientist, has put together a number of information materials to assist in Palmer amaranth identification.
Many other Amaranthus species are present in Iowa, including redroot pigweed (Amaranthus retroflexus), smooth pigweed (Amaranthus hybridus), waterhemp (Amaranthus tuberculatus), and spiny amaranth (Amaranthus spinosus), among others. These Amaranths have significant genetic variability, often making it nearly impossible to distinguish many individuals without presence of inflorescences. Luckily, or unluckily, Palmer amaranth is expected to begin flowering in Iowa in late June to early July.
While it has only been confirmed in five counties, "we suspect that it is present in much more of the state already," says Anderson. "Likely areas of infestation would be near cattle or swine operations that import feed or bedding from southern states, especially those that use cottonseed products or gin trash. Two of the known infestations are near commercial grain handling operations; Palmer amaranth seed may have been brought on trucks transporting grain from areas with Palmer amaranth infestations."
PALMER AMARANTH: Where is this new weed in Iowa? So far, only five counties in Iowa have confirmed Palmer amaranth infestations.
"Please keep an eye out for this weed while scouting this summer," says Anderson. "If you find a suspicious population of Amaranths, contact your local Iowa State University Extension Field Agronomist or Bob Hartzler (515-294-1164)."Meaghan Anderson is an Extension field agronomist serving east central Iowa. Anderson can be reached at [email protected] or 319-337-2145.